DESCRIPTION
The header file 〈elf.h〉 defines the format of ELF executable binary
files. Amongst these files are normal executable files, relocatable
object files, core files and shared libraries.
An executable file using the ELF file format consists of an ELF header,
followed by a program header table or a section header table, or both.
The ELF header is always at offset zero of the file. The program header
table and the section header table’s offset in the file are defined in
the ELF header. The two tables describe the rest of the particularities
of the file.
This header file describes the above mentioned headers as C structures
and also includes structures for dynamic sections, relocation sections
and symbol tables.
The following types are used for N-bit architectures (N=32,64, ElfN
stands for Elf32 or Elf64, uintN_t stands for uint32_t or uint64_t):
ElfN_Addr Unsigned program address, uintN_t
ElfN_Off Unsigned file offset, uintN_t
ElfN_Section Unsigned section index, uint16_t
ElfN_Versym Unsigned version symbol information, uint16_t
Elf_Byte unsigned char
ElfN_Half uint16_t
ElfN_Sword int32_t
ElfN_Word uint32_t
ElfN_Sxword int64_t
ElfN_Xword uint64_t
(Note: The *BSD terminology is a bit different. There Elf64_Half is twice
as large as Elf32_Half, and Elf64Quarter is used for uint16_t. In order
to avoid confusion these types are replaced by explicit ones in the
below.)
All data structures that the file format defines follow the “natural”
size and alignment guidelines for the relevant class. If necessary, data
structures contain explicit padding to ensure 4-byte alignment for 4-byte
objects, to force structure sizes to a multiple of 4, etc.
The ELF header is described by the type Elf32_Ehdr or Elf64_Ehdr:
#define EI_NIDENT 16
typedef struct {
unsigned char e_ident[EI_NIDENT];
uint16_t e_type;
uint16_t e_machine;
uint32_t e_version;
ElfN_Addr e_entry;
ElfN_Off e_phoff;
contents. Within this array everything is named by
macros, which start with the prefix EI_ and may con‐
tain values which start with the prefix ELF. The fol‐
lowing 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 vir‐
tual 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 sup‐
ported:
ELFDATANONE Unknown data format.
ELFDATA2LSB Two’s complement, little-
endian.
ELFDATA2MSB Two’s complement, big-endian.
EI_VERSION The version number of the ELF specifica‐
tion:
EV_NONE Invalid version.
EV_CURRENT Current version.
EI_OSABI This byte identifies the operating system
and ABI to which the object is targeted.
Some fields in other ELF structures have
flags and values that have platform spe‐
cific meanings; the interpretation of
those fields is determined by the value of
this byte. E.g.:
EI_ABIVERSION
This byte identifies the version of the
ABI to which the object is targeted. This
field is used to distinguish among incom‐
patible versions of an ABI. The interpre‐
tation of this version number is dependent
on the ABI identified by the EI_OSABI
field. Applications conforming to this
specification use the value 0.
EI_PAD Start of padding. These bytes are
reserved and set to zero. Programs which
read them should ignore them. The value
for EI_PAD will change in the future if
currently unused bytes are given meanings.
EI_BRAND Start of architecture identification.
EI_NIDENT The size of the e_ident array.
e_type This member of the structure identifies the object
file type:
ET_NONE An unknown type.
ET_REL A relocatable file.
ET_EXEC An executable file.
ET_DYN A shared object.
ET_CORE A core file.
e_machine This member specifies the required architecture for an
individual file. E.g.:
EM_NONE An unknown machine.
EM_M32 AT&T WE 32100.
EM_SPARC Sun Microsystems SPARC.
EM_386 Intel 80386.
EM_68K Motorola 68000.
EM_88K Motorola 88000.
EM_860 Intel 80860.
EM_MIPS MIPS RS3000 (big-endian only).
EM_PARISC HP/PA.
EM_SPARC32PLUS SPARC with enhanced instruction set.
EM_PPC PowerPC.
EM_PPC64 PowerPC 64-bit.
EM_S390 IBM S/390
EM_ARM Advanced RISC Machines
EM_SH Renesas SuperH
EM_SPARCV9 SPARC v9 64-bit.
EM_IA_64 Intel Itanium
EM_X86_64 AMD x86-64
EM_VAX DEC Vax.
e_shoff This member holds the section header table’s file off‐
set in bytes. If the file has no section header table
this member holds zero.
e_flags This member holds processor-specific flags associated
with the file. Flag names take the form
EF_‘machine_flag’. Currently no flags have been
defined.
e_ehsize This member holds the ELF header’s size in bytes.
e_phentsize This member holds the size in bytes of one entry in
the file’s program header table; all entries are the
same size.
e_phnum This member holds the number of entries in the program
header table. Thus the product of e_phentsize and
e_phnum gives the table’s size in bytes. If a file
has no program header, e_phnum holds the value zero.
e_shentsize This member holds a sections header’s size in bytes.
A section header is one entry in the section header
table; all entries are the same size.
e_shnum This member holds the number of entries in the section
header table. Thus the product of e_shentsize and
e_shnum gives the section header table’s size in
bytes. If a file has no section header table, e_shnum
holds the value of zero.
e_shstrndx This member holds the section header table index of
the entry associated with the section name string ta‐
ble. If the file has no section name string table,
this member holds the value SHN_UNDEF.
SHN_UNDEF This value marks an undefined, missing,
irrelevant, or otherwise meaningless
section reference. For example, a sym‐
bol “defined” relative to section num‐
ber SHN_UNDEF is an undefined symbol.
SHN_LORESERVE This value specifies the lower bound of
the range of reserved indices.
SHN_LOPROC Values greater than or equal to
SHN_HIPROC are reserved for processor-
specific semantics.
SHN_HIPROC Values less than or equal to SHN_LOPROC
are reserved for processor-specific
semantics.
SHN_LORESERVE and SHN_HIRESERVE, inclu‐
sive; the values do not reference the
section header table. That is, the
section header table does not contain
entries for the reserved indices.
An executable or shared object file’s program header table is an array of
structures, each describing a segment or other information the system
needs to prepare the program for execution. An object file segment con‐
tains one or more sections. Program headers are meaningful only for exe‐
cutable 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 pro‐
gram header is described by the type Elf32_Phdr or Elf64_Phdr depending
on the architecture:
typedef struct {
uint32_t p_type;
Elf32_Off p_offset;
Elf32_Addr p_vaddr;
Elf32_Addr p_paddr;
uint32_t p_filesz;
uint32_t p_memsz;
uint32_t p_flags;
uint32_t p_align;
} Elf32_Phdr;
typedef struct {
uint32_t p_type;
uint32_t p_flags;
Elf64_Off p_offset;
Elf64_Addr p_vaddr;
Elf64_Addr p_paddr;
uint64_t p_filesz;
uint64_t p_memsz;
uint64_t p_align;
} Elf64_Phdr;
The main difference between the 32-bit and the 64-bit program header lies
in the location of the p_flags member in the total struct.
p_type This member of the Phdr struct tells what kind of segment
this array element describes or how to interpret the
array element’s information.
PT_NULL The array element is unused and the other
members’ values are undefined. This lets the
program header have ignored entries.
PT_LOAD The array element specifies a loadable seg‐
ment, described by p_filesz and p_memsz. The
bytes from the file are mapped to the begin‐
ning of the memory segment. If the segment’s
as an interpreter. This segment type is
meaningful only for executable files (though
it may occur for shared objects). However it
may not occur more than once in a file. If
it is present, it must precede any loadable
segment entry.
PT_NOTE The array element specifies the location and
size for auxiliary information.
PT_SHLIB This segment type is reserved but has unspec‐
ified 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. More‐
over, it may only occur if the program header
table is part of the memory image of the pro‐
gram. If it is present, it must precede any
loadable segment entry.
PT_LOPROC Values greater than or equal to PT_HIPROC are
reserved for processor-specific semantics.
PT_HIPROC Values less than or equal to PT_LOPROC are
reserved for processor-specific semantics.
p_offset This member holds the offset from the beginning of the
file at which the first byte of the segment resides.
p_vaddr This member holds the virtual address at which the first
byte of the segment resides in memory.
p_paddr On systems for which physical addressing is relevant,
this member is reserved for the segment’s physical
address. Under BSD this member is not used and must be
zero.
p_filesz This member holds the number of bytes in the file image
of the segment. It may be zero.
p_memsz This member holds the number of bytes in the memory image
of the segment. It may be zero.
p_flags This member holds flags relevant to the segment:
PF_X An executable segment.
PF_W A writable segment.
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. An object file does not have
sections for these special indices:
SHN_UNDEF This value marks an undefined, missing, irrelevant or oth‐
erwise meaningless section reference.
SHN_LORESERVE This value specifies the lower bound of the range of
reserved indices.
SHN_LOPROC Values greater than or equal to SHN_HIPROC are reserved
for processor-specific semantics.
SHN_HIPROC Values less than or equal to SHN_LOPROC are reserved for
processor-specific semantics.
SHN_ABS This value specifies the absolute value for the corre‐
sponding reference. For example, a symbol defined rela‐
tive 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 sym‐
bols, 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;
sh_name This member specifies the name of the section. Its
value is an index into the section header string ta‐
ble 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 mean‐
ing are determined solely by the pro‐
gram.
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 com‐
plete symbol table, it may contain many
symbols unnecessary for dynamic link‐
ing. 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 mul‐
tiple relocation sections.
SHT_HASH This section holds a symbol hash table.
An object participating in dynamic
linking must contain a symbol hash ta‐
ble. An object file may have only one
hash table.
SHT_DYNAMIC This section holds information for
dynamic linking. An object file may
have only one dynamic section.
SHT_NOTE This section holds information that
marks the file in some way.
SHT_NOBITS A section of this type occupies no
SHT_DYNSYM This section holds a minimal set of
dynamic linking symbols. An object
file can also contain a SHT_SYMTAB sec‐
tion.
SHT_LOPROC This value up to and including
SHT_HIPROC is reserved for processor-
specific semantics.
SHT_HIPROC This value down to and including
SHT_LOPROC is reserved for processor-
specific semantics.
SHT_LOUSER This value specifies the lower bound of
the range of indices reserved for
application programs.
SHT_HIUSER This value specifies the upper bound of
the range of indices reserved for
application programs. Section types
between SHT_LOUSER and SHT_HIUSER may
be used by the application, without
conflicting with current or future sys‐
tem-defined section types.
sh_flags Sections support one-bit flags that describe miscel‐
laneous 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 sec‐
tions 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 seman‐
tics.
sh_addr If this section appears in the memory image of a pro‐
cess, this member holds the address at which the sec‐
tion’s first byte should reside. Otherwise, the mem‐
ber contains zero.
sh_offset This member’s value holds the byte offset from the
beginning of the file to the first byte in the sec‐
tation 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 pos‐
itive integral powers of two are allowed. Values of
zero or one mean the section has no alignment con‐
straints.
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 sys‐
tem 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++ con‐
structor 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.
is SHF_ALLOC.
.fini This section holds executable instructions that con‐
tribute to the process termination code. When a program
exits normally the system arranges to execute the code
in this section. This section is of type SHT_PROGBITS.
The attributes used are SHF_ALLOC and SHF_EXECINSTR.
.got This section holds the global offset table. This sec‐
tion is of type SHT_PROGBITS. The attributes are pro‐
cessor-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 con‐
tribute to the process initialization code. When a pro‐
gram 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 inter‐
preter. If the file has a loadable segment that
includes the section, the section’s attributes will
include the SHF_ALLOC bit. Otherwise, that bit will be
off. This section is of type SHT_PROGBITS.
.line This section holds line number information for symbolic
debugging, which describes the correspondence between
the program source and the machine code. The contents
are unspecified. This section is of type SHT_PROGBITS.
No attribute types are used.
.note This section holds information in the “Note Section”
format described below. This section is of type
SHT_NOTE. No attribute types are used. OpenBSD native
executables usually contain a .note.openbsd.ident sec‐
tion to identify themselves, for the kernel to bypass
any compatibility ELF binary emulation tests when load‐
ing 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 con‐
vention, “NAME” is supplied by the section to which the
relocations apply. Thus a relocation section for .text
This section is of type SHT_PROGBITS. The attribute
used is SHF_ALLOC.
.rodata1 This section holds read-only data that typically con‐
tributes to a non-writable segment in the process image.
This section is of type SHT_PROGBITS. The attribute
used is SHF_ALLOC.
.shstrtab This section holds section names. This section is of
type SHT_STRTAB. No attribute types are used.
.strtab This section holds strings, most commonly the strings
that represent the names associated with symbol table
entries. If the file has a loadable segment that
includes the symbol string table, the section’s
attributes will include the SHF_ALLOC bit. Otherwise
the bit will be off. This section is of type
SHT_STRTAB.
.symtab This section holds a symbol table. If the file has a
loadable segment that includes the symbol table, the
section’s attributes will include the SHF_ALLOC bit.
Otherwise the bit will be off. This section is of type
SHT_SYMTAB.
.text This section holds the “text”, or executable instruc‐
tions, of a program. This section is of type
SHT_PROGBITS. The attributes used are SHF_ALLOC and
SHF_EXECINSTR.
String table sections hold null-terminated character sequences, commonly
called strings. The object file uses these strings to represent symbol
and section names. One references a string as an index into the string
table section. The first byte, which is index zero, is defined to hold a
null 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;
Otherwise, the symbol table has no name.
st_value This member gives the value of the associated symbol.
st_size Many symbols have associated sizes. This member holds
zero if the symbol has no size or an unknown size.
st_info This member specifies the symbol’s type and binding
attributes:
STT_NOTYPE The symbol’s type is not defined.
STT_OBJECT The symbol is associated with a data object.
STT_FUNC The symbol is associated with a function or
other executable code.
STT_SECTION The symbol is associated with a section.
Symbol table entries of this type exist pri‐
marily for relocation and normally have
STB_LOCAL bindings.
STT_FILE By convention, the symbol’s name gives the
name of the source file associated with the
object file. A file symbol has STB_LOCAL
bindings, its section index is SHN_ABS, and
it precedes the other STB_LOCAL symbols of
the file, if it is present.
STT_LOPROC This value up to and including STT_HIPROC is
reserved for processor-specific semantics.
STT_HIPROC This value down to and including STT_LOPROC
is reserved for processor-specific seman‐
tics.
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 sym‐
bol.
STB_WEAK Weak symbols resemble global symbols, but
their definitions have lower precedence.
STB_LOPROC This value up to and including STB_HIPROC is
ELF32_ST_INFO(bind, type) or
ELF64_ST_INFO(bind, type)
convert a binding and a type into an st_info
value.
st_other This member currently holds zero and has no defined mean‐
ing.
st_shndx Every symbol table entry is “defined” in relation to some
section. This member holds the relevant section header
table index.
Relocation is the process of connecting symbolic references with symbolic
definitions. Relocatable files must have information that describes how
to modify their section contents, thus allowing executable and shared
object files to hold the right information for a process’ program image.
Relocation entries are these data.
Relocation structures that do not need an addend:
typedef struct {
Elf32_Addr r_offset;
uint32_t r_info;
} Elf32_Rel;
typedef struct {
Elf64_Addr r_offset;
uint64_t r_info;
} Elf64_Rel;
Relocation structures that need an addend:
typedef struct {
Elf32_Addr r_offset;
uint32_t r_info;
int32_t r_addend;
} Elf32_Rela;
typedef struct {
Elf64_Addr r_offset;
uint64_t r_info;
int64_t r_addend;
} Elf64_Rela;
r_offset This member gives the location at which to apply the
relocation action. For a relocatable file, the value is
the byte offset from the beginning of the section to the
storage unit affected by the relocation. For an exe‐
cutable 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
typedef struct {
Elf32_Sword d_tag;
union {
Elf32_Word d_val;
Elf32_Addr d_ptr;
} d_un;
} Elf32_Dyn;
extern Elf32_Dyn _DYNAMIC[];
typedef struct {
Elf64_Sxword d_tag;
union {
Elf64_Xword d_val;
Elf64_Addr d_ptr;
} d_un;
} Elf64_Dyn;
extern Elf64_Dyn _DYNAMIC[];
d_tag This member may have any of the following values:
DT_NULL Marks end of dynamic section
DT_NEEDED String table offset to name of a needed library
DT_PLTRELSZ Size in bytes of PLT relocs
DT_PLTGOT Address of PLT and/or GOT
DT_HASH Address of symbol hash table
DT_STRTAB Address of string table
DT_SYMTAB Address of symbol table
DT_RELA Address of Rela relocs table
DT_RELASZ Size in bytes of Rela table
DT_RELAENT Size in bytes of a Rela table entry
DT_STRSZ Size in bytes of string table
DT_SYMENT Size in bytes of a symbol table entry
DT_INIT Address of the initialization function
DT_FINI Address of the termination function
DT_SONAME String table offset to name of shared object
DT_RPATH String table offset to library search path
DT_TEXTREL Absence of this indicates no relocs should
apply to a non-writable segment
DT_JMPREL Address of reloc entries solely for the PLT
DT_BIND_NOW Instruct dynamic linker to process all relocs
before transferring control to the executable
DT_RUNPATH String table offset to library search path
DT_LOPROC Start of processor-specific semantics
DT_HIPROC End of processor-specific semantics
d_val This member represents integer values with various 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.
SEE ALSO
as(1), gdb(1), ld(1), objdump(1), execve(2), core(5)
Hewlett-Packard, Elf-64 Object File Format.
Santa Cruz Operation, System V Application Binary Interface.
Unix System Laboratories, "Object Files", Executable and Linking Format
(ELF).
HISTORY
OpenBSD ELF support first appeared in OpenBSD 1.2, although not all sup‐
ported platforms use it as the native binary file format. ELF in itself
first appeared in AT&T System V UNIX. The ELF format is an adopted stan‐
dard.
AUTHORS
The original version of this manual page was written by Jeroen Ruigrok
van der Werven 〈asmodai@FreeBSD.org〉 with inspiration from BSDi’s BSD/OS
elf manpage.
BSD July 31, 1999 BSD
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