ELF(5) - man - phpman

Che Dong

ELF(5) Linux Programmer's 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 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 tar‐
geted. This field is used to distinguish among incompatible versions of an
ABI. The interpretation of this version number is dependent on the ABI identi‐
fied 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 sec‐
tion 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_LORE‐‐
SERVE (0xff00), this member holds SHN_XINDEX (0xffff) and the real index of the sec‐
tion 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 execu‐
tion. 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 de‐
scribes 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 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 pro‐
gram 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. More‐
over, 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 seg‐
ment entry.

PT_LOPROC, PT_HIPROC
Values in the inclusive range [PT_LOPROC, PT_HIPROC] are reserved for pro‐
cessor-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. 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 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 in‐
dices are reserved: the initial entry and the indices between SHN_LORESERVE and SHN_HIRE‐‐
SERVE. 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 pro‐
cessor-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 re‐
serves 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 associ‐
ated 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 ta‐
ble 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 multi‐
ple 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 proces‐
sor-specific semantics.

SHT_LOUSER
This value specifies the lower bound of the range of indices reserved for ap‐
plication programs.

SHT_HIUSER
This value specifies the upper bound of the range of indices reserved for ap‐
plication programs. Section types between SHT_LOUSER and SHT_HIUSER may be
used by the application, without conflicting with current or future system-de‐
fined 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 ad‐
dress 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 sec‐
tion 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 sec‐
tion 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_DYN‐‐
SYM. 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 struc‐
tures. 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 attri‐
bute used is SHF_ALLOC.

.init This section holds executable instructions that contribute to the process initializa‐
tion 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 un‐
specified. 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 con‐
tent. 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 sec‐
tion is of type SHT_PROGBITS. The only attribute used is SHF_EXECINSTR. This indi‐
cates 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 load‐
able 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 nor‐
mally 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 load‐
able 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 nor‐
mally 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 associ‐
ated 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. Other‐
wise, 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 ex‐
ist 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 proces‐
sor-specific semantics.

STB_LOCAL
Local symbols are not visible outside the object file containing their defini‐
tion. Local symbols of the same name may exist in multiple files without in‐
terfering with each other.

STB_GLOBAL
Global symbols are visible to all object files being combined. One file's def‐
inition 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 prece‐
dence.

STB_LOPROC, STB_HIPROC
Values in the inclusive range [STB_LOPROC, STB_HIPROC] are reserved for proces‐
sor-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 defini‐
tions 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. Relo‐
catable 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 relo‐
catable 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-spe‐
cific. 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, respec‐
tively, 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 in‐
formation. 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 library search path (deprecated)

DT_SYMBOLIC Alert linker to search this shared object before the executable for sym‐
bols

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 pro‐
cessor-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 automat‐
ically 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 align‐
ment. 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 de‐
buggers. 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:

• word 0: OS descriptor (ELF_NOTE_OS_LINUX, ELF_NOTE_OS_GNU, and so on)`
• word 1: major version of the ABI
• word 2: minor version of the ABI
• word 3: subminor version of the ABI

NT_GNU_HWCAP
Synthetic hwcap information. The desc field begins with two words:

• word 0: number of entries
• word 1: bit mask of enabled entries

Then follow variable-length entries, one byte followed by a null-termi‐
nated hwcap name string. The byte gives the bit number to test if en‐
abled, (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.

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 Architecture Processor Supple‐
ment.

COLOPHON
This page is part of release 5.10 of the Linux man-pages project. A description of the
project, information about reporting bugs, and the latest version of this page, can be found
at https://www.kernel.org/doc/man-pages/.

Linux 2020-12-21 ELF(5)