ELF(5) File Formats Manual ELF(5)
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 objects.
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 de-
fined in the ELF header. The two tables describe the rest of the par-
ticularities 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.
Basic types
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, and so on.
ELF header (Ehdr)
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 how to interpret the file, inde-
pendent 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 bi-
nary:
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 proces-
sor-specific data in the file. Currently, these encod-
ings are supported:
ELFDATANONE Unknown data format.
ELFDATA2LSB Two's complement, little-endian.
ELFDATA2MSB Two's complement, big-endian.
EI_VERSION
The seventh byte is the version number of the ELF speci-
fication:
EV_NONE Invalid version.
EV_CURRENT Current version.
EI_OSABI
The eighth 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. For example:
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
The ninth byte identifies the version of the ABI to which
the object is targeted. This field is used to distin-
guish among incompatible versions of an ABI. The inter-
pretation of this version number is dependent on the ABI
identified by the EI_OSABI field. Applications conform-
ing 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_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 individ-
ual file. For example:
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.
If the number of entries in the program header table is larger
than or equal to PN_XNUM (0xffff), this member holds PN_XNUM
(0xffff) and the real number of entries in the program header
table is held in the sh_info member of the initial entry in sec-
tion header table. Otherwise, the sh_info member of the initial
entry contains the value zero.
PN_XNUM
This is defined as 0xffff, the largest number e_phnum can
have, specifying where the actual number of program head-
ers is assigned.
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.
If the number of entries in the section header table is larger
than or equal to SHN_LORESERVE (0xff00), e_shnum holds the value
zero and the real number of entries in the section header table
is held in the sh_size member of the initial entry in section
header table. Otherwise, the sh_size member of the initial en-
try in the section header table 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.
If the index of section name string table section is larger than
or equal to SHN_LORESERVE (0xff00), this member holds SHN_XINDEX
(0xffff) and the real index of the section name string table
section is held in the sh_link member of the initial entry in
section header table. Otherwise, the sh_link member of the ini-
tial entry in section header table contains the value zero.
Program header (Phdr)
An executable or shared object file's program header table is an array
of structures, each describing a segment or other information the sys-
tem 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 structure indicates 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, de-
scribed by p_filesz and p_memsz. The bytes from the
file are mapped to the beginning of the memory seg-
ment. If the segment's memory size p_memsz is larger
than the file size p_filesz, the "extra" bytes are de-
fined 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 informa-
tion.
PT_INTERP
The array element specifies the location and size of a
null-terminated pathname to invoke as an interpreter.
This segment type is meaningful only for executable
files (though it may occur for shared objects). How-
ever it may not occur more than once in a file. If it
is present, it must precede any loadable segment en-
try.
PT_NOTE
The array element specifies the location of notes
(ElfN_Nhdr).
PT_SHLIB
This segment type is reserved but has unspecified se-
mantics. 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 ta-
ble is part of the memory image of the program. If it
is present, it must precede any loadable segment en-
try.
PT_LOPROC
PT_HIPROC
Values in the inclusive range [PT_LOPROC, PT_HIPROC]
are reserved for processor-specific semantics.
PT_GNU_STACK
GNU extension which is used by the Linux kernel to
control the state of the stack via the flags set in
the p_flags member.
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 mem-
ber 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 a bit mask of 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_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. Other-
wise, p_align should be a positive, integral power of two, and
p_vaddr should equal p_offset, modulo p_align.
Section header (Shdr)
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 struc-
tures. 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 sec-
tion header table indices are reserved: the initial entry and the in-
dices between SHN_LORESERVE and SHN_HIRESERVE. The initial entry is
used in ELF extensions for e_phnum, e_shnum, and e_shstrndx; in other
cases, each field in the initial entry is set to zero. 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
SHN_HIPROC
Values greater in the inclusive range [SHN_LOPROC, SHN_HIPROC]
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 re-
location.
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 head-
ers.
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 SHT_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 ad-
dends, 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 par-
ticipating 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 notes (ElfN_Nhdr).
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 con-
ceptual 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 reloca-
tion sections.
SHT_SHLIB
This section is reserved but has unspecified semantics.
SHT_DYNSYM
This section holds a minimal set of dynamic linking sym-
bols. An object file can also contain a SHT_SYMTAB sec-
tion.
SHT_LOPROC
SHT_HIPROC
Values in the inclusive range [SHT_LOPROC, SHT_HIPROC]
are reserved for processor-specific semantics.
SHT_LOUSER
This value specifies the lower bound of the range of in-
dices reserved for application programs.
SHT_HIUSER
This value specifies the upper bound of the range of in-
dices reserved for application programs. Section types
between SHT_LOUSER and SHT_HIUSER may be used by the ap-
plication, without conflicting with current or future
system-defined section types.
sh_flags
Sections support one-bit flags that describe miscellaneous at-
tributes. 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 sec-
tions.
SHF_EXECINSTR
This section contains executable machine instructions.
SHF_MASKPROC
All bits included in this mask are reserved for proces-
sor-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 sec-
tion type is SHT_NOBITS, the section occupies sh_size bytes in
the file. A section of type SHT_NOBITS may have a nonzero size,
but it occupies no space in the file.
sh_link
This member holds a section header table index link, whose in-
terpretation depends on the section type.
sh_info
This member holds extra information, whose interpretation de-
pends 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. The value 0 or
1 means that 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 sec-
tion 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 pro-
gram'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 pro-
gram'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 con-
tents 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 com-
monly the strings that represent the names associated with sym-
bol table entries. This section is of type SHT_STRTAB. The at-
tribute type used is SHF_ALLOC.
.dynsym
This section holds the dynamic linking symbol table. This sec-
tion 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 sec-
tion is of type SHT_PROGBITS. The attributes used are SHF_ALLOC
and SHF_EXECINSTR.
.gnu.version
This section holds the version symbol table, an array of
ElfN_Half elements. This section is of type SHT_GNU_versym.
The attribute type used is SHF_ALLOC.
.gnu.version_d
This section holds the version symbol definitions, a table of
ElfN_Verdef structures. This section is of type SHT_GNU_verdef.
The attribute type used is SHF_ALLOC.
.gnu.version_r
This section holds the version symbol needed elements, a table
of ElfN_Verneed structures. This section is of type
SHT_GNU_versym. The attribute type used is SHF_ALLOC.
.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_EXECIN-
STR.
.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 debug-
ging, 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 various notes. This section is of type
SHT_NOTE. No attribute types are used.
.note.ABI-tag
This section is used to declare the expected run-time ABI of the
ELF image. It may include the operating system name and its
run-time versions. This section is of type SHT_NOTE. The only
attribute used is SHF_ALLOC.
.note.gnu.build-id
This section is used to hold an ID that uniquely identifies the
contents of the ELF image. Different files with the same build
ID should contain the same executable content. See the
--build-id option to the GNU linker (ld (1)) for more details.
This section is of type SHT_NOTE. The only attribute used is
SHF_ALLOC.
.note.GNU-stack
This section is used in Linux object files for declaring stack
attributes. This section is of type SHT_PROGBITS. The only at-
tribute used is SHF_EXECINSTR. This indicates to the GNU linker
that the object file requires an executable stack.
.note.openbsd.ident
OpenBSD native executables usually contain this section to iden-
tify themselves so the kernel can 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 sec-
tion 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 sec-
tion 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 nonwritable 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 nonwritable 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 rep-
resent the names associated with symbol table entries. If the
file has a loadable segment that includes the symbol string ta-
ble, 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 and symbol tables
String table sections hold null-terminated character sequences, com-
monly 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 byte ('\0'). Similarly, a string table's last byte is
defined to hold a null byte, 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 ta-
ble 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 differ-
ent 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 nonzero, it represents a string table
index that gives the symbol name. Otherwise, the symbol 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 exe-
cutable 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 sym-
bol 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
STT_HIPROC
Values in the inclusive range [STT_LOPROC, STT_HIPROC]
are 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 com-
bined. 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 defini-
tions have lower precedence.
STB_LOPROC
STB_HIPROC
Values in the inclusive range [STB_LOPROC, STB_HIPROC]
are reserved for processor-specific semantics.
There are macros for packing and unpacking the binding and type
fields:
ELF32_ST_BIND(info)
ELF64_ST_BIND(info)
Extract a binding from an st_info value.
ELF32_ST_TYPE(info)
ELF64_ST_TYPE(info)
Extract a type from an st_info value.
ELF32_ST_INFO(bind, type)
ELF64_ST_INFO(bind, type)
Convert a binding and a type into an st_info value.
st_other
This member defines the symbol visibility.
STV_DEFAULT
Default symbol visibility rules. Global and weak symbols
are available to other modules; references in the local
module can be interposed by definitions in other modules.
STV_INTERNAL
Processor-specific hidden class.
STV_HIDDEN
Symbol is unavailable to other modules; references in the
local module always resolve to the local symbol (i.e.,
the symbol can't be interposed by definitions in other
modules).
STV_PROTECTED
Symbol is available to other modules, but references in
the local module always resolve to the local symbol.
There are macros for extracting the visibility type:
ELF32_ST_VISIBILITY(other) or ELF64_ST_VISIBILITY(other)
st_shndx
Every symbol table entry is "defined" in relation to some sec-
tion. This member holds the relevant section header table in-
dex.
Relocation entries (Rel & Rela)
Relocation is the process of connecting symbolic references with sym-
bolic definitions. Relocatable files must have information that de-
scribes 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.
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.
Dynamic tags (Dyn)
The .dynamic section contains a series of structures that hold relevant
dynamic linking information. The d_tag member controls the interpreta-
tion 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 relocation entries
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 relocation table
DT_RELASZ Size in bytes of the Rela relocation table
DT_RELAENT Size in bytes of a Rela relocation 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 (depre-
cated)
DT_SYMBOLIC Alert linker to search this shared object before the
executable for symbols
DT_REL Address of Rel relocation table
DT_RELSZ Size in bytes of Rel relocation table
DT_RELENT Size in bytes of a Rel table entry
DT_PLTREL Type of relocation entry to which the PLT refers
(Rela or Rel)
DT_DEBUG Undefined use for debugging
DT_TEXTREL Absence of this entry indicates that no relocation
entries should apply to a nonwritable segment
DT_JMPREL Address of relocation entries associated solely with
the PLT
DT_BIND_NOW Instruct dynamic linker to process all relocations
before transferring control to the executable
DT_RUNPATH String table offset to library search path
DT_LOPROC
DT_HIPROC Values in the inclusive range [DT_LOPROC, DT_HIPROC]
are reserved for processor-specific semantics
d_val This member represents integer values with various interpreta-
tions.
d_ptr This member represents program virtual addresses. When inter-
preting 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 sec-
tion. This is automatically populated by the linker.
Notes (Nhdr)
ELF notes allow for appending arbitrary information for the system to
use. They are largely used by core files (e_type of ET_CORE), but many
projects define their own set of extensions. For example, the GNU tool
chain uses ELF notes to pass information from the linker to the C li-
brary.
Note sections contain a series of notes (see the struct definitions be-
low). Each note is followed by the name field (whose length is defined
in n_namesz) and then by the descriptor field (whose length is defined
in n_descsz) and whose starting address has a 4 byte alignment. Nei-
ther field is defined in the note struct due to their arbitrary
lengths.
An example for parsing out two consecutive notes should clarify their
layout in memory:
void *memory, *name, *desc;
Elf64_Nhdr *note, *next_note;
/* The buffer is pointing to the start of the section/segment. */
note = memory;
/* If the name is defined, it follows the note. */
name = note->n_namesz == 0 ? NULL : memory + sizeof(*note);
/* If the descriptor is defined, it follows the name
(with alignment). */
desc = note->n_descsz == 0 ? NULL :
memory + sizeof(*note) + ALIGN_UP(note->n_namesz, 4);
/* The next note follows both (with alignment). */
next_note = memory + sizeof(*note) +
ALIGN_UP(note->n_namesz, 4) +
ALIGN_UP(note->n_descsz, 4);
Keep in mind that the interpretation of n_type depends on the namespace
defined by the n_namesz field. If the n_namesz field is not set (e.g.,
is 0), then there are two sets of notes: one for core files and one for
all other ELF types. If the namespace is unknown, then tools will usu-
ally fallback to these sets of notes as well.
typedef struct {
Elf32_Word n_namesz;
Elf32_Word n_descsz;
Elf32_Word n_type;
} Elf32_Nhdr;
typedef struct {
Elf64_Word n_namesz;
Elf64_Word n_descsz;
Elf64_Word n_type;
} Elf64_Nhdr;
n_namesz
The length of the name field in bytes. The contents will imme-
diately follow this note in memory. The name is null termi-
nated. For example, if the name is "GNU", then n_namesz will be
set to 4.
n_descsz
The length of the descriptor field in bytes. The contents will
immediately follow the name field in memory.
n_type Depending on the value of the name field, this member may have
any of the following values:
Core files (e_type = ET_CORE)
Notes used by all core files. These are highly operating
system or architecture specific and often require close co-
ordination with kernels, C libraries, and debuggers. These
are used when the namespace is the default (i.e., n_namesz
will be set to 0), or a fallback when the namespace is un-
known.
NT_PRSTATUS prstatus struct
NT_FPREGSET fpregset struct
NT_PRPSINFO prpsinfo struct
NT_PRXREG prxregset struct
NT_TASKSTRUCT task structure
NT_PLATFORM String from sysinfo(SI_PLATFORM)
NT_AUXV auxv array
NT_GWINDOWS gwindows struct
NT_ASRS asrset struct
NT_PSTATUS pstatus struct
NT_PSINFO psinfo struct
NT_PRCRED prcred struct
NT_UTSNAME utsname struct
NT_LWPSTATUS lwpstatus struct
NT_LWPSINFO lwpinfo struct
NT_PRFPXREG fprxregset struct
NT_SIGINFO siginfo_t (size might increase over
time)
NT_FILE Contains information about mapped
files
NT_PRXFPREG user_fxsr_struct
NT_PPC_VMX PowerPC Altivec/VMX registers
NT_PPC_SPE PowerPC SPE/EVR registers
NT_PPC_VSX PowerPC VSX registers
NT_386_TLS i386 TLS slots (struct user_desc)
NT_386_IOPERM x86 io permission bitmap (1=deny)
NT_X86_XSTATE x86 extended state using xsave
NT_S390_HIGH_GPRS s390 upper register halves
NT_S390_TIMER s390 timer register
NT_S390_TODCMP s390 time-of-day (TOD) clock compara-
tor register
NT_S390_TODPREG s390 time-of-day (TOD) programmable
register
NT_S390_CTRS s390 control registers
NT_S390_PREFIX s390 prefix register
NT_S390_LAST_BREAK s390 breaking event address
NT_S390_SYSTEM_CALL s390 system call restart data
NT_S390_TDB s390 transaction diagnostic block
NT_ARM_VFP ARM VFP/NEON registers
NT_ARM_TLS ARM TLS register
NT_ARM_HW_BREAK ARM hardware breakpoint registers
NT_ARM_HW_WATCH ARM hardware watchpoint registers
NT_ARM_SYSTEM_CALL ARM system call number
n_name = GNU
Extensions used by the GNU tool chain.
NT_GNU_ABI_TAG
Operating system (OS) ABI information. The desc
field will be 4 words:
[0] OS descriptor (ELF_NOTE_OS_LINUX,
ELF_NOTE_OS_GNU, and so on)`
[1] major version of the ABI
[2] minor version of the ABI
[3] subminor version of the ABI
NT_GNU_HWCAP
Synthetic hwcap information. The desc field begins
with two words:
[0] number of entries
[1] bit mask of enabled entries
Then follow variable-length entries, one byte fol-
lowed by a null-terminated hwcap name string. The
byte gives the bit number to test if enabled, (1U <<
bit) & bit mask.
NT_GNU_BUILD_ID
Unique build ID as generated by the GNU ld(1)
--build-id option. The desc consists of any nonzero
number of bytes.
NT_GNU_GOLD_VERSION
The desc contains the GNU Gold linker version used.
Default/unknown namespace (e_type != ET_CORE)
These are used when the namespace is the default (i.e.,
n_namesz will be set to 0), or a fallback when the name-
space is unknown.
NT_VERSION A version string of some sort.
NT_ARCH Architecture information.
NOTES
ELF first appeared in System V. The ELF format is an adopted standard.
The extensions for e_phnum, e_shnum, and e_shstrndx respectively are
Linux extensions. Sun, BSD, and AMD64 also support them; for further
information, look under SEE ALSO.
SEE ALSO
as(1), elfedit(1), gdb(1), ld(1), nm(1), objcopy(1), objdump(1),
patchelf(1), readelf(1), size(1), strings(1), strip(1), execve(2),
dl_iterate_phdr(3), core(5), ld.so(8)
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).
Sun Microsystems, Linker and Libraries Guide.
AMD64 ABI Draft, System V Application Binary Interface AMD64 Architec-
ture Processor Supplement.
Linux man-pages 6.7 2024-02-25 ELF(5)
Generated by dwww version 1.16 on Tue Dec 16 14:51:22 CET 2025.