elf — format of Executable and Linking Format (ELF) files
#include <elf.h>
The header file (elf.h) defines the format of
ELF executable binary files. Amongst these files are normal
executable files, relocatable object files, core files and
shared 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
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Note |
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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_identThis 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_MAG0The first byte of the magic number. It must be filled with
ELFMAG0. (0: 0x7f)EI_MAG1The second byte of the magic number. It must be filled with
ELFMAG1. (1: 'E')EI_MAG2The third byte of the magic number. It must be filled with
ELFMAG2. (2: 'L')EI_MAG3The fourth byte of the magic number. It must be filled with
ELFMAG3. (3: 'F')EI_CLASSThe fifth byte identifies the architecture for this binary:
ELFCLASSNONEThis class is invalid.
ELFCLASS32This defines the 32-bit architecture. It supports machines with files and virtual address spaces up to 4 Gigabytes.
ELFCLASS64This defines the 64-bit architecture.
EI_DATAThe sixth byte specifies the data encoding of the processor-specific data in the file. Currently these encodings are supported:
ELFDATANONEUnknown data format.
ELFDATA2LSBTwo's complement, little-endian.
ELFDATA2MSBTwo's complement, big-endian.
EI_VERSIONThe version number of the ELF specification:
EV_NONEInvalid version.
EV_CURRENTCurrent version.
EI_OSABIThis 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_NONESame as ELFOSABI_SYSV
ELFOSABI_SYSVUNIX System V ABI.
ELFOSABI_HPUXHP-UX ABI.
ELFOSABI_NETBSDNetBSD ABI.
ELFOSABI_LINUXLinux ABI.
ELFOSABI_SOLARISSolaris ABI.
ELFOSABI_IRIXIRIX ABI.
ELFOSABI_FREEBSDFreeBSD ABI.
ELFOSABI_TRU64TRU64 UNIX ABI.
ELFOSABI_ARMARM architecture ABI.
ELFOSABI_STANDALONEStand-alone (embedded) ABI.
EI_ABIVERSIONThis 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_PADStart 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_BRANDStart of architecture identification.
EI_NIDENTThe size of the e_ident array.
e_typeThis member of the structure identifies the object file type:
ET_NONEAn unknown type.
ET_RELA relocatable file.
ET_EXECAn executable file.
ET_DYNA shared object.
ET_COREA core file.
e_machineThis member specifies the required architecture for an individual file. E.g.:
EM_NONEAn unknown machine.
EM_M32AT&T WE 32100.
EM_SPARCSun Microsystems SPARC.
EM_386Intel 80386.
EM_68KMotorola 68000.
EM_88KMotorola 88000.
EM_860Intel 80860.
EM_MIPSEM_PARISCHP/PA.
EM_SPARC32PLUSSPARC with enhanced instruction set.
EM_PPCPowerPC.
EM_PPC64PowerPC 64-bit.
EM_S390IBM S/390
EM_ARMAdvanced RISC Machines
EM_SHRenesas SuperH
EM_SPARCV9SPARC v9 64-bit.
EM_IA_64Intel Itanium
EM_X86_64AMD x86-64
EM_VAXDEC Vax.
e_versionThis member identifies the file version:
EV_NONEInvalid version.
EV_CURRENTCurrent version.
e_entryThis 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_phoffThis member holds the program header table's file offset in bytes. If the file has no program header table, this member holds zero.
e_shoffThis member holds the section header table's file offset in bytes. If the file has no section header table this member holds zero.
e_flagsThis member holds processor-specific flags associated with the file. Flag names take the form EF_`machine_flag'. Currently no flags have been defined.
e_ehsizeThis member holds the ELF header's size in bytes.
e_phentsizeThis member holds the size in bytes of one entry in the file's program header table; all entries are the same size.
e_phnumThis 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_shentsizeThis 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_shnumThis 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_shstrndxThis 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.
SHN_UNDEFThis value marks an undefined, missing, irrelevant, or otherwise meaningless section reference. For example, a symbol “defined” relative to section number
SHN_UNDEFis an undefined symbol.SHN_LORESERVEThis value specifies the lower bound of the range of reserved indices.
SHN_LOPROCValues greater than or equal to
SHN_HIPROCare reserved for processor-specific semantics.SHN_HIPROCValues less than or equal to
SHN_LOPROCare reserved for processor-specific semantics.SHN_ABSThis value specifies absolute values for the corresponding reference. For example, symbols defined relative to section number
SHN_ABShave absolute values and are not affected by relocation.SHN_COMMONSymbols defined relative to this section are common symbols, such as Fortran COMMON or unallocated C external variables.
SHN_HIRESERVEThis value specifies the upper bound of the range of reserved indices between
SHN_LORESERVEandSHN_HIRESERVE, inclusive; the values do not reference the section header table. That is, the section header table doesnotcontain 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_typeThis member of the Phdr struct tells what kind of segment this array element describes or how to interpret the array element's information.
PT_NULLThe array element is unused and the other members' values are undefined. This lets the program header have ignored entries.
PT_LOADThe array element specifies a loadable segment, described by
p_fileszandp_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 thep_vaddrmember.PT_DYNAMICThe array element specifies dynamic linking information.
PT_INTERPThe 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_NOTEThe array element specifies the location and size for auxiliary information.
PT_SHLIBThis segment type is reserved but has unspecified semantics. Programs that contain an array element of this type do not conform to the ABI.
PT_PHDRThe 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 only occur 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_LOPROCValues greater than or equal to
PT_HIPROCare reserved for processor-specific semantics.PT_HIPROCValues less than or equal to
PT_LOPROCare reserved for processor-specific semantics.
p_offsetThis member holds the offset from the beginning of the file at which the first byte of the segment resides.
p_vaddrThis member holds the virtual address at which the first byte of the segment resides in memory.
p_paddrOn 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_fileszThis member holds the number of bytes in the file image of the segment. It may be zero.
p_memszThis member holds the number of bytes in the memory image of the segment. It may be zero.
p_flagsThis member holds flags relevant to the segment:
PF_XAn executable segment.
PF_WA writable segment.
PF_RA 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_alignThis 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. An object file does not have sections for these special indices:
SHN_UNDEFThis value marks an undefined, missing, irrelevant or otherwise meaningless section reference.
SHN_LORESERVEThis value specifies the lower bound of the range of reserved indices.
SHN_LOPROCValues greater than or equal to SHN_HIPROC are reserved for
processor-specific semantics.
SHN_HIPROCValues less than or equal to SHN_LOPROC are reserved for
processor-specific semantics.
SHN_ABSThis 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_COMMONSymbols defined relative to this section are common symbols, such as FORTRAN COMMON or unallocated C external variables.
SHN_HIRESERVEThis 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_nameThis 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_typeThis member categorizes the section's contents and semantics.
SHT_NULLThis value marks the section header as inactive. It does not have an associated section. Other members of the section header have undefined values.
SHT_PROGBITSThis section holds information defined by the program, whose format and meaning are determined solely by the program.
SHT_SYMTABThis section holds a symbol table. Typically,
SHT_SYMTABprovides 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 aSHT_DYNSYMsection.SHT_STRTABThis section holds a string table. An object file may have multiple string table sections.
SHT_RELAThis section holds relocation entries with explicit addends, such as type
Elf32_Relafor the 32-bit class of object files. An object may have multiple relocation sections.SHT_HASHThis 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_DYNAMICThis section holds information for dynamic linking. An object file may have only one dynamic section.
SHT_NOTEThis section holds information that marks the file in some way.
SHT_NOBITSA section of this type occupies no space in the file but otherwise resembles
SHT_PROGBITS. Although this section contains no bytes, thesh_offsetmember contains the conceptual file offset.SHT_RELThis section holds relocation offsets without explicit addends, such as type
Elf32_Relfor the 32-bit class of object files. An object file may have multiple relocation sections.SHT_SHLIBThis section is reserved but has unspecified semantics.
SHT_DYNSYMThis section holds a minimal set of dynamic linking symbols. An object file can also contain a
SHT_SYMTABsection.SHT_LOPROCThis value up to and including
SHT_HIPROCis reserved for processor-specific semantics.SHT_HIPROCThis value down to and including
SHT_LOPROCis reserved for processor-specific semantics.SHT_LOUSERThis value specifies the lower bound of the range of indices reserved for application programs.
SHT_HIUSERThis value specifies the upper bound of the range of indices reserved for application programs. Section types between
SHT_LOUSERandSHT_HIUSERmay be used by the application, without conflicting with current or future system-defined section types.
sh_flagsSections 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_WRITEThis section contains data that should be writable during process execution.
SHF_ALLOCThis 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_EXECINSTRThis section contains executable machine instructions.
SHF_MASKPROCAll bits included in this mask are reserved for processor-specific semantics.
sh_addrIf 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_offsetThis 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_sizeThis 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 non-zero size,
but it occupies no space in the file.
sh_linkThis member holds a section header table index link, whose interpretation depends on the section type.
sh_infoThis member holds extra information, whose interpretation depends on the section type.
sh_addralignSome 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_entsizeSome 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:
.bssThis 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.
.commentThis section holds version control information. This
section is of type SHT_PROGBITS. No attribute types are
used.
.ctorsThis 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.
.dataThis 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.
.data1This 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.
.debugThis section holds information for symbolic
debugging. The contents are unspecified. This section
is of type SHT_PROGBITS.
No attribute types are used.
.dtorsThis 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.
.dynamicThis 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.
.dynstrThis 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.
.dynsymThis section holds the dynamic linking symbol table.
This section is of type SHT_DYNSYM. The attribute used is
SHF_ALLOC.
.finiThis 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.
.gotThis section holds the global offset table. This
section is of type SHT_PROGBITS. The attributes are
processor-specific.
.hashThis section holds a symbol hash table. This section
is of type SHT_HASH. The
attribute used is SHF_ALLOC.
.initThis 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.
.interpThis 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.
.lineThis 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.
.noteThis 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
section to identify themselves, for the kernel to
bypass any compatibility ELF binary emulation tests
when loading the file.
.pltThis section holds the procedure linkage table. This
section is of type SHT_PROGBITS. The attributes are
processor-specific.
.relNAMEThis 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.
.relaNAMEThis 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.
.rodataThis section holds read-only data that typically
contributes to a non-writable segment in the process
image. This section is of type SHT_PROGBITS. The attribute used is
SHF_ALLOC.
.rodata1This section holds read-only data that typically
contributes to a non-writable segment in the process
image. This section is of type SHT_PROGBITS. The attribute used is
SHF_ALLOC.
.shstrtabThis section holds section names. This section is of
type SHT_STRTAB. No
attribute types are used.
.strtabThis 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.
.symtabThis 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.
.textThis 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_nameThis member holds an index into the object file's symbol string table, which holds character representations of the symbol names. If the value is non-zero, it represents a string table index that gives the symbol name. Otherwise, the symbol table has no name.
st_valueThis member gives the value of the associated symbol.
st_sizeMany symbols have associated sizes. This member holds zero if the symbol has no size or an unknown size.
st_infoThis member specifies the symbol's type and binding attributes:
STT_NOTYPEThe symbol's type is not defined.
STT_OBJECTThe symbol is associated with a data object.
STT_FUNCThe symbol is associated with a function or other executable code.
STT_SECTIONThe symbol is associated with a section. Symbol table entries of this type exist primarily for relocation and normally have
STB_LOCALbindings.STT_FILEBy convention, the symbol's name gives the name of the source file associated with the object file. A file symbol has
STB_LOCALbindings, its section index isSHN_ABS, and it precedes the otherSTB_LOCALsymbols of the file, if it is present.STT_LOPROCThis value up to and including
STT_HIPROCis reserved for processor-specific semantics.STT_HIPROCThis value down to and including
STT_LOPROCis reserved for processor-specific semantics.
STB_LOCALLocal 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_GLOBALGlobal 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_WEAKWeak symbols resemble global symbols, but their definitions have lower precedence.
STB_LOPROCThis value up to and including
STB_HIPROCis reserved for processor-specific semantics.STB_HIPROCThis value down to and including
STB_LOPROCis reserved for processor-specific semantics.There are macros for packing and unpacking the binding and type fields:
ELF32_ST_BIND(info) orELF64_ST_BIND(info) extract a binding from an st_info value.
ELF32_ST_TYPE(info) orELF64_ST_TYPE(info)extract a type from an st_info value.
ELF32_ST_INFO(bind, type) orELF64_ST_INFO(bind, type)convert a binding and a type into an st_info value.
st_otherThis member currently holds zero and has no defined meaning.
st_shndxEvery 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_offsetThis 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_infoThis 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_addendThis 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_tagThis member may have any of the following values:
DT_NULLMarks end of dynamic section
DT_NEEDEDString table offset to name of a needed library
DT_PLTRELSZSize in bytes of PLT relocs
DT_PLTGOTAddress of PLT and/or GOT
DT_HASHAddress of symbol hash table
DT_STRTABAddress of string table
DT_SYMTABAddress of symbol table
DT_RELAAddress of Rela relocs table
DT_RELASZSize in bytes of Rela table
DT_RELAENTSize in bytes of a Rela table entry
DT_STRSZSize in bytes of string table
DT_SYMENTSize in bytes of a symbol table entry
DT_INITAddress of the initialization function
DT_FINIAddress of the termination function
DT_SONAMEString table offset to name of shared object
DT_RPATHString table offset to library search path (deprecated)
DT_SYMBOLICAlert linker to search this shared object before the executable for symbols
DT_RELAddress of Rel relocs table
DT_RELSZSize in bytes of Rel table
DT_RELENTSize in bytes of a Rel table entry
DT_PLTRELType of reloc the PLT refers (Rela or Rel)
DT_DEBUGUndefined use for debugging
DT_TEXTRELAbsence of this indicates no relocs should apply to a non-writable segment
DT_JMPRELAddress of reloc entries solely for the PLT
DT_BIND_NOWInstruct dynamic linker to process all relocs before transferring control to the executable
DT_RUNPATHString table offset to library search path
DT_LOPROCStart of processor-specific semantics
DT_HIPROCEnd of processor-specific semantics
d_valThis member represents integer values with various interpretations.
d_ptrThis 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.
_DYNAMICArray containing all the dynamic structures in the .dynamic section. This is automatically populated by the linker.
OpenBSD ELF
support first appeared in OpenBSD 1.2, although not all
supported platforms use it as the native binary file format.
ELF in itself first appeared in Version V AT&T UNIX™. The ELF
format is an adopted standard.
as(1), gdb(1), ld(1), objdump(1), execve(2), core(5)
, Elf-64 Object File Format.
, System V Application Binary Interface.
, “Object Files”, Executable and Linking Format (ELF).
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