Provided by:
manpages_2.17-1_all 
NAME
elf - format of Executable and Linking Format (ELF) files
SYNOPSIS
#include <elf.h>
DESCRIPTION
The header file 〈elf.h〉 defines the format of ELF executable binary
files. Amongst these files are normal executable files, relocatable
object files, core files and shared libraries.
An executable file using the ELF file format consists of an ELF header,
followed by a program header table or a section header table, or both.
The ELF header is always at offset zero of the file. The program header
table and the section header table’s offset in the file are defined in
the ELF header. The two tables describe the rest of the particularities
of the file.
This header file describes the above mentioned headers as C structures
and also includes structures for dynamic sections, relocation sections
and symbol tables.
The following types are used for N-bit architectures (N=32,64, ElfN
stands for Elf32 or Elf64, uintN_t stands for uint32_t or uint64_t):
ElfN_Addr Unsigned program address, uintN_t
ElfN_Off Unsigned file offset, uintN_t
ElfN_Section Unsigned section index, uint16_t
ElfN_Versym Unsigned version symbol information, uint16_t
Elf_Byte unsigned char
ElfN_Half uint16_t
ElfN_Sword int32_t
ElfN_Word uint32_t
ElfN_Sxword int64_t
ElfN_Xword uint64_t
(Note: The *BSD terminology is a bit different. There Elf64_Half is twice
as large as Elf32_Half, and Elf64Quarter is used for uint16_t. In order
to avoid confusion these types are replaced by explicit ones in the
below.)
All data structures that the 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.
The ELF header is described by the type Elf32_Ehdr or Elf64_Ehdr:
#define EI_NIDENT 16
typedef struct {
unsigned char e_ident[EI_NIDENT];
uint16_t e_type;
uint16_t e_machine;
uint32_t e_version;
ElfN_Addr e_entry;
ElfN_Off e_phoff;
ElfN_Off e_shoff;
uint32_t e_flags;
uint16_t e_ehsize;
uint16_t e_phentsize;
uint16_t e_phnum;
uint16_t e_shentsize;
uint16_t e_shnum;
uint16_t e_shstrndx;
} ElfN_Ehdr;
The fields have the following meanings:
e_ident This array of bytes specifies to interpret the file,
independent of the processor or the file’s remaining
contents. Within this array everything is named by
macros, which start with the prefix EI_ and may
contain values which start with the prefix ELF. The
following macros are defined:
EI_MAG0 The first byte of the magic number. It
must be filled with ELFMAG0. (0: 0x7f)
EI_MAG1 The second byte of the magic number. It
must be filled with ELFMAG1. (1: ’E’)
EI_MAG2 The third byte of the magic number. It
must be filled with ELFMAG2. (2: ’L’)
EI_MAG3 The fourth byte of the magic number. It
must be filled with ELFMAG3. (3: ’F’)
EI_CLASS The fifth byte identifies the architecture
for this binary:
ELFCLASSNONE This class is invalid.
ELFCLASS32 This defines the 32-bit
architecture. It supports
machines with files and
virtual address spaces up to
4 Gigabytes.
ELFCLASS64 This defines the 64-bit
architecture.
EI_DATA The sixth byte specifies the data encoding
of the processor-specific data in the
file. Currently these encodings are
supported:
ELFDATANONE Unknown data format.
ELFDATA2LSB Two’s complement, little-
endian.
ELFDATA2MSB Two’s complement, big-endian.
EI_VERSION The version number of the ELF
specification:
EV_NONE Invalid version.
EV_CURRENT Current version.
EI_OSABI This byte identifies the operating system
and ABI to which the object is targeted.
Some fields in other ELF structures have
flags and values that have platform
specific meanings; the interpretation of
those fields is determined by the value of
this byte. E.g.:
ELFOSABI_NONE Same as ELFOSABI_SYSV
ELFOSABI_SYSV UNIX System V ABI.
ELFOSABI_HPUX HP-UX ABI.
ELFOSABI_NETBSD NetBSD ABI.
ELFOSABI_LINUX Linux ABI.
ELFOSABI_SOLARIS Solaris ABI.
ELFOSABI_IRIX IRIX ABI.
ELFOSABI_FREEBSD FreeBSD ABI.
ELFOSABI_TRU64 TRU64 UNIX ABI.
ELFOSABI_ARM ARM architecture ABI.
ELFOSABI_STANDALONE Stand-alone
(embedded) ABI.
EI_ABIVERSION
This byte identifies the version of the
ABI to which the object is targeted. This
field is used to distinguish among
incompatible versions of an ABI. The
interpretation of this version number is
dependent on the ABI identified by the
EI_OSABI field. Applications conforming
to this specification use the value 0.
EI_PAD Start of padding. These bytes are
reserved and set to zero. Programs which
read them should ignore them. The value
for EI_PAD will change in the future if
currently unused bytes are given meanings.
EI_BRAND Start of architecture identification.
EI_NIDENT The size of the e_ident array.
e_type This member of the structure identifies the object
file type:
ET_NONE An unknown type.
ET_REL A relocatable file.
ET_EXEC An executable file.
ET_DYN A shared object.
ET_CORE A core file.
e_machine This member specifies the required architecture for an
individual file. E.g.:
EM_NONE An unknown machine.
EM_M32 AT&T WE 32100.
EM_SPARC Sun Microsystems SPARC.
EM_386 Intel 80386.
EM_68K Motorola 68000.
EM_88K Motorola 88000.
EM_860 Intel 80860.
EM_MIPS MIPS RS3000 (big-endian only).
EM_PARISC HP/PA.
EM_SPARC32PLUS SPARC with enhanced instruction set.
EM_PPC PowerPC.
EM_PPC64 PowerPC 64-bit.
EM_S390 IBM S/390
EM_ARM Advanced RISC Machines
EM_SH Renesas SuperH
EM_SPARCV9 SPARC v9 64-bit.
EM_IA_64 Intel Itanium
EM_X86_64 AMD x86-64
EM_VAX DEC Vax.
e_version This member identifies the file version:
EV_NONE Invalid version.
EV_CURRENT Current version.
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’. Currently no flags have been
defined.
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, e_phnum holds the value zero.
e_shentsize This member holds a sections 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 of 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.
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.
SHN_LORESERVE This value specifies the lower bound of
the range of reserved indices.
SHN_LOPROC Values greater than or equal to
SHN_HIPROC are reserved for processor-
specific semantics.
SHN_HIPROC Values less than or equal to SHN_LOPROC
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 indices 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 indices.
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. 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. The
ELF program header is described by the type Elf32_Phdr or Elf64_Phdr
depending on the architecture:
typedef struct {
uint32_t p_type;
Elf32_Off p_offset;
Elf32_Addr p_vaddr;
Elf32_Addr p_paddr;
uint32_t p_filesz;
uint32_t p_memsz;
uint32_t p_flags;
uint32_t p_align;
} Elf32_Phdr;
typedef struct {
uint32_t p_type;
uint32_t p_flags;
Elf64_Off p_offset;
Elf64_Addr p_vaddr;
Elf64_Addr p_paddr;
uint64_t p_filesz;
uint64_t p_memsz;
uint64_t p_align;
} Elf64_Phdr;
The main difference between the 32-bit and the 64-bit program header lies
in the location of the p_flags member in the total struct.
p_type This member of the Phdr struct tells what kind of segment
this array element describes or how to interpret the
array element’s information.
PT_NULL The array element is unused and the other
members’ values are undefined. This lets the
program header 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.
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). However it
may not occur more than once in a file. If
it is present, it must precede any loadable
segment entry.
PT_NOTE The array element specifies the location and
size for auxiliary information.
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 only occur 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.
PT_LOPROC Values greater than or equal to PT_HIPROC are
reserved for processor-specific semantics.
PT_HIPROC Values less than or equal to PT_LOPROC are
reserved for processor-specific semantics.
p_offset This member holds the offset from the beginning of the
file at which the first byte of the segment resides.
p_vaddr This member holds 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. Under BSD this member is not used and must be
zero.
p_filesz This member holds the number of bytes in the file image
of the segment. It may be zero.
p_memsz This member holds the number of bytes in the memory image
of the segment. It may be zero.
p_flags This member holds flags relevant to the segment:
PF_X An executable segment.
PF_W A writable segment.
PF_R A readable segment.
A text segment commonly has the flags PF_X and PF_R. A
data segment commonly has PF_X, PF_W and PF_R.
p_align This member holds the value to which the segments are
aligned in memory and in the file. Loadable process
segments must have congruent values for p_vaddr and
p_offset, modulo the page size. Values of zero and one
mean no alignment is required. Otherwise, p_align should
be a positive, integral power of two, and p_vaddr should
equal p_offset, modulo p_align.
A file’s section header table lets one locate all the file’s sections.
The section header table is an array of Elf32_Shdr or Elf64_Shdr
structures. The ELF header’s e_shoff member gives the byte offset from
the beginning of the file to the section header table. e_shnum holds the
number of entries the section header table contains. e_shentsize holds
the size in bytes of each entry.
A section header table index is a subscript into this array. Some
section header table indices are reserved. An object file does not have
sections for these special indices:
SHN_UNDEF This value marks an undefined, missing, irrelevant or
otherwise meaningless section reference.
SHN_LORESERVE This value specifies the lower bound of the range of
reserved indices.
SHN_LOPROC Values greater than or equal to SHN_HIPROC are reserved
for processor-specific semantics.
SHN_HIPROC Values less than or equal to SHN_LOPROC are reserved for
processor-specific semantics.
SHN_ABS This value specifies the absolute value for the
corresponding reference. For example, a symbol defined
relative to section number SHN_ABS has an absolute value
and is 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 indices. The system reserves indices between
SHN_LORESERVE and SHN_HIRESERVE, inclusive. The section
header table does not contain entries for the reserved
indices.
The section header has the following structure:
typedef struct {
uint32_t sh_name;
uint32_t sh_type;
uint32_t sh_flags;
Elf32_Addr sh_addr;
Elf32_Off sh_offset;
uint32_t sh_size;
uint32_t sh_link;
uint32_t sh_info;
uint32_t sh_addralign;
uint32_t sh_entsize;
} Elf32_Shdr;
typedef struct {
uint32_t sh_name;
uint32_t sh_type;
uint64_t sh_flags;
Elf64_Addr sh_addr;
Elf64_Off sh_offset;
uint64_t sh_size;
uint32_t sh_link;
uint32_t sh_info;
uint64_t sh_addralign;
uint64_t sh_entsize;
} Elf64_Shdr;
No real differences exist between the 32-bit and 64-bit section headers.
sh_name This member specifies the name of the section. Its
value is an index into the section header string
table section, giving the location of a null-
terminated string.
sh_type This member categorizes the section’s contents and
semantics.
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 This section holds information defined
by the program, whose format and
meaning are determined solely by the
program.
SHT_SYMTAB This section holds a symbol table.
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. An object file can also
contain a SHN_DYNSYM section.
SHT_STRTAB This section holds a string table. An
object file may have multiple string
table sections.
SHT_RELA This section holds relocation entries
with explicit addends, such as type
Elf32_Rela for the 32-bit class of
object files. An object may have
multiple relocation sections.
SHT_HASH This section holds a symbol hash table.
An object participating in dynamic
linking must contain a symbol hash
table. An object file may have only
one hash table.
SHT_DYNAMIC This section holds information for
dynamic linking. An object file may
have only one dynamic section.
SHT_NOTE This section holds information that
marks the file in some way.
SHT_NOBITS A section of this type occupies no
space in the file but otherwise
resembles SHN_PROGBITS. Although this
section contains no bytes, the
sh_offset member contains the
conceptual file offset.
SHT_REL This section holds relocation offsets
without explicit addends, such as type
Elf32_Rel for the 32-bit class of
object files. An object file may have
multiple relocation sections.
SHT_SHLIB This section is reserved but has
unspecified semantics.
SHT_DYNSYM This section holds a minimal set of
dynamic linking symbols. An object
file can also contain a SHN_SYMTAB
section.
SHT_LOPROC This value up to and including
SHT_HIPROC is reserved for processor-
specific semantics.
SHT_HIPROC This value down to and including
SHT_LOPROC is reserved for processor-
specific semantics.
SHT_LOUSER This value specifies the lower bound of
the range of indices reserved for
application programs.
SHT_HIUSER This value specifies the upper bound of
the range of indices 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.
sh_flags Sections support one-bit flags that describe
miscellaneous attributes. 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 This section contains data that should
be writable during process execution.
SHF_ALLOC This 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 This section contains executable
machine instructions.
SHF_MASKPROC All bits included in this mask are
reserved for processor-specific
semantics.
sh_addr If this section appears in the memory image of a
process, this member holds the address at which the
section’s first byte should reside. Otherwise, the
member contains zero.
sh_offset This member’s value holds the byte offset from the
beginning of the file to the first byte in the
section. One section type, SHT_NOBITS, occupies no
space in the file, and its sh_offset member locates
the conceptual placement in the file.
sh_size This member holds 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.
sh_info This member holds extra information, whose
interpretation depends on the section type.
sh_addralign Some sections have address alignment constraints. 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 zero,
modulo the value of sh_addralign. Only zero and
positive integral powers of two are allowed. Values
of zero or one mean the section has no alignment
constraints.
sh_entsize Some sections hold a table of fixed-sized entries,
such as a symbol table. For such a section, this
member gives the size in bytes for each entry. This
member contains zero if the section does not hold a
table of fixed-size entries.
Various sections hold program and control information:
.bss This section holds uninitialized data that contributes
to the program’s memory image. By definition, the
system initializes the data with zeros when the program
begins to run. This section is of type SHT_NOBITS. The
attribute types are SHF_ALLOC and SHF_WRITE.
.comment This section holds version control information. This
section is of type SHT_PROGBITS. No attribute types are
used.
.ctors This section holds initialized pointers to the C++
constructor functions. This section is of type
SHT_PROGBITS. The attribute types are SHF_ALLOC and
SHF_WRITE.
.data This section holds initialized data that contribute to
the program’s memory image. This section is of type
SHT_PROGBITS. The attribute types are SHF_ALLOC and
SHF_WRITE.
.data1 This section holds initialized data that contribute to
the program’s memory image. This section is of type
SHT_PROGBITS. The attribute types are SHF_ALLOC and
SHF_WRITE.
.debug This section holds information for symbolic debugging.
The contents are unspecified. This section is of type
SHT_PROGBITS. No attribute types are used.
.dtors This section holds initialized pointers to the C++
destructor functions. This section is of type
SHT_PROGBITS. The attribute types are SHF_ALLOC and
SHF_WRITE.
.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.
This section is of type SHT_DYNAMIC. See the attributes
above.
.dynstr This section holds strings needed for dynamic linking,
most commonly the strings that represent the names
associated with symbol table entries. This section is
of type SHT_STRTAB. The attribute type used is
SHF_ALLOC.
.dynsym This section holds the dynamic linking symbol table.
This section is of type SHT_DYNSYM. The attribute used
is SHF_ALLOC.
.fini This section holds executable instructions that
contribute to the process termination code. When a
program exits normally the system arranges to execute
the code in this section. This section is of type
SHT_PROGBITS. The attributes used are SHF_ALLOC and
SHF_EXECINSTR.
.got This section holds the global offset table. This
section is of type SHT_PROGBITS. The attributes are
processor-specific.
.hash This section holds a symbol hash table. This section is
of type SHT_HASH. The attribute used is SHF_ALLOC.
.init This section holds executable instructions that
contribute to the process initialization code. When a
program starts to run the system arranges to execute the
code in this section before calling the main program
entry point. This section is of type SHT_PROGBITS. The
attributes used are SHF_ALLOC and SHF_EXECINSTR.
.interp This section holds the pathname 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. This section is of type SHT_PROGBITS.
.line This section holds line number information for symbolic
debugging, which describes the correspondence between
the program source and the machine code. The contents
are unspecified. This section is of type SHT_PROGBITS.
No attribute types are used.
.note This section holds information in the “Note Section”
format described below. This section is of type
SHT_NOTE. No attribute types are used. OpenBSD native
executables usually contain a .note.openbsd.ident
section to identify themselves, for the kernel to bypass
any compatibility ELF binary emulation tests when
loading the file.
.plt This section holds the procedure linkage table. This
section is of type SHT_PROGBITS. The attributes are
processor-specific.
.relNAME This section holds relocation information as described
below. If the file has a loadable segment that includes
relocation, the section’s attributes will include the
SHF_ALLOC bit. Otherwise the bit will be off. By
convention, “NAME” is supplied by the section to which
the relocations apply. Thus a relocation section for
.text normally would have the name .rel.text. This
section is of type SHT_REL.
.relaNAME This section holds relocation information as described
below. If the file has a loadable segment that includes
relocation, the section’s attributes will include the
SHF_ALLOC bit. Otherwise the bit will be off. By
convention, “NAME” is supplied by the section to which
the relocations apply. Thus a relocation section for
.text normally would have the name .rela.text. This
section is of type SHT_RELA.
.rodata This section holds read-only data that typically
contributes to a non-writable segment in the process
image. This section is of type SHT_PROGBITS. The
attribute used is SHF_ALLOC.
.rodata1 This section holds read-only data that typically
contributes to a non-writable segment in the process
image. This section is of type SHT_PROGBITS. The
attribute used is SHF_ALLOC.
.shstrtab This section holds section names. This section is of
type SHT_STRTAB. No attribute types are used.
.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
the bit will be off. This section is of type
SHT_STRTAB.
.symtab This section holds a symbol table. If the file has a
loadable segment that includes the symbol table, the
section’s attributes will include the SHF_ALLOC bit.
Otherwise the bit will be off. This section is of type
SHT_SYMTAB.
.text This section holds the “text”, or executable
instructions, of a program. This section is of type
SHT_PROGBITS. The attributes used are SHF_ALLOC and
SHF_EXECINSTR.
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. Similarly, a string table’s last byte is defined to hold
a null character, ensuring null termination for all strings.
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.
typedef struct {
uint32_t st_name;
Elf32_Addr st_value;
uint32_t st_size;
unsigned char st_info;
unsigned char st_other;
uint16_t st_shndx;
} Elf32_Sym;
typedef struct {
uint32_t st_name;
unsigned char st_info;
unsigned char st_other;
uint16_t st_shndx;
Elf64_Addr st_value;
uint64_t st_size;
} Elf64_Sym;
The 32-bit and 64-bit versions have the same members, just in a different
order.
st_name This member holds an index into the object file’s symbol
string table, which holds 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 has no name.
st_value This member gives the value of the associated symbol.
st_size Many symbols have associated sizes. This member holds
zero if the symbol has no size or an unknown size.
st_info This member specifies the symbol’s type and binding
attributes:
STT_NOTYPE The symbol’s type is not defined.
STT_OBJECT The symbol is associated with a data object.
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 bindings.
STT_FILE By convention, the symbol’s name gives the
name of the source file associated with the
object file. A file symbol has STB_LOCAL
bindings, its section index is SHN_ABS, and
it precedes the other STB_LOCAL symbols of
the file, if it is present.
STT_LOPROC This value up to and including STT_HIPROC is
reserved for processor-specific semantics.
STT_HIPROC This value down to and including STT_LOPROC
is reserved for processor-specific
semantics.
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
symbol.
STB_WEAK Weak symbols resemble global symbols, but
their definitions have lower precedence.
STB_LOPROC This value up to and including STB_HIPROC is
reserved for processor-specific semantics.
STB_HIPROC This value down to and including STB_LOPROC
is reserved for processor-specific semantics.
There are macros for packing and unpacking
the binding and type fields:
ELF32_ST_BIND(info) or ELF64_ST_BIND(info)
extract a binding from an st_info value.
ELF32_ST_TYPE(info) or ELF64_ST_TYPE(info)
extract a type from an st_info value.
ELF32_ST_INFO(bind, type) or
ELF64_ST_INFO(bind, type)
convert a binding and a type into an st_info
value.
st_other This member currently holds zero 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.
Relocation is the process of connecting symbolic references with
symbolic definitions. 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’ program image. Relocation entries are these data.
Relocation structures that do not need an addend:
typedef struct {
Elf32_Addr r_offset;
uint32_t r_info;
} Elf32_Rel;
typedef struct {
Elf64_Addr r_offset;
uint64_t r_info;
} Elf64_Rel;
Relocation structures that need an addend:
typedef struct {
Elf32_Addr r_offset;
uint32_t r_info;
int32_t r_addend;
} Elf32_Rela;
typedef struct {
Elf64_Addr r_offset;
uint64_t r_info;
int64_t r_addend;
} Elf64_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 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. 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
ELF_[32|64]_R_TYPE or ELF[32|64]_R_SYM, respectively,
to the entry’s r_info member.
r_addend This member specifies a constant addend used to
compute the value to be stored into the relocatable
field.
The .dynamic section contains a series of structures that hold
relevant dynamic linking information. The d_tag member controls the
interpretation of d_un.
typedef struct {
Elf32_Sword d_tag;
union {
Elf32_Word d_val;
Elf32_Addr d_ptr;
} d_un;
} Elf32_Dyn;
extern Elf32_Dyn _DYNAMIC[];
typedef struct {
Elf64_Sxword d_tag;
union {
Elf64_Xword d_val;
Elf64_Addr d_ptr;
} d_un;
} Elf64_Dyn;
extern Elf64_Dyn _DYNAMIC[];
d_tag This member may have any of the following values:
DT_NULL Marks end of dynamic section
DT_NEEDED String table offset to name of a needed
library
DT_PLTRELSZ Size in bytes of PLT relocs
DT_PLTGOT Address of PLT and/or GOT
DT_HASH Address of symbol hash table
DT_STRTAB Address of string table
DT_SYMTAB Address of symbol table
DT_RELA Address of Rela relocs table
DT_RELASZ Size in bytes of Rela table
DT_RELAENT Size in bytes of a Rela table entry
DT_STRSZ Size in bytes of string table
DT_SYMENT Size in bytes of a symbol table entry
DT_INIT Address of the initialization function
DT_FINI Address of the termination function
DT_SONAME String table offset to name of shared object
DT_RPATH String table offset to library search path
(deprecated)
DT_SYMBOLIC Alert linker to search this shared object
before the executable for symbols
DT_REL Address of Rel relocs table
DT_RELSZ Size in bytes of Rel table
DT_RELENT Size in bytes of a Rel table entry
DT_PLTREL Type of reloc the PLT refers (Rela or Rel)
DT_DEBUG Undefined use for debugging
DT_TEXTREL Absence of this indicates no relocs should
apply to a non-writable segment
DT_JMPREL Address of reloc entries solely for the PLT
DT_BIND_NOW Instruct dynamic linker to process all
relocs before transferring control to the
executable
DT_RUNPATH String table offset to library search path
DT_LOPROC Start of processor-specific semantics
DT_HIPROC End of processor-specific semantics
d_val This member represents integer values with various
interpretations.
d_ptr This member represents program virtual addresses. When
interpreting these addresses, the actual address should
be computed based on the original file value and memory
base address. Files do not contain relocation entries to
fixup these addresses.
_DYNAMIC
Array containing all the dynamic structures in the
.dynamic section. This is automatically populated by the
linker.
SEE ALSO
as(1), gdb(1), ld(1), objdump(1), execve(2), core(5)
Hewlett-Packard, Elf-64 Object File Format.
Santa Cruz Operation, System V Application Binary Interface.
Unix System Laboratories, "Object Files", Executable and Linking Format
(ELF).
HISTORY
OpenBSD ELF support first appeared in OpenBSD 1.2, although not all
supported platforms use it as the native binary file format. ELF in
itself first appeared in AT&T System V UNIX. The ELF format is an
adopted standard.
AUTHORS
The original version of this manual page was written by Jeroen Ruigrok
van der Werven 〈asmodai@FreeBSD.org〉 with inspiration from BSDi’s BSD/OS
elf manpage.