elf
ELF(5) BSD File Formats Manual ELF(5)
NAME
elf - format of ELF executable binary 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 con-
tain 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 architec-
ture 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 encod-
ing 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.:
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 (embed-
ded) ABI.
EI_ABIVERSION
This 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 mean-
ings.
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 HPPA.
EM_SPARC32PLUS SPARC with enhanced instruction set.
EM_PPC PowerPC.
EM_SPARCV9 SPARC v9 64-bit.
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 pro-
gram 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 sec-
tion 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, miss-
ing, irrelevant, or otherwise meaning-
less section reference. For example,
a symbol “defined” relative to section
number SHN_UNDEF is an undefined sym-
bol.
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 reloca-
tion.
SHN_COMMON Symbols defined relative to this sec-
tion are common symbols, such as For-
tran 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 con-
tains 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 seg-
ment 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
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 pro-
gram header table appear in ascending order,
sorted on the p_vaddr member.
PT_DYNAMIC The array element specifies dynamic linking
information.
PT_INTERP The array element specifies the location and
size of a null-terminated path name to
invoke as an interpreter. This segment type
is meaningful only for executable files
(though it may occur for shared objects).
However it may not occur more than once in a
file. If it is present, it must precede any
loadable segment entry.
PT_NOTE The array element specifies the location and
size for auxiliary information.
PT_SHLIB This segment type is reserved but has
unspecified semantics. Programs that con-
tain 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 ta-
ble itself, both in the file and in the mem-
ory 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 pre-
cede any loadable segment entry.
PT_LOPROC Values greater than or equal to PT_HIPROC
are reserved for processor-specific seman-
tics.
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.
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 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
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 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;
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;
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 seman-
tics.
SHT_NULL This value marks the section header as inac-
tive. It does not have an associated sec-
tion. 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. Typi-
cally, SHT_SYMTAB provides symbols for link
editing, though it may also be used for
dynamic linking. As a complete symbol ta-
ble, it may contain many symbols unnecessary
for dynamic linking. An object file can
also contain a SHN_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 sec-
tions.
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
SHN_PROGBITS. Although this section con-
tains no bytes, the sh_offset member con-
tains the conceptual file offset.
SHT_REL This section holds relocation offsets with-
out 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 SHN_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 seman-
tics.
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 applica-
tion, 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 sec-
tions.
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 begin-
ning 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 non-zero 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 double-
word 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 initial-
izes the data with zeros when the program begins to run.
This section is of type SHT_NOBITS. The attribute types are
SHF_ALLOC and SHF_WRITE.
.comment This section holds version control information. This section
is of type SHT_PROGBITS. No attribute types are used.
.ctors This section holds initialized pointers to the C++ construc-
tor functions. This section is of type SHT_PROGBITS. The
attribute types are SHF_ALLOC and SHF_WRITE.
.data This section holds initialized data that contribute to the
program’s memory image. This section is of type
SHT_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 sec-
tion’s attributes will include the SHF_ALLOC bit. Whether
the SHF_WRITE bit is set is processor-specific. This section
is of type SHT_DYNAMIC. See the attributes above.
.dynstr This section holds strings needed for dynamic linking, most
commonly the strings that represent the names associated with
symbol table entries. This section is of type SHT_STRTAB.
The attribute type used is SHF_ALLOC.
.dynsym This section holds the dynamic linking symbol table. This
section is of type SHT_DYNSYM. The attribute used is
SHF_ALLOC.
.fini This section holds executable instructions that contribute to
the process termination code. When a program exits normally
the system arranges to execute the code in this section.
This section is of type SHT_PROGBITS. The attributes used
are SHF_ALLOC and SHF_EXECINSTR.
.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
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 emula-
tion tests when loading the file.
.plt This section holds the procedure linkage table. This section
is of type SHT_PROGBITS. The attributes are processor-spe-
cific.
.relNAME This section holds relocation information as described below.
If the file has a loadable segment that includes relocation,
the section’s attributes will include the SHF_ALLOC bit.
Otherwise the bit will be off. By convention, “NAME” is sup-
plied 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 sup-
plied 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 non-writable 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 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 load-
able segment that includes the symbol table, the section’s
attributes will include the SHF_ALLOC bit. Otherwise the bit
will be off. This section is of type SHT_SYMTAB.
.text This section holds the “text”, or executable instructions, of
a program. This section is of type SHT_PROGBITS. The
attributes used are SHF_ALLOC and SHF_EXECINSTR.
String 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 character. Similarly, a string table’s last byte is defined to
hold a null character, ensuring null termination for all strings.
An object file’s symbol table holds information needed to locate and
relocate a program’s symbolic definitions and references. A symbol ta-
ble index is a subscript into this array.
typedef struct {
uint32_t st_name;
Elf32_Addr st_value;
uint32_t st_size;
unsigned char st_info;
unsigned char st_other;
uint16_t st_shndx;
} Elf32_Sym;
typedef struct {
uint32_t st_name;
unsigned char st_info;
unsigned char st_other;
uint16_t st_shndx;
Elf64_Addr st_value;
uint64_t st_size;
} Elf64_Sym;
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 non-zero, 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 with-
out 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 ref-
erence 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.
ELF64_ST_TYPE(info) or ELF32_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 currently holds zero and has no defined meaning.
st_shndx Every symbol table entry is “defined” in relation to some sec-
tion. This member holds the relevant section header table
index.
Relocation is the process of connecting symbolic references with sym-
bolic definitions. Relocatable files must have information that
describes how to modify their section contents, thus allowing 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 reloca-
tion action. For a relocatable file, the value is the byte
offset from the beginning of the section to the storage unit
affected by the relocation. For an executable file or shared
object, the value is the virtual address of the storage unit
affected by the relocation.
r_info This member gives both the symbol table index with respect to
which the relocation must be made and the type of relocation
to apply. Relocation types are processor-specific. When the
text refers to a relocation entry’s relocation type or symbol
table index, it means the result of applying
ELF_[32|64]_R_TYPE or ELF[32|64]_R_SYM, respectively, to the
entry’s r_info member.
r_addend This member specifies a constant addend used to compute the
value to be stored into the relocatable field.
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
standard.
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
