Notes on the Flat-Text Transcription

The content of this transcription differs from the content of the
original document in the following ways.

1. Page breaks and pagination have been omitted.
2. As a result of the above, the page numbers have been left out of
   the table of contents, and the index has been omitted entirely.
   (Unlike a Postscript document, a text file can be searched.)
3. The contents of the title page and the footer text has been placed
   at the beginning.
4. The lines and boxes in the original figures and tables have been
   adapted.
5. Differing fonts have, of necessity, been elided. For the most part,
   the context is sufficient to understand the meaning. In a few
   places, however, the original document used italics to implicitly
   indicate that the text stood for a variable string. In these cases,
   I have used <angle brackets> around the text to indicate this.
   There are no occurrences of angle brackets in the original.
6. The original contains three errors which are not immediately
   obvious as such upon a casual reading, but which can be
   unambiguously identified as such and the proper contents
   determined. I have taken the liberty to correct these errors. Their
   locations are marked in the text by a {*}. Any other (seeming)
   errors I have let stand.

Any other differences between the contents of this file and the
original are my responsibility. Direct notices of such errors to
breadbox@muppetlabs.com.

Brian Raiter
[Last edited Fri Jul 23 1999]

   ________________________________________________________________


		 EXECUTABLE AND LINKABLE FORMAT (ELF)

	     Portable Formats Specification, Version 1.1
		    Tool Interface Standards (TIS)

   ________________________________________________________________


   =========================== Contents ===========================


   PREFACE
1. OBJECT FILES
   Introduction
   ELF Header
   Sections
   String Table
   Symbol Table
   Relocation
2. PROGRAM LOADING AND DYNAMIC LINKING
   Introduction
   Program Header
   Program Loading
   Dynamic Linking
3. C LIBRARY
   C Library

   ________________________________________________________________


			       PREFACE

   ________________________________________________________________


		  ELF: Executable and Linking Format

The Executable and Linking Format was originally developed and
published by UNIX System Laboratories (USL) as part of the Application
Binary Interface (ABI). The Tool Interface Standards committee (TIS)
has selected the evolving ELF standard as a portable object file
format that works on 32-bit Intel Architecture environments for a
variety of operating systems.

The ELF standard is intended to streamline software development by
providing developers with a set of binary interface definitions that
extend across multiple operating environments. This should reduce the
number of different interface implementations, thereby reducing the
need for recoding and recompiling code.


			 About This Document

This document is intended for developers who are creating object or
executable files on various 32-bit environment operating systems. It
is divided into the following three parts:

* Part 1, ``Object Files'' describes the ELF object file format for
  the three main types of object files.
* Part 2, ``Program Loading and Dynamic Linking'' describes the object
  file information and system actions that create running programs.
* Part 3, ``C Library'' lists the symbols contained in libsys, the
  standard ANSI C and libc routines, and the global data symbols
  required by the libc routines.

NOTE: References to X86 architecture have been changed to Intel
Architecture.

   ________________________________________________________________


			   1. OBJECT FILES

   ________________________________________________________________


   ========================= Introduction =========================


Part 1 describes the iABI object file format, called ELF (Executable
and Linking Format). There are three main types of object files.

* A relocatable file holds code and data suitable for linking with
  other object files to create an executable or a shared object file.
* An executable file holds a program suitable for execution; the file
  specifies how exec(BA_OS) creates a program's process image.
* A shared object file holds code and data suitable for linking in two
  contexts. First, the link editor [see ld(SD_CMD)] may process it
  with other relocatable and shared object files to create another
  object file. Second, the dynamic linker combines it with an
  executable file and other shared objects to create a process image.

Created by the assembler and link editor, object files are binary
representations of programs intended to execute directly on a
processor. Programs that require other abstract machines, such as
shell scripts, are excluded.

After the introductory material, Part 1 focuses on the file format and
how it pertains to building programs. Part 2 also describes parts of
the object file, concentrating on the information necessary to execute
a program.


			     File Format

Object files participate in program linking (building a program) and
program execution (running a program). For convenience and efficiency,
the object file format provides parallel views of a file's contents,
reflecting the differing needs of these activities. Figure 1-1 shows
an object file's organization.

+ Figure 1-1: Object File Format

  Linking View                      Execution View
  ============                      ==============
  ELF header                        ELF header
  Program header table (optional)   Program header table
  Section 1                         Segment 1
  ...                               Segment 2
  Section n                         ...
  Section header table              Section header table (optional)

An ELF header resides at the beginning and holds a ``road map''
describing the file's organization. Sections hold the bulk of object
file information for the linking view: instructions, data, symbol
table, relocation information, and so on. Descriptions of special
sections appear later in Part 1. Part 2 discusses segments and the
program execution view of the file.

A program header table, if present, tells the system how to create a
process image. Files used to build a process image (execute a program)
must have a program header table; relocatable files do not need one. A
section header table contains information describing the file's
sections. Every section has an entry in the table; each entry gives
information such as the section name, the section size, etc. Files
used during linking must have a section header table; other object
files may or may not have one.

NOTE: Although the figure shows the program header table immediately
after the ELF header, and the section header table following the
sections, actual files may differ. Moreover, sections and segments
have no specified order. Only the ELF header has a fixed position in
the file.


			 Data Representation

As described here, the object file format supports various processors
with 8-bit bytes and 32-bit architectures. Nevertheless, it is
intended to be extensible to larger (or smaller) architectures.
Object files therefore represent some control data with a
machine-independent format, making it possible to identify object
files and interpret their contents in a common way. Remaining data in
an object file use the encoding of the target processor, regardless of
the machine on which the file was created.

+ Figure 1-2: 32-Bit Data Types

  Name           Size Alignment   Purpose
  ====           ==== =========   =======
  Elf32_Addr      4       4       Unsigned program address
  Elf32_Half      2       2       Unsigned medium integer
  Elf32_Off       4       4       Unsigned file offset
  Elf32_Sword     4       4       Signed large integer
  Elf32_Word      4       4       Unsigned large integer
  unsigned char   1       1       Unsigned small integer

All data structures that the object file format defines follow the
``natural'' size and alignment guidelines for the relevant class. If
necessary, data structures contain explicit padding to ensure 4-byte
alignment for 4-byte objects, to force structure sizes to a multiple
of 4, etc. Data also have suitable alignment from the beginning of the
file. Thus, for example, a structure containing an Elf32_Addr member
will be aligned on a 4-byte boundary within the file.

For portability reasons, ELF uses no bit-fields.


   ========================== ELF Header ==========================


Some object file control structures can grow, because the ELF header
contains their actual sizes. If the object file format changes, a
program may encounter control structures that are larger or smaller
than expected. Programs might therefore ignore``extra'' information.
The treatment of ``missing'' information depends on context and will
be specified when and if extensions are defined.

+ Figure 1-3: ELF Header

  #define EI_NIDENT       16

  typedef struct {
      unsigned char       e_ident[EI_NIDENT];
      Elf32_Half          e_type;
      Elf32_Half          e_machine;
      Elf32_Word          e_version;
      Elf32_Addr          e_entry;
      Elf32_Off           e_phoff;
      Elf32_Off           e_shoff;
      Elf32_Word          e_flags;
      Elf32_Half          e_ehsize;
      Elf32_Half          e_phentsize;
      Elf32_Half          e_phnum;
      Elf32_Half          e_shentsize;
      Elf32_Half          e_shnum;
      Elf32_Half          e_shstrndx;
  } Elf32_Ehdr;

* e_ident

  The initial bytes mark the file as an object file and provide
  machine-independent data with which to decode and interpret the
  file's contents. Complete descriptions appear below, in ``ELF
  Identification.''

* e_type

  This member identifies the object file type.

               Name        Value  Meaning
               ====        =====  =======
               ET_NONE         0  No file type
               ET_REL          1  Relocatable file
               ET_EXEC         2  Executable file
               ET_DYN          3  Shared object file
	       ET_CORE         4  Core file
	       ET_LOPROC  0xff00  Processor-specific
	       ET_HIPROC  0xffff  Processor-specific

  Although the core file contents are unspecified, type ET_CORE is
  reserved to mark the file. Values from ET_LOPROC through ET_HIPROC
  (inclusive) are reserved for processor-specific semantics. Other
  values are reserved and will be assigned to new object file types as
  necessary.

* e_machine

  This member's value specifies the required architecture for an
  individual file.

                    Name      Value  Meaning
	            ====      =====  =======
                    EM_NONE       0  No machine
		    EM_M32        1  AT&T WE 32100
                    EM_SPARC      2  SPARC
                    EM_386        3  Intel 80386
                    EM_68K        4  Motorola 68000
                    EM_88K        5  Motorola 88000
                    EM_860        7  Intel 80860
                    EM_MIPS       8  MIPS RS3000

  Other values are reserved and will be assigned to new machines as
  necessary. Processor-specific ELF names use the machine name to
  distinguish them. For example, the flags mentioned below use the
  prefix EF_; a flag named WIDGET for the EM_XYZ machine would be
  called EF_XYZ_WIDGET.

* e_version

  This member identifies the object file version.

                 Name         Value  Meaning
                 ====         =====  =======
                 EV_NONE          0  Invalid version
		 EV_CURRENT       1  Current version

  The value 1 signifies the original file format; extensions will
  create new versions with higher numbers. The value of EV_CURRENT,
  though given as 1 above, will change as necessary to reflect the
  current version number.

* e_entry

  This member gives the virtual address to which the system first
  transfers control, thus starting the process. If the file has no
  associated entry point, this member holds zero.

* e_phoff

  This member holds the program header table's file offset in bytes.
  If the file has no program header table, this member holds zero.

* e_shoff

  This member holds the section header table's file offset in bytes.
  If the file has no section header table, this member holds zero.

* e_flags

  This member holds processor-specific flags associated with the file.
  Flag names take the form EF_<machine>_<flag>. See ``Machine
  Information'' for flag definitions.

* e_ehsize

  This member holds the ELF header's size in bytes.

* e_phentsize

  This member holds the size in bytes of one entry in the file's
  program header table; all entries are the same size.

* e_phnum

  This member holds the number of entries in the program header
  table. Thus the product of e_phentsize and e_phnum gives the table's
  size in bytes. If a file has no program header table, e_phnum holds
  the value zero.

* e_shentsize

  This member holds a section header's size in bytes. A section header
  is one entry in the section header table; all entries are the same
  size.

* e_shnum

  This member holds the number of entries in the section header table.
  Thus the product of e_shentsize and e_shnum gives the section header
  table's size in bytes. If a file has no section header table,
  e_shnum holds the value zero.

* e_shstrndx

  This member holds the section header table index of the entry
  associated with the section name string table. If the file has no
  section name string table, this member holds the value SHN_UNDEF.
  See ``Sections'' and ``String Table'' below for more information.


			  ELF Identification

As mentioned above, ELF provides an object file framework to support
multiple processors, multiple data encodings, and multiple classes of
machines. To support this object file family, the initial bytes of the
file specify how to interpret the file, independent of the processor
on which the inquiry is made and independent of the file's remaining
contents.

The initial bytes of an ELF header (and an object file) correspond to
the e_ident member.

+ Figure 1-4: e_ident[] Identification Indexes

  Name           Value  Purpose
  ====           =====  =======
  EI_MAG0	     0  File identification
  EI_MAG1	     1  File identification
  EI_MAG2	     2  File identification
  EI_MAG3	     3  File identification
  EI_CLASS	     4  File class
  EI_DATA	     5  Data encoding
  EI_VERSION	     6  File version
  EI_PAD	     7  Start of padding bytes
  EI_NIDENT	    16  Size of e_ident[]

These indexes access bytes that hold the following values.

* EI_MAG0 to EI_MAG3

  A file's first 4 bytes hold a ``magic number,'' identifying the file
  as an ELF object file.

                  Name       Value  Position
                  ====       =====  ========
		  ELFMAG0    0x7f   e_ident[EI_MAG0]
                  ELFMAG1    'E'    e_ident[EI_MAG1]
                  ELFMAG2    'L'    e_ident[EI_MAG2]
                  ELFMAG3    'F'    e_ident[EI_MAG3]

* EI_CLASS

  The next byte, e_ident[EI_CLASS], identifies the file's class, or
  capacity.

                 Name           Value  Meaning
                 ====           =====  =======
                 ELFCLASSNONE       0  Invalid class
                 ELFCLASS32         1  32-bit objects
		 ELFCLASS64         2  64-bit objects

  The file format is designed to be portable among machines of various
  sizes, without imposing the sizes of the largest machine on the
  smallest. Class ELFCLASS32 supports machines with files and virtual
  address spaces up to 4 gigabytes; it uses the basic types defined
  above.

  Class ELFCLASS64 is reserved for 64-bit architectures. Its
  appearance here shows how the object file may change, but the 64-bit
  format is otherwise unspecified. Other classes will be defined as
  necessary, with different basic types and sizes for object file
  data.

* EI_DATA

  Byte e_ident[EI_DATA] specifies the data encoding of the
  processor-specific data in the object file. The following encodings
  are currently defined.

             Name           Value  Meaning
             ====           =====  =======
	     ELFDATANONE        0  Invalid data encoding
             ELFDATA2LSB        1  See below
             ELFDATA2MSB        2  See below

  More information on these encodings appears below. Other values are
  reserved and will be assigned to new encodings as necessary.

* EI_VERSION

  Byte e_ident[EI_VERSION] specifies the ELF header version number.
  Currently this, value must be EV_CURRENT, as explained above for
  e_version.

* EI_PAD

  This value marks the beginning of the unused bytes in e_ident. These
  bytes are reserved and set to zero; programs that read object files
  should ignore them.  The value of EI_PAD will change in the future
  if currently unused bytes are given meanings.

A file's data encoding specifies how to interpret the basic objects in
a file. As described above, class ELFCLASS32 files use objects that
occupy 1, 2, and 4 bytes. Under the defined encodings, objects are
represented as shown below. Byte numbers appear in the upper left
corners.

Encoding ELFDATA2LSB specifies 2's complement values, with the least
significant byte occupying the lowest address.

+ Figure 1-5: Data Encoding ELFDATA2LSB

               0------+
      0x0102   |  01  |
               +------+
               0------1------+
    0x010204   |  02  |  01  |
               +------+------+
               0------1------2------3------+
  0x01020304   |  04  |  03  |  02  |  01  |
               +------+------+------+------+

ELFDATA2MSB specifies 2's complement values, with the most significant
byte occupying the lowest address.

+ Figure 1-6: Data Encoding ELFDATA2MSB

               0------+
      0x0102   |  01  |
               +------+
               0------1------+
    0x010204   |  01  |  02  |
               +------+------+
               0------1------2------3------+
  0x01020304   |  01  |  02  |  03  |  04  |
               +------+------+------+------+


			 Machine Information

For file identification in e_ident, the 32-bit Intel Architecture
requires the following values.

+ Figure 1-7: 32-bit Intel Architecture Identification, e_ident 

  Position           Value
  ========           =====
  e_ident[EI_CLASS]  ELFCLASS32
  e_ident[EI_DATA]   ELFDATA2LSB

Processor identification resides in the ELF header's e_machine member
and must have the value EM_386.

The ELF header's e_flags member holds bit flags associated with the
file. The 32-bit Intel Architecture defines no flags; so this member
contains zero.


   =========================== Sections ===========================


An object file's section header table lets one locate all the file's
sections. The section header table is an array of Elf32_Shdr
structures as described below. A section header table index is a
subscript into this array. The ELF header's e_shoff member gives the
byte offset from the beginning of the file to the section header
table; e_shnum tells how many entries the section header table
contains; e_shentsize gives the size in bytes of each entry.

Some section header table indexes are reserved; an object file will
not have sections for these special indexes.

+ Figure 1-8: Special Section Indexes

  Name             Value
  ====             =====
  SHN_UNDEF            0
  SHN_LORESERVE   0xff00
  SHN_LOPROC      0xff00
  SHN_HIPROC      0xff1f
  SHN_ABS         0xfff1
  SHN_COMMON      0xfff2
  SHN_HIRESERVE   0xffff

* SHN_UNDEF

  This value marks an undefined, missing, irrelevant, or otherwise
  meaningless section reference. For example, a symbol ``defined''
  relative to section number SHN_UNDEF is an undefined symbol.

NOTE: Although index 0 is reserved as the undefined value, the section
header table contains an entry for index 0. That is, if the e_shnum
member of the ELF header says a file has 6 entries in the section
header table, they have the indexes 0 through 5. The contents of the
initial entry are specified later in this section.

* SHN_LORESERVE

  This value specifies the lower bound of the range of reserved
  indexes.

* SHN_LOPROC through SHN_HIPROC

  Values in this inclusive range are reserved for processor-specific
  semantics.

* SHN_ABS

  This value specifies absolute values for the corresponding
  reference. For example, symbols defined relative to section number
  SHN_ABS have absolute values and are not affected by relocation.

* SHN_COMMON

  Symbols defined relative to this section are common symbols, such as
  FORTRAN COMMON or unallocated C external variables.

* SHN_HIRESERVE

  This value specifies the upper bound of the range of reserved
  indexes. The system reserves indexes between SHN_LORESERVE and
  SHN_HIRESERVE, inclusive; the values do not reference the section
  header table. That is, the section header table does not contain
  entries for the reserved indexes.

Sections contain all information in an object file, except the ELF
header, the program header table, and the section header
table. Moreover, object files' sections satisfy several conditions.

* Every section in an object file has exactly one section header
  describing it. Section headers may exist that do not have a section.
* Each section occupies one contiguous (possibly empty) sequence of
  bytes within a file.
* Sections in a file may not overlap. No byte in a file resides in
  more than one section.
* An object file may have inactive space. The various headers and the
  sections might not ``cover'' every byte in an object file. The
  contents of the inactive data are unspecified.

A section header has the following structure.

+ Figure 1-9: Section Header

  typedef struct {
      Elf32_Word	sh_name;
      Elf32_Word	sh_type;
      Elf32_Word	sh_flags;
      Elf32_Addr	sh_addr;
      Elf32_Off		sh_offset;
      Elf32_Word	sh_size;
      Elf32_Word	sh_link;
      Elf32_Word	sh_info;
      Elf32_Word	sh_addralign;
      Elf32_Word	sh_entsize;
  } Elf32_Shdr;

* sh_name

  This member specifies the name of the section. Its value is an index
  into the section header string table section [see ``String Table''
  below], giving the location of a null-terminated string.

* sh_type

  This member categorizes the section's contents and semantics.
  Section types and their descriptions appear below.

* sh_flags

  Sections support 1-bit flags that describe miscellaneous attributes.
  Flag definitions appear below.

* sh_addr

  If the section will appear in the memory image of a process, this
  member gives the address at which the section's first byte should
  reside. Otherwise, the member contains 0.

* sh_offset

  This member's value gives the byte offset from the beginning of the
  file to the first byte in the section. One section type, SHT_NOBITS
  described below, occupies no space in the file, and its sh_offset
  member locates the conceptual placement in the file.

* sh_size

  This member gives the section's size in bytes.  Unless the section
  type is SHT_NOBITS, the section occupies sh_size bytes in the file.
  A section of type SHT_NOBITS may have a non-zero size, but it
  occupies no space in the file.

* sh_link

  This member holds a section header table index link, whose
  interpretation depends on the section type. A table below describes
  the values.

* sh_info

  This member holds extra information, whose interpretation depends on
  the section type. A table below describes the values.

* sh_addralign

  Some sections have address alignment constraints. For example, if a
  section holds a doubleword, the system must ensure doubleword
  alignment for the entire section. That is, the value of sh_addr must
  be congruent to 0, modulo the value of sh_addralign. Currently, only
  0 and positive integral powers of two are allowed. Values 0 and 1
  mean the section has no alignment constraints.

* sh_entsize

  Some sections hold a table of fixed-size entries, such as a symbol
  table. For such a section, this member gives the size in bytes of
  each entry. The member contains 0 if the section does not hold a
  table of fixed-size entries.

A section header's sh_type member specifies the section's semantics.

+ Figure 1-10: Section Types, sh_type

  Name               Value
  ====               =====
  SHT_NULL               0
  SHT_PROGBITS           1
  SHT_SYMTAB             2
  SHT_STRTAB	         3
  SHT_RELA	         4
  SHT_HASH	         5
  SHT_DYNAMIC            6
  SHT_NOTE	         7
  SHT_NOBITS	         8
  SHT_REL	         9
  SHT_SHLIB             10
  SHT_DYNSYM            11
  SHT_LOPROC    0x70000000
  SHT_HIPROC    0x7fffffff
  SHT_LOUSER    0x80000000
  SHT_HIUSER    0xffffffff

* SHT_NULL

  This value marks the section header as inactive; it does not have an
  associated section. Other members of the section header have
  undefined values.

* SHT_PROGBITS

  The section holds information defined by the program, whose format
  and meaning are determined solely by the program.

* SHT_SYMTAB and SHT_DYNSYM

  These sections hold a symbol table. Currently, an object file may
  have only one section of each type, but this restriction may be
  relaxed in the future. Typically, SHT_SYMTAB provides symbols for
  link editing, though it may also be used for dynamic linking. As a
  complete symbol table, it may contain many symbols unnecessary for
  dynamic linking. Consequently, an object file may also contain a
  SHT_DYNSYM section, which holds a minimal set of dynamic linking
  symbols, to save space. See ``Symbol Table'' below for details.

* SHT_STRTAB

  The section holds a string table. An object file may have multiple
  string table sections. See ``String Table'' below for details.

* SHT_RELA

  The section holds relocation entries with explicit addends, such as
  type Elf32_Rela for the 32-bit class of object files. An object file
  may have multiple relocation sections.  See ``Relocation'' below for
  details.

* SHT_HASH

  The section holds a symbol hash table. All objects participating in
  dynamic linking must contain a symbol hash table. Currently, an
  object file may have only one hash table, but this restriction may
  be relaxed in the future. See ``Hash Table'' in Part 2 for details.

* SHT_DYNAMIC

  The section holds information for dynamic linking. Currently, an
  object file may have only one dynamic section, but this restriction
  may be relaxed in the future. See ``Dynamic Section'' in Part 2 for
  details.

* SHT_NOTE

  The section holds information that marks the file in some way. See
  ``Note Section'' in Part 2 for details.

* SHT_NOBITS

  A section of this type occupies no space in the file but otherwise
  resembles SHT_PROGBITS. Although this section contains no bytes, the
  sh_offset member contains the conceptual file offset.

* SHT_REL

  The section holds relocation entries without explicit addends, such
  as type Elf32_Rel for the 32-bit class of object files. An object
  file may have multiple relocation sections. See ``Relocation'' below
  for details.

* SHT_SHLIB

  This section type is reserved but has unspecified
  semantics. Programs that contain a section of this type do not
  conform to the ABI.

* SHT_LOPROC through SHT_HIPROC

  Values in this inclusive range are reserved for processor-specific
  semantics.

* SHT_LOUSER

  This value specifies the lower bound of the range of indexes
  reserved for application programs.

* SHT_HIUSER

  This value specifies the upper bound of the range of indexes
  reserved for application programs. Section types between SHT_LOUSER
  and SHT_HIUSER may be used by the application, without conflicting
  with current or future system-defined section types.

Other section type values are reserved. As mentioned before, the
section header for index 0 (SHN_UNDEF) exists, even though the index
marks undefined section references. This entry holds the following.

+ Figure 1-11: Section Header Table Entry: Index 0

  Name            Value    Note
  ====            =====    ====
  sh_name           0      No name
  sh_type        SHT_NULL  Inactive
  sh_flags          0      No flags
  sh_addr           0      No address
  sh_offset         0      No file offset
  sh_size           0      No size
  sh_link	SHN_UNDEF  No link information
  sh_info	    0      No auxiliary information
  sh_addralign	    0      No alignment
  sh_entsize        0      No entries

A section header's sh_flags member holds 1-bit flags that describe the
section's attributes. Defined values appear below; other values are
reserved.

+ Figure 1-12: Section Attribute Flags, sh_flags

  Name                Value
  ====                =====
  SHF_WRITE             0x1
  SHF_ALLOC             0x2
  SHF_EXECINSTR         0x4
  SHF_MASKPROC   0xf0000000

If a flag bit is set in sh_flags, the attribute is ``on'' for the
section. Otherwise, the attribute is ``off'' or does not apply.
Undefined attributes are set to zero.

* SHF_WRITE

  The section contains data that should be writable during process
  execution.

* SHF_ALLOC

  The section occupies memory during process execution. Some control
  sections do not reside in the memory image of an object file; this
  attribute is off for those sections.

* SHF_EXECINSTR

  The section contains executable machine instructions.

* SHF_MASKPROC

  All bits included in this mask are reserved for processor-specific
  semantics.

Two members in the section header, sh_link and sh_info, hold special
information, depending on section type.

+ Figure 1-13: sh_link and sh_info Interpretation

  sh_type      sh_link                        sh_info
  =======      =======                        =======
  SHT_DYNAMIC  The section header index of    0
               the string table used by
               entries in the section.
  SHT_HASH     The section header index of    0
               the symbol table to which the
               hash table applies.
  SHT_REL,     The section header index of    The section header index of
  SHT_RELA     the associated symbol table.   the section to which the
                                              relocation applies.
  SHT_SYMTAB,  The section header index of    One greater than the symbol
  SHT_DYNSYM   the associated string table.   table index of the last local
                                              symbol (binding STB_LOCAL).
  other        SHN_UNDEF                      0


			   Special Sections

Various sections hold program and control information. Sections in the
list below are used by the system and have the indicated types and
attributes.

+ Figure 1-14: Special Sections

  Name         Type           Attributes
  ====         ====           ==========
  .bss         SHT_NOBITS     SHF_ALLOC+SHF_WRITE
  .comment     SHT_PROGBITS   none
  .data        SHT_PROGBITS   SHF_ALLOC+SHF_WRITE
  .data1       SHT_PROGBITS   SHF_ALLOC+SHF_WRITE
  .debug       SHT_PROGBITS   none
  .dynamic     SHT_DYNAMIC    see below
  .dynstr      SHT_STRTAB     SHF_ALLOC
  .dynsym      SHT_DYNSYM     SHF_ALLOC
  .fini        SHT_PROGBITS   SHF_ALLOC+SHF_EXECINSTR
  .got         SHT_PROGBITS   see below
  .hash        SHT_HASH       SHF_ALLOC
  .init        SHT_PROGBITS   SHF_ALLOC+SHF_EXECINSTR
  .interp      SHT_PROGBITS   see below
  .line        SHT_PROGBITS   none
  .note        SHT_NOTE       none
  .plt         SHT_PROGBITS   see below
  .rel<name>   SHT_REL        see below
  .rela<name>  SHT_RELA       see below
  .rodata      SHT_PROGBITS   SHF_ALLOC
  .rodata1     SHT_PROGBITS   SHF_ALLOC
  .shstrtab    SHT_STRTAB     none
  .strtab      SHT_STRTAB     see below
  .symtab      SHT_SYMTAB     see below
  .text        SHT_PROGBITS   SHF_ALLOC+SHF_EXECINSTR

* .bss

  This section holds uninitialized data that contribute to the
  program's memory image. By definition, the system initializes the
  data with zeros when the program begins to run. The section occupies
  no file space, as indicated by the section type, SHT_NOBITS.

* .comment

  This section holds version control information.

* .data and .data1

  These sections hold initialized data that contribute to the
  program's memory image.

* .debug

  This section holds information for symbolic debugging. The contents
  are unspecified.

* .dynamic

  This section holds dynamic linking information. The section's
  attributes will include the SHF_ALLOC bit. Whether the SHF_WRITE bit
  is set is processor specific. See Part 2 for more information.

* .dynstr

  This section holds strings needed for dynamic linking, most commonly
  the strings that represent the names associated with symbol table
  entries. See Part 2 for more information.

* .dynsym

  This section holds the dynamic linking symbol table, as ``Symbol
  Table'' describes. See Part 2 for more information.

* .fini

  This section holds executable instructions that contribute to the
  process termination code. That is, when a program exits normally,
  the system arranges to execute the code in this section.

* .got

  This section holds the global offset table. See ``Special Sections''
  in Part 1 and ``Global Offset Table'' in Part 2 for more
  information.

* .hash

  This section holds a symbol hash table. See ``Hash Table'' in Part 2
  for more information.

* .init

  This section holds executable instructions that contribute to the
  process initialization code. That is, when a program starts to run,
  the system arranges to execute the code in this section before
  calling the main program entry point (called main for C programs).

* .interp

  This section holds the path name of a program interpreter. If the
  file has a loadable segment that includes the section, the section's
  attributes will include the SHF_ALLOC bit; otherwise, that bit will
  be off. See Part 2 for more information.

* .line

  This section holds line number information for symbolic debugging,
  which describes the correspondence between the source program and
  the machine code. The contents are unspecified.

* .note

  This section holds information in the format that ``Note Section''
  in Part 2 describes.

* .plt

  This section holds the procedure linkage table. See ``Special
  Sections'' in Part 1 and ``Procedure Linkage Table'' in Part 2 for
  more information.

* .rel<name> and .rela<name>

  These sections hold relocation information, as ``Relocation'' below
  describes. If the file has a loadable segment that includes
  relocation, the sections' attributes will include the SHF_ALLOC bit;
  otherwise, that bit will be off. Conventionally, <name> is supplied
  by the section to which the relocations apply. Thus a relocation
  section for .text normally would have the name .rel.text or
  .rela.text.

* .rodata and .rodata1

  These sections hold read-only data that typically contribute to a
  non-writable segment in the process image. See ``Program Header'' in
  Part 2 for more information.

* .shstrtab

  This section holds section names.

* .strtab

  This section holds strings, most commonly the strings that represent
  the names associated with symbol table entries. If the file has a
  loadable segment that includes the symbol string table, the
  section's attributes will include the SHF_ALLOC bit; otherwise, that
  bit will be off.

* .symtab

  This section holds a symbol table, as ``Symbol Table'' in this
  section describes. If the file has a loadable segment that includes
  the symbol table, the section's attributes will include the
  SHF_ALLOC bit; otherwise, that bit will be off.

* .text

  This section holds the ``text,'' or executable instructions, of a
  program.

Section names with a dot (.) prefix are reserved for the system,
although applications may use these sections if their existing
meanings are satisfactory. Applications may use names without the
prefix to avoid conflicts with system sections. The object file format
lets one define sections not in the list above. An object file may
have more than one section with the same name.

Section names reserved for a processor architecture are formed by
placing an abbreviation of the architecture name ahead of the section
name. The name should be taken from the architecture names used for
e_machine. For instance .FOO.psect is the psect section defined by the
FOO architecture. Existing extensions are called by their historical
names.

		       Pre-existing Extensions
		       =======================
			 .sdata     .tdesc
			 .sbss      .lit4
			 .lit8      .reginfo
			 .gptab     .liblist
			 .conflict


   ========================= String Table =========================


String table sections hold null-terminated character sequences,
commonly called strings. The object file uses these strings to
represent symbol and section names. One references a string as an
index into the string table section. The first byte, which is index
zero, is defined to hold a null character. Likewise, a string table's
last byte is defined to hold a null character, ensuring null
termination for all strings. A string whose index is zero specifies
either no name or a null name, depending on the context. An empty
string table section is permitted; its section header's sh_size member
would contain zero. Non-zero indexes are invalid for an empty string
table.

A section header's sh_name member holds an index into the section
header string table section, as designated by the e_shstrndx member of
the ELF header. The following figures show a string table with 25
bytes and the strings associated with various indexes.

       Index   +0   +1   +2   +3   +4   +5   +6   +7   +8   +9
       =====   ==   ==   ==   ==   ==   ==   ==   ==   ==   ==
          0    \0   n    a    m    e    .    \0   V    a    r     
         10    i    a    b    l    e    \0   a    b    l    e
         20    \0   \0   x    x    \0


+ Figure 1-15: String Table Indexes

  Index   String
  =====   ======
      0   none
      1   "name."
      7   "Variable"
     11   "able"
     16   "able"
     24   null string

As the example shows, a string table index may refer to any byte in
the section. A string may appear more than once; references to
substrings may exist; and a single string may be referenced multiple
times. Unreferenced strings also are allowed.


   ========================= Symbol Table =========================


An object file's symbol table holds information needed to locate and
relocate a program's symbolic definitions and references. A symbol
table index is a subscript into this array. Index 0 both designates
the first entry in the table and serves as the undefined symbol
index. The contents of the initial entry are specified later in this
section.

                             Name       Value
                             ====       =====
			     STN_UNDEF      0

A symbol table entry has the following format.

+ Figure 1-16: Symbol Table Entry

  typedef struct {
      Elf32_Word	st_name;
      Elf32_Addr	st_value;
      Elf32_Word	st_size;
      unsigned char	st_info;
      unsigned char	st_other;
      Elf32_Half	st_shndx;
  } Elf32_Sym;

* st_name

  This member holds an index into the object file's symbol string
  table, which holds the character representations of the symbol
  names. If the value is non-zero, it represents a string table index
  that gives the symbol name. Otherwise, the symbol table entry has no
  name.

NOTE: External C symbols have the same names in C and object files'
symbol tables.

* st_value

  This member gives the value of the associated symbol. Depending on
  the context, this may be an absolute value, an address, etc.;
  details appear below.

* st_size

  Many symbols have associated sizes. For example, a data object's
  size is the number of bytes contained in the object. This member
  holds 0 if the symbol has no size or an unknown size.

* st_info

  This member specifies the symbol's type and binding attributes.  A
  list of the values and meanings appears below. The following code
  shows how to manipulate the values.

    #define ELF32_ST_BIND(i)	((i)>>4)
    #define ELF32_ST_TYPE(i)	((i)&0xf)
    #define ELF32_ST_INFO(b, t)	(((b)<<4)+((t)&0xf))

* st_other

  This member currently holds 0 and has no defined meaning.

* st_shndx

  Every symbol table entry is ``defined'' in relation to some section;
  this member holds the relevant section header table index. As Figure
  1-8 {*} and the related text describe, some section indexes indicate
  special meanings.

A symbol's binding determines the linkage visibility and behavior.

+ Figure 1-17: Symbol Binding, ELF32_ST_BIND

  Name        Value
  ====        =====
  STB_LOCAL       0
  STB_GLOBAL      1
  STB_WEAK        2
  STB_LOPROC     13
  STB_HIPROC     15

* STB_LOCAL

  Local symbols are not visible outside the object file containing
  their definition. Local symbols of the same name may exist in
  multiple files without interfering with each other.

* STB_GLOBAL

  Global symbols are visible to all object files being combined. One
  file's definition of a global symbol will satisfy another file's
  undefined reference to the same global symbol.

* STB_WEAK

  Weak symbols resemble global symbols, but their definitions have
  lower precedence.

* STB_LOPROC through STB_HIPROC

  Values in this inclusive range are reserved for processor-specific
  semantics.

Global and weak symbols differ in two major ways.

* When the link editor combines several relocatable object files, it
  does not allow multiple definitions of STB_GLOBAL symbols with the
  same name. On the other hand, if a defined global symbol exists, the
  appearance of a weak symbol with the same name will not cause an
  error. The link editor honors the global definition and ignores the
  weak ones. Similarly, if a common symbol exists (i.e., a symbol
  whose st_shndx field holds SHN_COMMON), the appearance of a weak
  symbol with the same name will not cause an error. The link editor
  honors the common definition and ignores the weak ones.

* When the link editor searches archive libraries, it extracts archive
  members that contain definitions of undefined global symbols. The
  member's definition may be either a global or a weak symbol. The
  link editor does not extract archive members to resolve undefined
  weak symbols. Unresolved weak symbols have a zero value.

In each symbol table, all symbols with STB_LOCAL binding precede the
weak and global symbols. As ``Sections'' above describes, a symbol
table section's sh_info section header member holds the symbol table
index for the first non-local symbol.

A symbol's type provides a general classification for the associated
entity.

+ Figure 1-18: Symbol Types, ELF32_ST_TYPE

  Name         Value
  ====         =====
  STT_NOTYPE       0
  STT_OBJECT       1
  STT_FUNC         2
  STT_SECTION      3
  STT_FILE         4
  STT_LOPROC      13
  STT_HIPROC      15

* STT_NOTYPE

  The symbol's type is not specified.

* STT_OBJECT

  The symbol is associated with a data object, such as a variable, an
  array, etc.

* STT_FUNC

  The symbol is associated with a function or other executable code.

* STT_SECTION

  The symbol is associated with a section. Symbol table entries of
  this type exist primarily for relocation and normally have STB_LOCAL
  binding.

* STT_FILE

  Conventionally, the symbol's name gives the name of the source file
  associated with the object file. A file symbol has STB_LOCAL
  binding, its section index is SHN_ABS, and it precedes the other
  STB_LOCAL symbols for the file, if it is present.

* STT_LOPROC through STT_HIPROC

  Values in this inclusive range are reserved for processor-specific
  semantics.

Function symbols (those with type STT_FUNC) in shared object files
have special significance. When another object file references a
function from a shared object, the link editor automatically creates a
procedure linkage table entry for the referenced symbol. Shared object
symbols with types other than STT_FUNC will not be referenced
automatically through the procedure linkage table.

If a symbol's value refers to a specific location within a section,
its section index member, st_shndx, holds an index into the section
header table. As the section moves during relocation, the symbol's
value changes as well, and references to the symbol continue to
``point'' to the same location in the program. Some special section
index values give other semantics.

* SHN_ABS

  The symbol has an absolute value that will not change because of
  relocation.

* SHN_COMMON

  The symbol labels a common block that has not yet been allocated.
  The symbol's value gives alignment constraints, similar to a
  section's sh_addralign member. That is, the link editor will
  allocate the storage for the symbol at an address that is a multiple
  of st_value. The symbol's size tells how many bytes are required.

* SHN_UNDEF

  This section table index means the symbol is undefined. When the
  link editor combines this object file with another that defines the
  indicated symbol, this file's references to the symbol will be
  linked to the actual definition.

As mentioned above, the symbol table entry for index 0 (STN_UNDEF) is
reserved; it holds the following.

+ Figure 1-19: Symbol Table Entry: Index 0

  Name        Value    Note
  ====        =====    ====
  st_name       0      No name
  st_value      0      Zero value
  st_size       0      No size
  st_info       0      No type, local binding
  st_other      0
  st_shndx  SHN_UNDEF  No section


			    Symbol Values

Symbol table entries for different object file types have slightly
different interpretations for the st_value member.

* In relocatable files, st_value holds alignment constraints for a
  symbol whose section index is SHN_COMMON.
* In relocatable files, st_value holds a section offset for a defined
  symbol. That is, st_value is an offset from the beginning of the
  section that st_shndx identifies.
* In executable and shared object files, st_value holds a virtual
  address. To make these files' symbols more useful for the dynamic
  linker, the section offset (file interpretation) gives way to a
  virtual address (memory interpretation) for which the section number
  is irrelevant.

Although the symbol table values have similar meanings for different
object files, the data allow efficient access by the appropriate
programs.


   ========================== Relocation ==========================


Relocation is the process of connecting symbolic references with
symbolic definitions. For example, when a program calls a function,
the associated call instruction must transfer control to the proper
destination address at execution. In other words, relocatable files
must have information that describes how to modify their section
contents, thus allowing executable and shared object files to hold the
right information for a process's program image. Relocation entries
are these data.

+ Figure 1-20: Relocation Entries

  typedef struct {
      Elf32_Addr	r_offset;
      Elf32_Word	r_info;
  } Elf32_Rel;

  typedef struct {
      Elf32_Addr	r_offset;
      Elf32_Word	r_info;
      Elf32_Sword	r_addend;
  } Elf32_Rela;

* r_offset

  This member gives the location at which to apply the relocation
  action. For a relocatable file, the value is the byte offset from
  the beginning of the section to the storage unit affected by the
  relocation. For an executable file or a shared object, the value is
  the virtual address of the storage unit affected by the relocation.

* r_info

  This member gives both the symbol table index with respect to which
  the relocation must be made, and the type of relocation to apply.
  For example, a call instruction's relocation entry would hold the
  symbol table index of the function being called. If the index is
  STN_UNDEF, the undefined symbol index, the relocation uses 0 as the
  ``symbol value.'' Relocation types are processor-specific. When the
  text refers to a relocation entry's relocation type or symbol table
  index, it means the result of applying ELF32_R_TYPE or ELF32_R_SYM,
  respectively, to the entry's r_info member.

    #define ELF32_R_SYM(i)	((i)>>8)
    #define ELF32_R_TYPE(i)	((unsigned char)(i))
    #define ELF32_R_INFO(s, t)	((s)<<8+(unsigned char)(t))

* r_addend

  This member specifies a constant addend used to compute the value to
  be stored into the relocatable field.

As shown above, only Elf32_Rela entries contain an explicit
addend. Entries of type Elf32_Rel store an implicit addend in the
location to be modified. Depending on the processor architecture, one
form or the other might be necessary or more convenient. Consequently,
an implementation for a particular machine may use one form
exclusively or either form depending on context.

A relocation section references two other sections: a symbol table and
a section to modify. The section header's sh_info and sh_link members,
described in ``Sections'' above, specify these relationships.
Relocation entries for different object files have slightly different
interpretations for the r_offset member.

* In relocatable files, r_offset holds a section offset. That is, the
  relocation section itself describes how to modify another section in
  the file; relocation offsets designate a storage unit within the
  second section.
* In executable and shared object files, r_offset holds a virtual
  address. To make these files' relocation entries more useful for the
  dynamic linker, the section offset (file interpretation) gives way
  to a virtual address (memory interpretation).

Although the interpretation of r_offset changes for different object
files to allow efficient access by the relevant programs, the
relocation types' meanings stay the same.


			   Relocation Types

Relocation entries describe how to alter the following instruction and
data fields (bit numbers appear inthe lower box corners).

+ Figure 1-21: Relocatable Fields 

    +---------------------------+
    |          word32           |
   31---------------------------0


* word32

  This specifies a 32-bit field occupying 4 bytes with arbitrary byte
  alignment. These values use the same byte order as other word values
  in the 32-bit Intel Architecture.

                           3------2------1------0------+
	     0x01020304    |  01  |  02  |  03  |  04  |
                          31------+------+------+------0

Calculations below assume the actions are transforming a relocatable
file into either an executable or a shared object file. Conceptually,
the link editor merges one or more relocatable files to form the
output. It first decides how to combine and locate the input files,
then updates the symbol values, and finally performs the relocation.
Relocations applied to executable or shared object files are similar
and accomplish the same result. Descriptions below use the following
notation.

* A

  This means the addend used to compute the value of the relocatable
  field.

* B

  This means the base address at which a shared object has been loaded
  into memory during execution. Generally, a shared object file is
  built with a 0 base virtual address, but the execution address will
  be different.

* G

  This means the offset into the global offset table at which the
  address of the relocation entry's symbol will reside during
  execution. See ``Global Offset Table'' in Part 2 for more
  information.

* GOT

  This means the address of the global offset table. See ``Global
  Offset Table'' in Part 2 for more information.

* L

  This means the place (section offset or address) of the procedure
  linkage table entry for a symbol. A procedure linkage table entry
  redirects a function call to the proper destination. The link editor
  builds the initial procedure linkage table, and the dynamic linker
  modifies the entries during execution. See ``Procedure Linkage
  Table'' in Part 2 for more information.

* P

  This means the place (section offset or address) of the storage unit
  being relocated (computed using r_offset).

* S

  This means the value of the symbol whose index resides in the
  relocation entry.

A relocation entry's r_offset value designates the offset or virtual
address of the first byte of the affected storage unit. The relocation
type specifies which bits to change and how to calculate their
values. The SYSTEM V architecture uses only Elf32_Rel relocation
entries, the field to be relocated holds the addend. In all cases, the
addend and the computed result use the same byte order.

+ Figure 1-22: Relocation Types

  Name            Value  Field   Calculation
  ====            =====  =====   ===========
  R_386_NONE        0    none    none
  R_386_32	    1    word32  S + A
  R_386_PC32	    2    word32  S + A - P
  R_386_GOT32	    3    word32  G + A - P
  R_386_PLT32	    4    word32  L + A - P
  R_386_COPY	    5    none    none
  R_386_GLOB_DAT    6    word32  S
  R_386_JMP_SLOT    7    word32  S
  R_386_RELATIVE    8    word32  B + A
  R_386_GOTOFF	    9    word32  S + A - GOT
  R_386_GOTPC	   10    word32  GOT + A - P

Some relocation types have semantics beyond simple calculation.

* R_386_GOT32

  This relocation type computes the distance from the base of the
  global offset table to the symbol's global offset table entry. It
  additionally instructs the link editor to build a global offset
  table.

* R_386_PLT32

  This relocation type computes the address of the symbol's procedure
  linkage table entry and additionally instructs the link editor to
  build a procedure linkage table.

* R_386_COPY

  The link editor creates this relocation type for dynamic linking.
  Its offset member refers to a location in a writable segment. The
  symbol table index specifies a symbol that should exist both in the
  current object file and in a shared object. During execution, the
  dynamic linker copies data associated with shared object's symbol to
  the location specified by the offset.

* R_386_GLOB_DAT

  This relocation type is used to set a global offset table entry to
  the address of the specified symbol. The special relocation type
  allows one to determine the correspondence between symbols and
  global offset table entries.

* R_386_JMP_SLOT {*}

  The link editor creates this relocation type for dynamic linking.
  Its offset member gives the location of a procedure linkage table
  entry. The dynamic linker modifies the procedure linkage table entry
  to transfer control to the designated symbol's address [see
  ``Procedure Linkage Table'' in Part 2].

* R_386_RELATIVE

  The link editor creates this relocation type for dynamic linking.
  Its offset member gives a location within a shared object that
  contains a value representing a relative address. The dynamic linker
  computes the corresponding virtual address by adding the virtual
  address at which the shared object was loaded to the relative
  address. Relocation entries for this type must specify 0 for the
  symbol table index.

* R_386_GOTOFF

  This relocation type computes the difference between a symbol's
  value and the address of the global offset table. It additionally
  instructs the link editor to build the global offset table.


* R_386_GOTPC

  This relocation type resembles R_386_PC32, except it uses the
  address of the global offset table in its calculation. The symbol
  referenced in this relocation normally is _GLOBAL_OFFSET_TABLE_,
  which additionally instructs the link editor to build the global
  offset table.

   ________________________________________________________________


		2. PROGRAM LOADING AND DYNAMIC LINKING

   ________________________________________________________________


   ========================= Introduction =========================


Part 2 describes the object file information and system actions that
create running programs. Some information here applies to all systems;
other information is processor-specific.

Executable and shared object files statically represent programs. To
execute such programs, the system uses the files to create dynamic
program representations, or process images. A process image has
segments that hold its text, data, stack, and so on. The major
sections in this part discuss the following.

* Program header. This section complements Part 1, describing object
  file structures that relate directly to program execution. The
  primary data structure, a program header table, locates segment
  images within the file and contains other information necessary to
  create the memory image for the program.
* Program loading. Given an object file, the system must load it into
  memory for the program to run.
* Dynamic linking. After the system loads the program, it must
  complete the process image by resolving symbolic references among
  the object files that compose the process.

NOTE: There are naming conventions for ELF constants that have
specified processor ranges. Names such as DT_, PT_, for
processor-specific extensions, incorporate the name of the processor:
DT_M32_SPECIAL, for example. Pre-existing processor extensions not
using this convention will be supported.

		       Pre-existing Extensions
		       =======================
			      DT_JMP_REL


   ======================== Program Header ========================


An executable or shared object file's program header table is an array
of structures, each describing a segment or other information the
system needs to prepare the program for execution. An object file
segment contains one or more sections, as ``Segment Contents''
describes below. Program headers are meaningful only for executable
and shared object files. A file specifies its own program header size
with the ELF header's e_phentsize and e_phnum members [see ``ELF
Header'' in Part 1].

+ Figure 2-1: Program Header

  typedef struct {
      Elf32_Word	p_type;
      Elf32_Off		p_offset;
      Elf32_Addr	p_vaddr;
      Elf32_Addr	p_paddr;
      Elf32_Word	p_filesz;
      Elf32_Word	p_memsz;
      Elf32_Word	p_flags;
      Elf32_Word	p_align;
  } Elf32_Phdr;

* p_type

  This member tells what kind of segment this array element describes
  or how to interpret the array element's information. Type values and
  their meanings appear below.

* p_offset

  This member gives the offset from the beginning of the file at which
  the first byte of the segment resides.

* p_vaddr

  This member gives the virtual address at which the first byte of the
  segment resides in memory.

* p_paddr

  On systems for which physical addressing is relevant, this member is
  reserved for the segment's physical address. Because System V
  ignores physical addressing for application programs, this member
  has unspecified contents for executable files and shared objects.

* p_filesz

  This member gives the number of bytes in the file image of the
  segment; it may be zero.

* p_memsz

  This member gives the number of bytes in the memory image of the
  segment; it may be zero.

* p_flags

  This member gives flags relevant to the segment. Defined flag values
  appear below.

* p_align

  As ``Program Loading'' later in this part describes, loadable
  process segments must have congruent values for p_vaddr and
  p_offset, modulo the page size. This member gives the value to which
  the segments are aligned in memory and in the file. Values 0 and 1
  mean no alignment is required. Otherwise, p_align should be a
  positive, integral power of 2, and p_vaddr should equal p_offset,
  modulo p_align.

Some entries describe process segments; others give supplementary
information and do not contribute to the process image.  Defined
entries may appear in any order, except as explicitly noted
below. Segment type values follow; other values are reserved for
future use.

+ Figure 2-2: Segment Types, p_type

  Name             Value
  ====             =====
  PT_NULL              0
  PT_LOAD              1
  PT_DYNAMIC           2
  PT_INTERP            3
  PT_NOTE              4
  PT_SHLIB             5
  PT_PHDR              6
  PT_LOPROC   0x70000000
  PT_HIPROC   0x7fffffff

* PT_NULL

  The array element is unused; other members' values are undefined.
  This type lets the program header table have ignored entries.

* PT_LOAD

  The array element specifies a loadable segment, described by
  p_filesz and p_memsz. The bytes from the file are mapped to the
  beginning of the memory segment. If the segment's memory size
  (p_memsz) is larger than the file size (p_filesz), the ``extra''
  bytes are defined to hold the value 0 and to follow the segment's
  initialized area. The file size may not be larger than the memory
  size. Loadable segment entries in the program header table appear in
  ascending order, sorted on the p_vaddr member.

* PT_DYNAMIC

  The array element specifies dynamic linking information. See
  ``Dynamic Section'' below for more information.

* PT_INTERP

  The array element specifies the location and size of a
  null-terminated path name to invoke as an interpreter. This segment
  type is meaningful only for executable files (though it may occur
  for shared objects); it may not occur more than once in a file. If
  it is present, it must precede any loadable segment entry. See
  ``Program Interpreter'' below for further information.

* PT_NOTE

  The array element specifies the location and size of auxiliary
  information. See ``Note Section'' below for details.

* PT_SHLIB

  This segment type is reserved but has unspecified semantics.
  Programs that contain an array element of this type do not conform
  to the ABI.

* PT_PHDR

  The array element, if present, specifies the location and size of
  the program header table itself, both in the file and in the memory
  image of the program. This segment type may not occur more than once
  in a file. Moreover, it may occur only if the program header table
  is part of the memory image of the program. If it is present, it
  must precede any loadable segment entry. See ``Program Interpreter''
  below for further information.

* PT_LOPROC through PT_HIPROC

  Values in this inclusive range are reserved for processor-specific
  semantics.

NOTE: Unless specifically required elsewhere, all program header
segment types are optional. That is, a file's program header table may
contain only those elements relevant to its contents.


			     Base Address

Executable and shared object files have a base address, which is the
lowest virtual address associated with the memory image of the
program's object file. One use of the base address is to relocate the
memory image of the program during dynamic linking.

An executable or shared object file's base address is calculated
during execution from three values: the memory load address, the
maximum page size, and the lowest virtual address of a program's
loadable segment. As ``Program Loading'' in this chapter describes,
the virtual addresses in the program headers might not represent the
actual virtual addresses of the program's memory image. To compute the
base address, one determines the memory address associated with the
lowest p_vaddr value for a PT_LOAD segment. One then obtains the base
address by truncating the memory address to the nearest multiple of
the maximum page size. Depending on the kind of file being loaded into
memory, the memory address might or might not match the p_vaddr
values.

As ``Sections'' in Part 1 describes, the .bss section has the type
SHT_NOBITS. Although it occupies no space in the file, it contributes
to the segment's memory image. Normally, these uninitialized data
reside at the end of the segment, thereby making p_memsz larger than
p_filesz in the associated program header element.


			     Note Section

Sometimes a vendor or system builder needs to mark an object file with
special information that other programs will check for conformance,
compatibility, etc. Sections of type SHT_NOTE and program header
elements of type PT_NOTE can be used for this purpose. The note
information in sections and program header elements holds any number
of entries, each of which is an array of 4-byte words in the format of
the target processor. Labels appear below to help explain note
information organization, but they are not part of the specification.

+ Figure 2-3: Note Information

  namesz
  descsz
  type
  name ...
  desc ...

* namesz and name

  The first namesz bytes in name contain a null-terminated character
  representation of the entry's owner or originator. There is no
  formal mechanism for avoiding name conflicts. By convention, vendors
  use their own name, such as ``XYZ Computer Company,'' as the
  identifier. If no name is present, namesz contains 0. Padding is
  present, if necessary, to ensure 4-byte alignment for the
  descriptor. Such padding is not included in namesz.

* descsz and desc

  The first descsz bytes in desc hold the note descriptor. The ABI
  places no constraints on a descriptor's contents. If no descriptor
  is present, descsz contains 0. Padding is present, if necessary, to
  ensure 4-byte alignment for the next note entry. Such padding is not
  included in descsz.

* type

  This word gives the interpretation of the descriptor. Each
  originator controls its own types; multiple interpretations of a
  single type value may exist. Thus, a program must recognize both the
  name and the type to ``understand'' a descriptor. Types currently
  must be non-negative. The ABI does not define what descriptors mean.

To illustrate, the following note segment holds two entries.

+ Figure 2-4: Example Note Segment

           +0   +1   +2   +3
          -------------------
  namesz           7
  descsz           0           No descriptor
    type           1
    name   X    Y    Z    spc 
           C    o    \0   pad
  namesz           7
  descsz           8
    type           3
    name   X    Y    Z    spc
           C    o    \0   pad
    desc         word0
                 word1

NOTE: The system reserves note information with no name (namesz==0)
and with a zero-length name (name[0]=='\0') but currently defines no
types. All other names must have at least one non-null character.

NOTE: Note information is optional. The presence of note information
does not affect a program's ABI conformance, provided the information
does not affect the program's execution behavior. Otherwise, the
program does not conform to the ABI and has undefined behavior.


   ======================= Program Loading ========================


As the system creates or augments a process image, it logically copies
a file's segment to a virtual memory segment. When--and if--the system
physically reads the file depends on the program's execution behavior,
system load, etc. A process does not require a physical page unless it
references the logical page during execution, and processes commonly
leave many pages unreferenced. Therefore delaying physical reads
frequently obviates them, improving system performance. To obtain this
efficiency in practice, executable and shared object files must have
segment images whose file offsets and virtual addresses are congruent,
modulo the page size.

Virtual addresses and file offsets for the SYSTEM V architecture
segments are congruent modulo 4 KB (0x1000) or larger powers of 2.
Because 4 KB is the maximum page size, the files will be suitable for
paging regardless of physical page size.

+ Figure 2-5: Executable File

           File Offset   File                  Virtual Address
           ===========   ====                  ===============
                     0   ELF header
  Program header table
                         Other information
                 0x100   Text segment          0x8048100
                         ...
                         0x2be00 bytes         0x8073eff
               0x2bf00   Data segment          0x8074f00
                         ...
                         0x4e00 bytes          0x8079cff
               0x30d00   Other information
                         ...

+ Figure 2-6: Program Header Segments

  Member    Text	 Data
  ======    ====         ====
  p_type    PT_LOAD      PT_LOAD
  p_offset  0x100	 0x2bf00
  p_vaddr   0x8048100	 0x8074f00
  p_paddr   unspecified	 unspecified
  p_filesz  0x2be00	 0x4e00
  p_memsz   0x2be00	 0x5e24
  p_flags   PF_R+PF_X    PF_R+PF_W+PF_X
  p_align   0x1000	 0x1000

Although the example's file offsets and virtual addresses are
congruent modulo 4 KB for both text and data, up to four file pages
hold impure text or data (depending on page size and file system block
size).

* The first text page contains the ELF header, the program header
  table, and other information.
* The last text page holds a copy of the beginning of data.
* The first data page has a copy of the end of text.
* The last data page may contain file information not relevant to the
  running process.

Logically, the system enforces the memory permissions as if each
segment were complete and separate; segments' addresses are adjusted
to ensure each logical page in the address space has a single set of
permissions. In the example above, the region of the file holding the
end of text and the beginning of data will be mapped twice: at one
virtual address for text and at a different virtual address for data.

The end of the data segment requires special handling for
uninitialized data, which the system defines to begin with zero
values. Thus if a file's last data page includes information not in
the logical memory page, the extraneous data must be set to zero, not
the unknown contents of the executable file. ``Impurities'' in the
other three pages are not logically part of the process image; whether
the system expunges them is unspecified. The memory image for this
program follows, assuming 4 KB (0x1000) pages.

+ Figure 2-7: Process Image Segments

  Virtual Address  Contents            Segment
  ===============  ========            =======
        0x8048000  Header padding      Text
                   0x100 bytes
        0x8048100  Text segment
                   ...
                   0x2be00 bytes
        0x8073f00  Data padding
                   0x100 bytes
        0x8074000  Text padding        Data
                   0xf00 bytes
        0x8074f00  Data segment
                   ...
                   0x4e00 bytes
        0x8079d00  Uninitialized data
                   0x1024 zero bytes
        0x807ad24  Page padding
                   0x2dc zero bytes

One aspect of segment loading differs between executable files and
shared objects. Executable file segments typically contain absolute
code. To let the process execute correctly, the segments must reside
at the virtual addresses used to build the executable file. Thus the
system uses the p_vaddr values unchanged as virtual addresses.

On the other hand, shared object segments typically contain
position-independent code. This lets a segment's virtual address
change from one process to another, without invalidating execution
behavior. Though the system chooses virtual addresses for individual
processes, it maintains the segments' relative positions. Because
position-independent code uses relative addressing between segments,
the difference between virtual addresses in memory must match the
difference between virtual addresses in the file. The following table
shows possible shared object virtual address assignments for several
processes, illustrating constant relative positioning. The table also
illustrates the base address computations.

+ Figure 2-8: Example Shared Object Segment Addresses

  Sourc             Text        Data  Base Address
  =====             ====        ====  ============
  File             0x200     0x2a400           0x0
  Process 1   0x80000200  0x8002a400    0x80000000
  Process 2   0x80081200  0x800ab400    0x80081000
  Process 3   0x900c0200  0x900ea400    0x900c0000
  Process 4   0x900c6200  0x900f0400    0x900c6000


   ======================= Dynamic Linking ========================


			 Program Interpreter

An executable file may have one PT_INTERP program header element.
During exec(BA_OS), the system retrieves a path name from the
PT_INTERP segment and creates the initial process image from the
interpreter file's segments. That is, instead of using the original
executable file's segment images, the system composes a memory image
for the interpreter. It then is the interpreter's responsibility to
receive control from the system and provide an environment for the
application program.

The interpreter receives control in one of two ways. First, it may
receive a file descriptor to read the executable file, positioned at
the beginning. It can use this file descriptor to read and/or map the
executable file's segments into memory. Second, depending on the
executable file format, the system may load the executable file into
memory instead of giving the interpreter an open file descriptor. With
the possible exception of the file descriptor, the interpreter's
initial process state matches what the executable file would have
received. The interpreter itself may not require a second interpreter.
An interpreter may be either a shared object or an executable file.

* A shared object (the normal case) is loaded as position-independent,
  with addresses that may vary from one process to another; the system
  creates its segments in the dynamic segment area used by mmap(KE_OS)
  and related services. Consequently, a shared object interpreter
  typically will not conflict with the original executable file's
  original segment addresses.

* An executable file is loaded at fixed addresses; the system creates
  its segments using the virtual addresses from the program header
  table. Consequently, an executable file interpreter's virtual
  addresses may collide with the first executable file; the
  interpreter is responsible for resolving conflicts.


			    Dynamic Linker

When building an executable file that uses dynamic linking, the link
editor adds a program header element of type PT_INTERP to an
executable file, telling the system to invoke the dynamic linker as
the program interpreter.

NOTE: The locations of the system provided dynamic linkers are
processor-specific.

Exec(BA_OS) and the dynamic linker cooperate to create the process
image for the program, which entails the following actions:

* Adding the executable file's memory segments to the process image;
* Adding shared object memory segments to the process image;
* Performing relocations for the executable file and its shared
  objects;
* Closing the file descriptor that was used to read the executable
  file, if one was given to the dynamic linker;
* Transferring control to the program, making it look as if the
  program had received control directly from exec(BA_OS).

The link editor also constructs various data that assist the dynamic
linker for executable and shared object files. As shown above in
``Program Header,'' these data reside in loadable segments, making
them available during execution. (Once again, recall the exact segment
contents are processor-specific. See the processor supplement for
complete information.)

* A .dynamic section with type SHT_DYNAMIC holds various data. The
  structure residing at the beginning of the section holds the
  addresses of other dynamic linking information.

* The .hash section with type SHT_HASH holds a symbol hash table.

* The .got and .plt sections with type SHT_PROGBITS hold two separate
  tables: the global offset table and the procedure linkage table.
  Sections below explain how the dynamic linker uses and changes the
  tables to create memory images for object files.

Because every ABI-conforming program imports the basic system services
from a shared object library, the dynamic linker participates in every
ABI-conforming program execution.

As ``Program Loading'' explains in the processor supplement, shared
objects may occupy virtual memory addresses that are different from
the addresses recorded in the file's program header table. The dynamic
linker relocates the memory image, updating absolute addresses before
the application gains control. Although the absolute address values
would be correct if the library were loaded at the addresses specified
in the program header table, this normally is not the case.

If the process environment [see exec(BA_OS)] contains a variable named
LD_BIND_NOW with a non-null value, the dynamic linker processes all
relocation before transferring control to the program. For example,
all the following environment entries would specify this behavior.

* LD_BIND_NOW=1
* LD_BIND_NOW=on
* LD_BIND_NOW=off

Otherwise, LD_BIND_NOW either does not occur in the environment or has
a null value. The dynamic linker is permitted to evaluate procedure
linkage table entries lazily, thus avoiding symbol resolution and
relocation overhead for functions that are not called. See ``Procedure
Linkage Table'' in this part for more information.


			   Dynamic Section

If an object file participates in dynamic linking, its program header
table will have an element of type PT_DYNAMIC. This ``segment''
contains the .dynamic section. A special symbol, _DYNAMIC, labels the
section, which contains an array of the following structures.

+ Figure 2-9: Dynamic Structure

  typedef struct {
      Elf32_Sword d_tag;
      union {
          Elf32_Sword	d_val;
          Elf32_Addr	d_ptr;
      } d_un;
  } Elf32_Dyn;

  extern Elf32_Dyn _DYNAMIC[];

For each object with this type, d_tag controls the interpretation of
d_un.

* d_val

  These Elf32_Word objects represent integer values with various
  interpretations.

* d_ptr

  These Elf32_Addr objects represent program virtual addresses. As
  mentioned previously, a file's virtual addresses might not match the
  memory virtual addresses during execution. When interpreting
  addresses contained in the dynamic structure, the dynamic linker
  computes actual addresses, based on the original file value and the
  memory base address. For consistency, files do not contain
  relocation entries to ``correct'' addresses in the dynamic
  structure.

The following table summarizes the tag requirements for executable and
shared object files. If a tag is marked ``mandatory,'' then the
dynamic linking array for an ABI-conforming file must have an entry of
that type. Likewise, ``optional'' means an entry for the tag may
appear but is not required.

+ Figure 2-10: Dynamic Array Tags, d_tag

  Name               Value  d_un         Executable   Shared Object
  ====               =====  ====         ==========   =============
  DT_NULL                0  ignored	 mandatory    mandatory
  DT_NEEDED		 1  d_val	 optional     optional
  DT_PLTRELSZ		 2  d_val	 optional     optional
  DT_PLTGOT		 3  d_ptr	 optional     optional
  DT_HASH		 4  d_ptr	 mandatory    mandatory
  DT_STRTAB		 5  d_ptr	 mandatory    mandatory
  DT_SYMTAB		 6  d_ptr	 mandatory    mandatory
  DT_RELA		 7  d_ptr	 mandatory    optional
  DT_RELASZ		 8  d_val	 mandatory    optional
  DT_RELAENT		 9  d_val	 mandatory    optional
  DT_STRSZ		10  d_val	 mandatory    mandatory
  DT_SYMENT		11  d_val	 mandatory    mandatory
  DT_INIT		12  d_ptr	 optional     optional
  DT_FINI		13  d_ptr	 optional     optional
  DT_SONAME		14  d_val	 ignored      optional
  DT_RPATH		15  d_val	 optional     ignored
  DT_SYMBOLIC		16  ignored	 ignored      optional
  DT_REL		17  d_ptr	 mandatory    optional
  DT_RELSZ		18  d_val	 mandatory    optional
  DT_RELENT		19  d_val	 mandatory    optional
  DT_PLTREL		20  d_val	 optional     optional
  DT_DEBUG		21  d_ptr	 optional     ignored
  DT_TEXTREL		22  ignored	 optional     optional
  DT_JMPREL		23  d_ptr	 optional     optional
  DT_LOPROC     0x70000000  unspecified  unspecified  unspecified
  DT_HIPROC     0x7fffffff  unspecified  unspecified  unspecified

* DT_NULL

  An entry with a DT_NULL tag marks the end of the _DYNAMIC array.

* DT_NEEDED

  This element holds the string table offset of a null-terminated
  string, giving the name of a needed library. The offset is an index
  into the table recorded in the DT_STRTAB entry. See ``Shared Object
  Dependencies'' for more information about these names. The dynamic
  array may contain multiple entries with this type. These entries'
  relative order is significant, though their relation to entries of
  other types is not.

* DT_PLTRELSZ

  This element holds the total size, in bytes, of the relocation
  entries associated with the procedure linkage table. If an entry of
  type DT_JMPREL is present, a DT_PLTRELSZ must accompany it.

* DT_PLTGOT

  This element holds an address associated with the procedure linkage
  table and/or the global offset table. See this section in the
  processor supplement for details.

* DT_HASH

  This element holds the address of the symbol hash table, described
  in ``Hash Table.'' This hash table refers to the symbol table
  referenced by the DT_SYMTAB element.

* DT_STRTAB

  This element holds the address of the string table, described in
  Part 1. Symbol names, library names, and other strings reside in
  this table.

* DT_SYMTAB

  This element holds the address of the symbol table, described in
  Part 1, with Elf32_Sym entries for the 32-bit class of files.

* DT_RELA

  This element holds the address of a relocation table, described in
  Part 1. Entries in the table have explicit addends, such as
  Elf32_Rela for the 32-bit file class. An object file may have
  multiple relocation sections. When building the relocation table for
  an executable or shared object file, the link editor catenates those
  sections to form a single table. Although the sections remain
  independent in the object file, the dynamic linker sees a single
  table. When the dynamic linker creates the process image for an
  executable file or adds a shared object to the process image, it
  reads the relocation table and performs the associated actions. If
  this element is present, the dynamic structure must also have
  DT_RELASZ and DT_RELAENT elements. When relocation is ``mandatory''
  for a file, either DT_RELA or DT_REL may occur (both are permitted
  but not required).

* DT_RELASZ

  This element holds the total size, in bytes, of the DT_RELA
  relocation table.

* DT_RELAENT

  This element holds the size, in bytes, of the DT_RELA relocation
  entry.

* DT_STRSZ

  This element holds the size, in bytes, of the string table.

* DT_SYMENT

  This element holds the size, in bytes, of a symbol table entry.

* DT_INIT

  This element holds the address of the initialization function,
  discussed in ``Initialization and Termination Functions'' below.

* DT_FINI

  This element holds the address of the termination function,
  discussed in ``Initialization and Termination Functions'' below.

* DT_SONAME

  This element holds the string table offset of a null-terminated
  string, giving the name of the shared object. The offset is an index
  into the table recorded in the DT_STRTAB entry. See ``Shared Object
  Dependencies'' below for more information about these names.

* DT_RPATH

  This element holds the string table offset of a null-terminated
  search library search path string, discussed in ``Shared Object
  Dependencies.'' The offset is an index into the table recorded in
  the DT_STRTAB entry.

* DT_SYMBOLIC

  This element's presence in a shared object library alters the
  dynamic linker's symbol resolution algorithm for references within
  the library. Instead of starting a symbol search with the executable
  file, the dynamic linker starts from the shared object itself. If
  the shared object fails to supply the referenced symbol, the dynamic
  linker then searches the executable file and other shared objects as
  usual.

* DT_REL

  This element is similar to DT_RELA, except its table has implicit
  addends, such as Elf32_Rel for the 32-bit file class. If this
  element is present, the dynamic structure must also have DT_RELSZ
  and DT_RELENT elements.

* DT_RELSZ

  This element holds the total size, in bytes, of the DT_REL
  relocation table.

* DT_RELENT

  This element holds the size, in bytes, of the DT_REL relocation
  entry.

* DT_PLTREL

  This member specifies the type of relocation entry to which the
  procedure linkage table refers. The d_val member holds DT_REL or
  DT_RELA, as appropriate. All relocations in a procedure linkage
  table must use the same relocation.

* DT_DEBUG

  This member is used for debugging. Its contents are not specified
  for the ABI; programs that access this entry are not ABI-conforming.

* DT_TEXTREL

  This member's absence signifies that no relocation entry should
  cause a modification to a non-writable segment, as specified by the
  segment permissions in the program header table. If this member is
  present, one or more relocation entries might request modifications
  to a non-writable segment, and the dynamic linker can prepare
  accordingly.

* DT_JMPREL

  If present, this entries's d_ptr member holds the address of
  relocation entries associated solely with the procedure linkage
  table. Separating these relocation entries lets the dynamic linker
  ignore them during process initialization, if lazy binding is
  enabled. If this entry is present, the related entries of types
  DT_PLTRELSZ and DT_PLTREL must also be present.

* DT_LOPROC through DT_HIPROC

  Values in this inclusive range are reserved for processor-specific
  semantics.

Except for the DT_NULL element at the end of the array, and the
relative order of DT_NEEDED elements, entries may appear in any order.
Tag values not appearing in the table are reserved.


		      Shared Object Dependencies

When the link editor processes an archive library, it extracts library
members and copies them into the output object file. These statically
linked services are available during execution without involving the
dynamic linker. Shared objects also provide services, and the dynamic
linker must attach the proper shared object files to the process image
for execution. Thus executable and shared object files describe their
specific dependencies.

When the dynamic linker creates the memory segments for an object
file, the dependencies (recorded in DT_NEEDED entries of the dynamic
structure) tell what shared objects are needed to supply the program's
services. By repeatedly connecting referenced shared objects and their
dependencies, the dynamic linker builds a complete process image. When
resolving symbolic references, the dynamic linker examines the symbol
tables with a breadth-first search. That is, it first looks at the
symbol table of the executable program itself, then at the symbol
tables of the DT_NEEDED entries (in order), then at the second level
DT_NEEDED entries, and so on. Shared object files must be readable by
the process; other permissions are not required.

NOTE: Even when a shared object is referenced multiple times in the
dependency list, the dynamic linker will connect the object only once
to the process.

Names in the dependency list are copies either of the DT_SONAME
strings or the path names of the shared objects used to build the
object file. For example, if the link editor builds an executable file
using one shared object with a DT_SONAME entry of lib1 and another
shared object library with the path name /usr/lib/lib2, the executable
file will contain lib1 and /usr/lib/lib2 in its dependency list.

If a shared object name has one or more slash (/) characters anywhere
in the name, such as /usr/lib/lib2 above or directory/file, the
dynamic linker uses that string directly as the path name. If the name
has no slashes, such as lib1 above, three facilities specify shared
object path searching, with the following precedence.

* First, the dynamic array tag DT_RPATH may give a string that holds a
  list of directories, separated by colons (:). For example, the
  string /home/dir/lib:/home/dir2/lib: tells the dynamic linker to
  search first the directory /home/dir/lib, then /home/dir2/lib, and
  then the current directory to find dependencies.
* Second, a variable called LD_LIBRARY_PATH in the process environment
  [see exec(BA_OS)] may hold a list of directories as above,
  optionally followed by a semicolon (;) and another directory list.
  The following values would be equivalent to the previous example:
    LD_LIBRARY_PATH=/home/dir/lib:/home/dir2/lib:
    LD_LIBRARY_PATH=/home/dir/lib;/home/dir2/lib:
    LD_LIBRARY_PATH=/home/dir/lib:/home/dir2/lib:;
  All LD_LIBRARY_PATH directories are searched after those from
  DT_RPATH. Although some programs (such as the link editor) treat the
  lists before and after the semicolon differently, the dynamic linker
  does not. Nevertheless, the dynamic linker accepts the semicolon
  notation, with the semantics described above.
* Finally, if the other two groups of directories fail to locate the
  desired library, the dynamic linker searches /usr/lib.

NOTE: For security, the dynamic linker ignores environmental search
specifications (such as LD_LIBRARY_PATH) for set-user and set-group ID
programs. It does, however, search DT_RPATH directories and /usr/lib.


			 Global Offset Table

Position-independent code cannot, in general, contain absolute virtual
addresses. Global offset tables hold absolute addresses in private
data, thus making the addresses available without compromising the
position-independence and sharability of a program's text. A program
references its global offset table using position-independent
addressing and extracts absolute values, thus redirecting
position-independent references to absolute locations.

Initially, the global offset table holds information as required by
its relocation entries [see ``Relocation'' in Part 1]. After the
system creates memory segments for a loadable object file, the dynamic
linker processes the relocation entries, some of which will be type
R_386_GLOB_DAT referring to the global offset table. The dynamic
linker determines the associated symbol values, calculates their
absolute addresses, and sets the appropriate memory table entries to
the proper values. Although the absolute addresses are unknown when
the link editor builds an object file, the dynamic linker knows the
addresses of all memory segments and can thus calculate the absolute
addresses of the symbols contained therein.

If a program requires direct access to the absolute address of a
symbol, that symbol will have a global offset table entry. Because the
executable file and shared objects have separate global offset tables,
a symbol's address may appear in several tables. The dynamic linker
processes all the global offset table relocations before giving
control to any code in the process image, thus ensuring the absolute
addresses are available during execution.

The table's entry zero is reserved to hold the address of the dynamic
structure, referenced with the symbol _DYNAMIC. This allows a program,
such as the dynamic linker, to find its own dynamic structure without
having yet processed its relocation entries. This is especially
important for the dynamic linker, because it must initialize itself
without relying on other programs to relocate its memory image. On the
32-bit Intel Architecture, entries one and two in the global offset
table also are reserved. ``Procedure Linkage Table'' below describes
them.

The system may choose different memory segment addresses for the same
shared object in different programs; it may even choose different
library addresses for different executions of the same program.
Nonetheless, memory segments do not change addresses once the process
image is established. As long as a process exists, its memory segments
reside at fixed virtual addresses.

A global offset table's format and interpretation are
processor-specific. For the 32-bit Intel Architecture, the symbol
_GLOBAL_OFFSET_TABLE_ may be used to access the table.

+ Figure 2-11: Global Offset Table

  extern Elf32_Addr _GLOBAL_OFFSET_TABLE_[];

The symbol _GLOBAL_OFFSET_TABLE_ may reside in the middle of the .got
section, allowing both negative and non-negative ``subscripts'' into
the array of addresses.


		       Procedure Linkage Table

Much as the global offset table redirects position-independent address
calculations to absolute locations, the procedure linkage table
redirects position-independent function calls to absolute locations.
The link editor cannot resolve execution transfers (such as function
calls) from one executable or shared object to another. Consequently,
the link editor arranges to have the program transfer control to
entries in the procedure linkage table. On the SYSTEM V architecture,
procedure linkage tables reside in shared text, but they use addresses
in the private global offset table. The dynamic linker determines the
destinations' absolute addresses and modifies the global offset
table's memory image accordingly. The dynamic linker thus can redirect
the entries without compromising the position-independence and
sharability of the program's text. Executable files and shared object
files have separate procedure linkage tables.

+ Figure 2-12: Absolute Procedure Linkage Table {*}

  .PLT0:pushl   got_plus_4
        jmp     *got_plus_8
        nop; nop
        nop; nop
  .PLT1:jmp     *name1_in_GOT
        pushl   $offset
        jmp     .PLT0@PC
  .PLT2:jmp     *name2_in_GOT
        pushl   $offset
        jmp     .PLT0@PC
        ...

+ Figure 2-13: Position-Independent Procedure Linkage Table

  .PLT0:pushl   4(%ebx)
        jmp     *8(%ebx)
        nop; nop
        nop; nop
  .PLT1:jmp     *name1@GOT(%ebx)
        pushl   $offset
        jmp     .PLT0@PC
  .PLT2:jmp     *name2@GOT(%ebx)
        pushl   $offset
        jmp     .PLT0@PC
        ...

NOTE: As the figures show, the procedure linkage table instructions
use different operand addressing modes for absolute code and for
position-independent code. Nonetheless, their interfaces to the
dynamic linker are the same.

Following the steps below, the dynamic linker and the program
``cooperate'' to resolve symbolic references through the procedure
linkage table and the global offset table.

1. When first creating the memory image of the program, the dynamic
   linker sets the second and the third entries in the global offset
   table to special values. Steps below explain more about these
   values.
2. If the procedure linkage table is position-independent, the address
   of the global offset table must reside in %ebx. Each shared object
   file in the process image has its own procedure linkage table, and
   control transfers to a procedure linkage table entry only from
   within the same object file. Consequently, the calling function is
   responsible for setting the global offset table base register
   before calling the procedure linkage table entry.
3. For illustration, assume the program calls name1, which transfers
   control to the label .PLT1.
4. The first instruction jumps to the address in the global offset
   table entry for name1. Initially, the global offset table holds the
   address of the following pushl instruction, not the real address of
   name1.
5. Consequently, the program pushes a relocation offset (offset) on
   the stack. The relocation offset is a 32-bit, non-negative byte
   offset into the relocation table. The designated relocation entry
   will have type R_386_JMP_SLOT, and its offset will specify the
   global offset table entry used in the previous jmp instruction. The
   relocation entry also contains a symbol table index, thus telling
   the dynamic linker what symbol is being referenced, name1 in this
   case.
6. After pushing the relocation offset, the program then jumps to
   .PLT0, the first entry in the procedure linkage table. The pushl
   instruction places the value of the second global offset table
   entry (got_plus_4 or 4(%ebx)) on the stack, thus giving the dynamic
   linker one word of identifying information. The program then jumps
   to the address in the third global offset table entry (got_plus_8
   or 8(%ebx)), which transfers control to the dynamic linker.
7. When the dynamic linker receives control, it unwinds the stack,
   looks at the designated relocation entry, finds the symbol's value,
   stores the ``real'' address for name1 in its global offset table
   entry, and transfers control to the desired destination.
8. Subsequent executions of the procedure linkage table entry will
   transfer directly to name1, without calling the dynamic linker a
   second time. That is, the jmp instruction at .PLT1 will transfer to
   name1, instead of ``falling through'' to the pushl instruction.

The LD_BIND_NOW environment variable can change dynamic linking
behavior. If its value is non-null, the dynamic linker evaluates
procedure linkage table entries before transferring control to the
program. That is, the dynamic linker processes relocation entries of
type R_386_JMP_SLOT during process initialization. Otherwise, the
dynamic linker evaluates procedure linkage table entries lazily,
delaying symbol resolution and relocation until the first execution of
a table entry.

NOTE: Lazy binding generally improves overall application performance,
because unused symbols do not incur the dynamic linking overhead.
Nevertheless, two situations make lazy binding undesirable for some
applications. First, the initial reference to a shared object function
takes longer than subsequent calls, because the dynamic linker
intercepts the call to resolve the symbol. Some applications cannot
tolerate this unpredictability. Second, if an error occurs and the
dynamic linker cannot resolve the symbol, the dynamic linker will
terminate the program. Under lazy binding, this might occur at
arbitrary times. Once again, some applications cannot tolerate this
unpredictability. By turning off lazy binding, the dynamic linker
forces the failure to occur during process initialization, before the
application receives control.


			      Hash Table

A hash table of Elf32_Word objects supports symbol table access.
Labels appear below to help explain the hash table organization, but
they are not part of the specification.

+ Figure 2-14: Symbol Hash Table

  nbucket
  nchain
  bucket[0]
  ...
  bucket[nbucket - 1]
  chain[0]
  ...
  chain[nchain - 1]

The bucket array contains nbucket entries, and the chain array
contains nchain entries; indexes start at 0. Both bucket and chain
hold symbol table indexes. Chain table entries parallel the symbol
table. The number of symbol table entries should equal nchain; so
symbol table indexes also select chain table entries. A hashing
function (shown below) accepts a symbol name and returns a value that
may be used to compute a bucket index. Consequently, if the hashing
function returns the value x for some name, bucket[x%nbucket] gives an
index, y, into both the symbol table and the chain table. If the
symbol table entry is not the one desired, chain[y] gives the next
symbol table entry with the same hash value. One can follow the chain
links until either the selected symbol table entry holds the desired
name or the chain entry contains the value STN_UNDEF.

+ Figure 2-15: Hashing Function

  unsigned long
  elf_hash(const unsigned char *name)
  {
      unsigned long       h = 0, g;
  
      while (*name) {
          h = (h << 4) + *name++;
          if (g = h & 0xf0000000)
              h ^= g >> 24;
          h &= ~g;
      }
      return h;
  }


	       Initialization and Termination Functions

After the dynamic linker has built the process image and performed the
relocations, each shared object gets the opportunity to execute some
initialization code. These initialization functions are called in no
specified order, but all shared object initializations happen before
the executable file gains control.

Similarly, shared objects may have termination functions, which are
executed with the atexit(BA_OS) mechanism after the base process
begins its termination sequence. Once again, the order in which the
dynamic linker calls termination functions is unspecified.

Shared objects designate their initialization and termination
functions through the DT_INIT and DT_FINI entries in the dynamic
structure, described in ``Dynamic Section'' above. Typically, the code
for these functions resides in the .init and .fini sections, mentioned
in ``Sections'' of Part 1.

NOTE: Although the atexit(BA_OS) termination processing normally will
be done, it is not guaranteed to have executed upon process death. In
particular, the process will not execute the termination processing if
it calls _exit [see exit(BA_OS)] or if the process dies because it
received a signal that it neither caught nor ignored.

   ________________________________________________________________


			     3. C LIBRARY

   ________________________________________________________________


   ========================== C Library ===========================


The C library, libc, contains all of the symbols contained in libsys,
and, in addition, contains the routines listed in the following two
tables. The first table lists routines from the ANSI C standard.

+ Figure 3-1: libc Contents, Names without Synonyms

  abort        fputc        isprint      putc         strncmp
  abs	       fputs        ispunct      putchar      strncpy
  asctime      fread        isspace      puts         strpbrk
  atof	       freopen      isupper      qsort        strrchr
  atoi	       frexp        isxdigit     raise        strspn
  atol	       fscanf       labs         rand         strstr
  bsearch      fseek        ldexp        rewind       strtod
  clearerr     fsetpos      ldiv         scanf        strtok
  clock	       ftell        localtime    setbuf       strtol
  ctime	       fwrite       longjmp      setjmp       strtoul
  difftime     getc         mblen        setvbuf      tmpfile
  div	       getchar      mbstowcs     sprintf      tmpnam
  fclose       getenv       mbtowc       srand        tolower
  feof	       gets         memchr       sscanf       toupper
  ferror       gmtime       memcmp       strcat       ungetc
  fflush       isalnum      memcpy       strchr       vfprintf
  fgetc	       isalpha      memmove      strcmp       vprintf
  fgetpos      iscntrl      memset       strcpy       vsprintf
  fgets	       isdigit      mktime       strcspn      wcstombs
  fopen	       isgraph      perror       strlen       wctomb
  fprintf      islower      printf       strncat    

Additionally, libc holds the following services.

+ Figure 3-2: libc Contents, Names with Synonyms

  __assert     getdate      lockf **     sleep        tell ** 
  cfgetispeed  getopt       lsearch      strdup       tempnam
  cfgetospeed  getpass      memccpy      swab         tfind
  cfsetispeed  getsubopt    mkfifo       tcdrain      toascii
  cfsetospeed  getw         mktemp       tcflow       _tolower
  ctermid      hcreate      monitor      tcflush      tsearch
  cuserid      hdestroy     nftw         tcgetattr    _toupper
  dup2	       hsearch      nl_langinfo  tcgetpgrp    twalk
  fdopen       isascii      pclose       tcgetsid     tzset
  __filbuf     isatty       popen        tcsendbreak  _xftw
  fileno       isnan        putenv       tcsetattr    
  __flsbuf     isnand **    putw         tcsetpgrp    
  fmtmsg **    lfind        setlabel     tdelete      

  ** = Function is at Level 2 in the SVID Issue 3 and therefore at
       Level 2 in the ABI.

Besides the symbols listed in the With Synonyms table above, synonyms
of the form _<name> exist for <name> entries that are not listed with
a leading underscore prepended to their name. Thus libc contains both
getopt and _getopt, for example.

Of the routines listed above, the following are not defined elsewhere.

int __filbuf(FILE *f);
	This function returns the next input character for f, filling
	its buffer as appropriate. It returns EOF if an error occurs.

int __flsbuf(int x, FILE *f);
	This function flushes the output characters for f as if
	putc(x, f) had been called and then appends the value of x to
	the resulting output stream. It returns EOF if an error occurs
	and x otherwise.

int _xftw(int, char *, int (*)(char *, struct stat *, int), int);
	Calls to the ftw(BA_LIB) function are mapped to this function
	when applications are compiled. This function is identical to
	ftw(BA_LIB), except that _xftw() takes an interposed first
	argument, which must have the value 2.

See this chapter's other library sections for more SVID, ANSI C, and
POSIX facilities. See ``System Data Interfaces'' later in this chapter
for more information.


			 Global Data Symbols

The libc library requires that some global external data symbols be
defined for its routines to work properly. All the data symbols
required for the libsys library must be provided by libc, as well as
the data symbols listed in the table below.

For formal declarations of the data objects represented by these
symbols, see the System V Interface Definition, Third Edition or the
``Data Definitions'' section of Chapter 6 in the appropriate processor
supplement to the System V ABI.

For entries in the following table that are in <name>-_<name> form,
both symbols in each pair represent the same data. The underscore
synonyms are provided to satisfy the ANSI C standard.

+ Figure 3-3: libc Contents, Global External Data Symbols

  getdate_err		optarg
  _getdate_err		opterr
  __iob			optind
			optopt