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ELF(5) File Formats Manual ELF(5)
elf - format of Executable and Linking Format (ELF) files
#include <elf.h>
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 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.
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,
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 seventh byte is the version number of the ELF
specification:
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
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_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. 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 section 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 headers 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 entry 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 initial 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 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 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,
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 pathname 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 of notes
(ElfN_Nhdr).
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.
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
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 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. Otherwise, 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 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: the initial entry and
the indices 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 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 SHT_DYNSYM section. The index of the
associated string table section can be found in the
sh_link member.
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 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 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
SHT_SYMTAB section.
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
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
nonzero 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. 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 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.
.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_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 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 attribute 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
identify 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 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 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
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 and symbol tables
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 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 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 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
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
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
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
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
section. This member holds the relevant section header
table index.
Relocation entries (Rel & Rela)
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'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 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 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 search path for direct and
indirect library dependencies
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 search path for direct
library dependencies
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
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.
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 library.
Note sections contain a series of notes (see the struct
definitions below). 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. Neither 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 usually 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
immediately follow this note in memory. The name is null
terminated. 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 coordination 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 unknown.
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 comparator
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
followed 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 namespace is unknown.
NT_VERSION
A version string of some sort.
NT_ARCH
Architecture information.
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.
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
Architecture Processor Supplement.
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user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
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Linux man-pages 6.15 2025-06-28 ELF(5)
Pages that refer to this page: dl_iterate_phdr(3), end(3), core(5), ld.so(8)
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