Ubuntu Manpage: elf - format of Executable and Linking Format (ELF) files

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 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;
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 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. E.g.:

ELFOSABI_NONE Same as ELFOSABI_SYSV
ELFOSABI_SYSV UNIX System V ABI.
ELFOSABI_HPUX HP-UX ABI.
ELFOSABI_NETBSD NetBSD ABI.
ELFOSABI_LINUX Linux ABI.
ELFOSABI_SOLARIS Solaris ABI.
ELFOSABI_IRIX IRIX ABI.
ELFOSABI_FREEBSD FreeBSD ABI.
ELFOSABI_TRU64 TRU64 UNIX ABI.
ELFOSABI_ARM ARM architecture ABI.
ELFOSABI_STANDALONE Stand-alone (embedded) ABI.

EI_ABIVERSION
The ninth byte identifies the version of the ABI to which the object is targeted.
This field is used to distinguish among incompatible versions of an ABI. The
interpretation of this version number is dependent on the ABI identified by the
EI_OSABI field. Applications conforming to this specification use the value 0.

EI_PAD Start of padding. These bytes are reserved and set to zero. Programs which read
them should ignore them. The value for EI_PAD will change in the future if
currently unused bytes are given meanings.

EI_NIDENT The size of the e_ident array.

e_type This member of the structure identifies the object file type:

ET_NONE An unknown type.
ET_REL A relocatable file.
ET_EXEC An executable file.
ET_DYN A shared object.
ET_CORE A core file.

e_machine This member specifies the required architecture for an individual file. 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_version This member identifies the file version:

EV_NONE Invalid version.
EV_CURRENT Current version.

e_entry This member gives the virtual address to which the system first transfers control, thus
starting the process. If the file has no associated entry point, this member holds zero.

e_phoff This member holds the program header table's file offset in bytes. If the file has no
program header table, this member holds zero.

e_shoff This member holds the section header table's file offset in bytes. If the file has no
section header table this member holds zero.

e_flags This member holds processor-specific flags associated with the file. Flag names take the
form EF_`machine_flag'. Currently no flags have been defined.

e_ehsize This member holds the ELF header's size in bytes.

e_phentsize This member holds the size in bytes of one entry in the file's program header table; all
entries are the same size.

e_phnum This member holds the number of entries in the program header table. Thus the product of
e_phentsize and e_phnum gives the table's size in bytes. If a file has no program header,
e_phnum holds the value zero.

If the number of entries in the program header table is larger than or equal to PN_XNUM
(0xffff), this member holds PN_XNUM (0xffff) and the real number of entries in the program
header table is held in the sh_info member of the initial entry in section header table.
Otherwise, the sh_info member of the initial entry contains the value zero.

PN_XNUM This is defined as 0xffff, the largest number e_phnum can have, specifying where the
actual number of program headers is assigned.

e_shentsize This member holds a sections header's size in bytes. A section header is one entry in the
section header table; all entries are the same size.

e_shnum This member holds the number of entries in the section header table. Thus the product of
e_shentsize and e_shnum gives the section header table's size in bytes. If a file has no
section header table, e_shnum holds the value of zero.

If the number of entries in the section header table is larger than or equal to SHN_LORESERVE
(0xff00), e_shnum holds the value zero and the real number of entries in the section header
table is held in the sh_size member of the initial entry in section header table. Otherwise,
the sh_size member of the initial entry in the section header table holds the value zero.

e_shstrndx This member holds the section header table index of the entry associated with the section
name string table. If the file has no section name string table, this member holds the value
SHN_UNDEF.

If the index of section name string table section is larger than or equal to SHN_LORESERVE
(0xff00), this member holds SHN_XINDEX (0xffff) and the real index of the section name string
table section is held in the sh_link member of the initial entry in section header table.
Otherwise, the sh_link member of the initial entry in section header table contains the value
zero.

SHN_UNDEF This value marks an undefined, missing, irrelevant, or otherwise meaningless
section reference. For example, a symbol "defined" relative to section number
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_ABS This value specifies absolute values for the corresponding reference. For
example, symbols defined relative to section number SHN_ABS have absolute
values and are 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 between
SHN_LORESERVE and SHN_HIRESERVE, inclusive; 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 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 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 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 and size for auxiliary information.

PT_SHLIB This segment type is reserved but has unspecified semantics. Programs that
contain an array element of this type do not conform to the ABI.

PT_PHDR The array element, if present, specifies the location and size of the program
header table itself, both in the file and in the memory image of the program.
This segment type may not occur more than once in a file. Moreover, it may occur
only if the program header table is part of the memory image of the program. If
it is present, it must precede any loadable segment entry.

PT_LOPROC 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.

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_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.

A file's section header table lets one locate all the file's sections. The section header table is an
array of Elf32_Shdr or Elf64_Shdr structures. The ELF header's e_shoff member gives the byte offset from
the beginning of the file to the section header table. e_shnum holds the number of entries the section
header table contains. e_shentsize holds the size in bytes of each entry.

A section header table index is a subscript into this array. Some section header table indices are
reserved: the initial entry and the indices between SHN_LORESERVE and SHN_HIRESERVE. The initial entry
is used in ELF extensions for e_phnum, e_shnum and e_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 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 corresponding reference. For
example, a symbol defined relative to section number SHN_ABS has an absolute value
and is not affected by relocation.

SHN_COMMON Symbols defined relative to this section are common symbols, such as FORTRAN COMMON
or unallocated C external variables.

SHN_HIRESERVE This value specifies the upper bound of the range of reserved indices. The system
reserves indices between SHN_LORESERVE and SHN_HIRESERVE, inclusive. The section
header table does not contain entries for the reserved indices.

The section header has the following structure:

typedef struct {
uint32_t sh_name;
uint32_t sh_type;
uint32_t sh_flags;
Elf32_Addr sh_addr;
Elf32_Off sh_offset;
uint32_t sh_size;
uint32_t sh_link;
uint32_t sh_info;
uint32_t sh_addralign;
uint32_t sh_entsize;
} Elf32_Shdr;

typedef struct {
uint32_t sh_name;
uint32_t sh_type;
uint64_t sh_flags;
Elf64_Addr sh_addr;
Elf64_Off sh_offset;
uint64_t sh_size;
uint32_t sh_link;
uint32_t sh_info;
uint64_t sh_addralign;
uint64_t sh_entsize;
} Elf64_Shdr;

No real differences exist between the 32-bit and 64-bit section headers.

sh_name This member specifies the name of the section. Its value is an index into the section header
string table section, giving the location of a null-terminated string.

sh_type This member categorizes the section's contents and semantics.

SHT_NULL This value marks the section header as inactive. It does not have an associated
section. Other members of the section header have undefined values.

SHT_PROGBITS This section holds information defined by the program, whose format and meaning
are determined solely by the program.

SHT_SYMTAB This section holds a symbol table. Typically, SHT_SYMTAB provides symbols for
link editing, though it may also be used for dynamic linking. As a complete
symbol table, it may contain many symbols unnecessary for dynamic linking. An
object file can also contain a SHT_DYNSYM section.

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 information that marks the file in some way.

SHT_NOBITS A section of this type occupies no space in the file but otherwise resembles
SHT_PROGBITS. Although this section contains no bytes, the sh_offset member
contains the conceptual file offset.

SHT_REL This section holds relocation offsets without explicit addends, such as type
Elf32_Rel for the 32-bit class of object files. An object file may have
multiple relocation sections.

SHT_SHLIB This section is reserved but has unspecified semantics.

SHT_DYNSYM This section holds a minimal set of dynamic linking symbols. An object file can
also contain a SHT_SYMTAB section.

SHT_LOPROC 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 system-
defined section types.

sh_flags Sections support one-bit flags that describe miscellaneous attributes. If a flag bit is set in
sh_flags, the attribute is "on" for the section. Otherwise, the attribute is "off" or does not
apply. Undefined attributes are set to zero.

SHF_WRITE This section contains data that should be writable during process execution.

SHF_ALLOC This section occupies memory during process execution. Some control sections do
not reside in the memory image of an object file. This attribute is off for
those sections.

SHF_EXECINSTR This section contains executable machine instructions.

SHF_MASKPROC All bits included in this mask are reserved for processor-specific semantics.

sh_addr If this section appears in the memory image of a process, this member holds the address at
which the section's first byte should reside. Otherwise, the member contains zero.

sh_offset This member's value holds the byte offset from the beginning of the file to the first byte in
the section. One section type, SHT_NOBITS, occupies no space in the file, and its sh_offset
member locates the conceptual placement in the file.

sh_size This member holds the section's size in bytes. Unless the section type is SHT_NOBITS, the
section occupies sh_size bytes in the file. A section of type SHT_NOBITS may have a nonzero
size, but it occupies no space in the file.

sh_link This member holds a section header table index link, whose interpretation depends on the
section type.

sh_info This member holds extra information, whose interpretation depends on the section type.

sh_addralign
Some sections have address alignment constraints. If a section holds a doubleword, the system
must ensure doubleword alignment for the entire section. That is, the value of sh_addr must be
congruent to zero, modulo the value of sh_addralign. Only zero and positive integral powers of
two are allowed. Values of zero or one mean the section has no alignment constraints.

sh_entsize
Some sections hold a table of fixed-sized entries, such as a symbol table. For such a section,
this member gives the size in bytes for each entry. This member contains zero if the section
does not hold a table of fixed-size entries.

Various sections hold program and control information:

.bss This section holds uninitialized data that contributes to the program's memory image. By
definition, the system initializes the data with zeros when the program begins to run. This
section is of type SHT_NOBITS. The attribute types are SHF_ALLOC and SHF_WRITE.

.comment This section holds version control information. This section is of type SHT_PROGBITS. No
attribute types are used.

.ctors This section holds initialized pointers to the C++ constructor functions. This section is of
type SHT_PROGBITS. The attribute types are SHF_ALLOC and SHF_WRITE.

.data This section holds initialized data that contribute to the program's memory image. This
section is of type SHT_PROGBITS. The attribute types are SHF_ALLOC and SHF_WRITE.

.data1 This section holds initialized data that contribute to the program's memory image. This
section is of type SHT_PROGBITS. The attribute types are SHF_ALLOC and SHF_WRITE.

.debug This section holds information for symbolic debugging. The contents are unspecified. This
section is of type SHT_PROGBITS. No attribute types are used.

.dtors This section holds initialized pointers to the C++ destructor functions. This section is of
type SHT_PROGBITS. The attribute types are SHF_ALLOC and SHF_WRITE.

.dynamic This section holds dynamic linking information. The section's attributes will include the
SHF_ALLOC bit. Whether the SHF_WRITE bit is set is processor-specific. This section is of
type SHT_DYNAMIC. See the attributes above.

.dynstr This section holds strings needed for dynamic linking, most commonly the strings that represent
the names associated with symbol table entries. This section is of type SHT_STRTAB. The
attribute type used is SHF_ALLOC.

.dynsym This section holds the dynamic linking symbol table. This section is of type SHT_DYNSYM. The
attribute used is SHF_ALLOC.

.fini This section holds executable instructions that contribute to the process termination code.
When a program exits normally the system arranges to execute the code in this section. This
section is of type SHT_PROGBITS. The attributes used are SHF_ALLOC and SHF_EXECINSTR.

.gnu.version
This section holds the version symbol table, an array of ElfN_Half elements. This section is
of type SHT_GNU_versym. The attribute type used is SHF_ALLOC.

.gnu.version_d
This section holds the version symbol definitions, a table of ElfN_Verdef structures. This
section is of type SHT_GNU_verdef. The attribute type used is SHF_ALLOC.

.gnu.version_r
This section holds the version symbol needed elements, a table of ElfN_Verneed structures.
This section is of type SHT_GNU_versym. The attribute type used is SHF_ALLOC.

.got This section holds the global offset table. This section is of type SHT_PROGBITS. The
attributes are processor specific.

.hash This section holds a symbol hash table. This section is of type SHT_HASH. The attribute used
is SHF_ALLOC.

.init This section holds executable instructions that contribute to the process initialization code.
When a program starts to run the system arranges to execute the code in this section before
calling the main program entry point. This section is of type SHT_PROGBITS. The attributes
used are SHF_ALLOC and SHF_EXECINSTR.

.interp This section holds the pathname of a program interpreter. If the file has a loadable segment
that includes the section, the section's attributes will include the SHF_ALLOC bit. Otherwise,
that bit will be off. This section is of type SHT_PROGBITS.

.line This section holds line number information for symbolic debugging, which describes the
correspondence between the program source and the machine code. The contents are unspecified.
This section is of type SHT_PROGBITS. No attribute types are used.

.note This section holds information in the "Note Section" format. This section is of type SHT_NOTE.
No attribute types are used. OpenBSD native executables usually contain a .note.openbsd.ident
section to identify themselves, for the kernel to bypass any compatibility ELF binary emulation
tests when loading the file.

.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.

.plt This section holds the procedure linkage table. This section is of type SHT_PROGBITS. The
attributes are processor specific.

.relNAME This section holds relocation information as described below. If the file has a loadable
segment that includes relocation, the section's attributes will include the SHF_ALLOC bit.
Otherwise the bit will be off. By convention, "NAME" is supplied by the section to which the
relocations apply. Thus a relocation section for .text normally would have the name .rel.text.
This section is of type SHT_REL.

.relaNAME This section holds relocation information as described below. If the file has a loadable
segment that includes relocation, the section's attributes will include the SHF_ALLOC bit.
Otherwise the bit will be off. By convention, "NAME" is supplied by the section to which the
relocations apply. Thus a relocation section for .text normally would have the name
.rela.text. This section is of type SHT_RELA.

.rodata This section holds read-only data that typically contributes to a nonwritable segment in the
process image. This section is of type SHT_PROGBITS. The attribute used is SHF_ALLOC.

.rodata1 This section holds read-only data that typically contributes to a nonwritable segment in the
process image. This section is of type SHT_PROGBITS. The attribute used is SHF_ALLOC.

.shstrtab This section holds section names. This section is of type SHT_STRTAB. No attribute types are
used.

.strtab This section holds strings, most commonly the strings that represent the names associated with
symbol table entries. If the file has a loadable segment that includes the symbol string
table, the section's attributes will include the SHF_ALLOC bit. Otherwise the bit will be off.
This section is of type SHT_STRTAB.

.symtab This section holds a symbol table. If the file has a loadable segment that includes the symbol
table, the section's attributes will include the SHF_ALLOC bit. Otherwise the bit will be off.
This section is of type SHT_SYMTAB.

.text This section holds the "text", or executable instructions, of a program. This section is of
type SHT_PROGBITS. The attributes used are SHF_ALLOC and SHF_EXECINSTR.

String 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 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
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 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
semantics.

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

STB_GLOBAL Global symbols are visible to all object files being combined. One file's
definition of a global symbol will satisfy another file's undefined reference to
the same symbol.

STB_WEAK Weak symbols resemble global symbols, but their definitions have lower precedence.

STB_LOPROC This value up to and including STB_HIPROC is reserved for processor-specific
semantics.

STB_HIPROC This value down to and including STB_LOPROC is reserved for processor-specific
semantics.

There are macros for packing and unpacking the binding and type fields:

ELF32_ST_BIND(info) or ELF64_ST_BIND(info) extract a binding from an st_info value.

ELF32_ST_TYPE(info) or ELF64_ST_TYPE(info)
extract a type from an st_info value.

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 defines the symbol visibility.

STV_DEFAULT Default symbol visibility rules.
STV_INTERNAL Processor-specific hidden class.
STV_HIDDEN Symbol is unavailable in other modules.
STV_PROTECTED Not preemptible, not exported.

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 is the process of connecting symbolic references with symbolic definitions. Relocatable files
must have information that describes how to modify their section contents, thus allowing executable and
shared object files to hold the right information for a process's program image. Relocation entries are
these data.

Relocation structures that do not need an addend:

typedef struct {
Elf32_Addr r_offset;
uint32_t r_info;
} Elf32_Rel;

typedef struct {
Elf64_Addr r_offset;
uint64_t r_info;
} Elf64_Rel;

Relocation structures that need an addend:

typedef struct {
Elf32_Addr r_offset;
uint32_t r_info;
int32_t r_addend;
} Elf32_Rela;

typedef struct {
Elf64_Addr r_offset;
uint64_t r_info;
int64_t r_addend;
} Elf64_Rela;

r_offset This member gives the location at which to apply the relocation action. For a relocatable
file, the value is the byte offset from the beginning of the section to the storage unit
affected by the relocation. For an executable file or shared object, the value is the
virtual address of the storage unit affected by the relocation.

r_info This member gives both the symbol table index with respect to which the relocation must be
made and the type of relocation to apply. Relocation types are processor specific. When the
text refers to a relocation entry's relocation type or symbol table index, it means the
result of applying ELF[32|64]_R_TYPE or ELF[32|64]_R_SYM, respectively, to the entry's r_info
member.

r_addend This member specifies a constant addend used to compute the value to be stored into the
relocatable field.

The .dynamic section contains a series of structures that hold relevant dynamic linking information. The
d_tag member controls the interpretation of d_un.

typedef struct {
Elf32_Sword d_tag;
union {
Elf32_Word d_val;
Elf32_Addr d_ptr;
} d_un;
} Elf32_Dyn;
extern Elf32_Dyn _DYNAMIC[];

typedef struct {
Elf64_Sxword d_tag;
union {
Elf64_Xword d_val;
Elf64_Addr d_ptr;
} d_un;
} Elf64_Dyn;
extern Elf64_Dyn _DYNAMIC[];

d_tag This member may have any of the following values:

DT_NULL Marks end of dynamic section

DT_NEEDED String table offset to name of a needed library

DT_PLTRELSZ Size in bytes of PLT 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 (deprecated)

DT_SYMBOLIC Alert linker to search this shared object before the executable for symbols

DT_REL Address of Rel relocs table

DT_RELSZ Size in bytes of Rel table

DT_RELENT Size in bytes of a Rel table entry

DT_PLTREL Type of reloc the PLT refers (Rela or Rel)

DT_DEBUG Undefined use for debugging

DT_TEXTREL Absence of this indicates no relocs should apply to a nonwritable 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 interpretations.

d_ptr This member represents program virtual addresses. When interpreting these addresses, the
actual address should be computed based on the original file value and memory base address.
Files do not contain relocation entries to fixup these addresses.

_DYNAMIC Array containing all the dynamic structures in the .dynamic section. This is automatically
populated by the linker.

NOTES

       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 information, look under SEE ALSO.

COLOPHON

       This  page  is  part  of  release 3.54 of the Linux man-pages project.  A description of the project, and
       information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.

Linux                                              2013-04-17                                             ELF(5)