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
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 con-
tents. 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 Giga-
bytes.
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 depen-
dent 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 indi-
vidual 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'. Cur-
rently, 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 pro-
gram header table is held in the sh_info member of the ini-
tial 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 sec-
tion 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 sec-
tion 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 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 mem-
bers' values are undefined. This lets the pro-
gram 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 ele-
ment 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 seg-
ment:
PF_X An executable segment.
PF_W A writable segment.
PF_R A readable segment.
A text segment commonly has the flags PF_X and PF_R. A data
segment commonly has PF_X, PF_W, and PF_R.
p_align This member holds the value to which the segments are aligned
in memory and in the file. Loadable process segments must
have congruent values for p_vaddr and p_offset, modulo the
page size. Values of zero and one mean no alignment is
required. Otherwise, p_align should be a positive, integral
power of two, and p_vaddr should equal p_offset, modulo
p_align.
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
indices between SHN_LORESERVE and SHN_HIRESERVE. The initial entry is
used in ELF extensions for e_phnum, e_shnum and e_strndx; 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 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 inac-
tive. It does not have an associated section.
Other members of the section header have unde-
fined values.
SHT_PROGBITS This section holds information defined by the
program, whose format and meaning are deter-
mined 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 link-
ing. As a complete symbol table, it may con-
tain 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 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 con-
tain a SHT_SYMTAB section.
SHT_LOPROC, SHT_HIPROC
Values in the inclusive range [SHT_LOPROC,
SHT_HIPROC] are reserved for processor-spe-
cific semantics.
SHT_LOUSER This value specifies the lower bound of the
range of indices reserved for application pro-
grams.
SHT_HIUSER This value specifies the upper bound of the
range of indices reserved for application pro-
grams. 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 sec-
tion 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 initial-
izes 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++ construc-
tor 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_PROG-
BITS. 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_PROG-
BITS. 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_PROG-
BITS. 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 sec-
tion'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 ta-
ble 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 runtime ABI of
the ELF image. It may include the operating system name and
its runtime 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 compatibil-
ity 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-spe-
cific.
.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 load-
able 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
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 unde-
fined 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 inter-
posed by definitions in other modules.
STV_INTERNAL Processor-specific hidden class.
STV_HIDDEN Symbol is unavailable to other modules; ref-
erences 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 sym-
bolic definitions. Relocatable files must have information that
describes how to modify their section contents, thus allowing exe-
cutable 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 reloca-
tion 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 (dep-
recated)
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 reloca-
tion entries should apply to a nonwritable seg-
ment
DT_JMPREL Address of relocation entries associated solely
with the PLT
DT_BIND_NOW Instruct dynamic linker to process all reloca-
tions before transferring control to the exe-
cutable
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 interpre-
tations.
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 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
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 operat-
ing system or architecture specific and often require
close coordination with kernels, C libraries, and debug-
gers. 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 com-
parator 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:
o word 0: OS descriptor (ELF_NOTE_OS_LINUX,
ELF_NOTE_OS_GNU, and so on)`
o word 1: major version of the ABI
o word 2: minor version of the ABI
o word 3: subminor version of the ABI
NT_GNU_HWCAP
Synthetic hwcap information. The desc field
begins with two words:
o word 0: number of entries
o word 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.
NOTES
ELF first appeared in System V. The ELF format is an adopted standard.
The extensions for e_phnum, e_shnum and e_strndx respectively are Linux
extensions. Sun, BSD and AMD64 also support them; for further informa-
tion, look under SEE ALSO.
SEE ALSO
as(1), elfedit(1), gdb(1), ld(1), nm(1), objdump(1), readelf(1),
size(1), strings(1), strip(1), execve(2), dl_iterate_phdr(3), core(5)
Hewlett-Packard, Elf-64 Object File Format.
Santa Cruz Operation, System V Application Binary Interface.
UNIX System Laboratories, "Object Files", Executable and Linking Format
(ELF).
Sun Microsystems, Linker and Libraries Guide.
AMD64 ABI Draft, System V Application Binary Interface AMD64 Architec-
ture Processor Supplement.
COLOPHON
This page is part of release 4.15 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 2017-09-15 ELF(5)
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