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
[email protected].
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 [email protected]
.PLT2:jmp *name2_in_GOT
pushl $offset
jmp [email protected]
...
+ Figure 2-13: Position-Independent Procedure Linkage Table
.PLT0:pushl 4(%ebx)
jmp *8(%ebx)
nop; nop
nop; nop
.PLT1:jmp *[email protected](%ebx)
pushl $offset
jmp [email protected]
.PLT2:jmp *[email protected](%ebx)
pushl $offset
jmp [email protected]
...
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
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