path: root/man5/elf.5

.\"	$OpenBSD: elf.5,v 1.12 2003/10/27 20:23:58 jmc Exp $
.\"Copyright (c) 1999 Jeroen Ruigrok van der Werven
.\"All rights reserved.
.\"
.\"Redistribution and use in source and binary forms, with or without
.\"modification, are permitted provided that the following conditions
.\"are met:
.\"1. Redistributions of source code must retain the above copyright
.\"   notice, this list of conditions and the following disclaimer.
.\"2. Redistributions in binary form must reproduce the above copyright
.\"   notice, this list of conditions and the following disclaimer in the
.\"   documentation and/or other materials provided with the distribution.
.\"
.\"THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
.\"ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
.\"IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
.\"ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
.\"FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
.\"DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
.\"OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
.\"HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
.\"LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
.\"OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
.\"SUCH DAMAGE.
.\"
.\"	$FreeBSD: src/share/man/man5/elf.5,v 1.21 2001/10/01 16:09:23 ru Exp $
.\"
.\" Slightly adapted - aeb, 2004-01-01
.\" 2005-07-15, Mike Frysinger <vapier@gentoo.org>, various fixes
.\"
.Dd July 31, 1999
.Dt ELF 5
.Os
.Sh NAME
.Nm elf
.Nd format of Executable and Linking Format (ELF) files
.Sh SYNOPSIS
.\" .Fd #include <elf_abi.h>
.Fd #include <elf.h>
.Sh DESCRIPTION
The header file
.\" .Aq Pa elf_abi.h
.Aq Pa 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.
.Pp
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.
.Pp
.\" Applications which wish to process ELF binary files for their native
.\" architecture only should include
.\" .Aq Pa elf_abi.h
.\" in their source code.
.\" These applications should need to refer to
.\" all the types and structures by their generic names
.\" .Dq Elf_xxx
.\" and to the macros by
.\" .Dq ELF_xxx .
.\" Applications written this way can be compiled on any architecture,
.\" regardless of whether the host is 32-bit or 64-bit.
.\" .Pp
.\" Should an application need to process ELF files of an unknown
.\" architecture, then the application needs to explicitly use either
.\" .Dq Elf32_xxx
.\" or
.\" .Dq Elf64_xxx
.\" type and structure names.
.\" Likewise, the macros need to be identified by
.\" .Dq ELF32_xxx
.\" or
.\" .Dq ELF64_xxx .
.\" .Pp
This header file describes the above mentioned headers as C structures
and also includes structures for dynamic sections, relocation sections and
symbol tables.
.Pp
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):
.Bd -literal -offset indent
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
.\" Elf32_Size  Unsigned object size
.Ed
.Pp
(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.)
.Pp
All data structures that the file format defines follow the
.Dq 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.
.Pp
The ELF header is described by the type Elf32_Ehdr or Elf64_Ehdr:
.Bd -literal -offset indent
#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;
.Ed
.Pp
The fields have the following meanings:
.Bl -tag -width "e_phentsize" -offset indent
.It Dv 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
.Sy EI_
and may contain values which start with the prefix
.Sy ELF .
The following macros are defined:
.Bl -tag -width "EI_VERSION" \" EI_ABIVERSION
.It Dv EI_MAG0
The first byte of the magic number.
It must be filled with
.Sy ELFMAG0 .
(0: 0x7f)
.It Dv EI_MAG1
The second byte of the magic number.
It must be filled with
.Sy ELFMAG1 .
(1: 'E')
.It Dv EI_MAG2
The third byte of the magic number.
It must be filled with
.Sy ELFMAG2 .
(2: 'L')
.It Dv EI_MAG3
The fourth byte of the magic number.
It must be filled with
.Sy ELFMAG3 .
(3: 'F')
.It Dv EI_CLASS
The fifth byte identifies the architecture for this binary:
.Pp
.Bl -tag -width "ELFCLASSNONE" -compact
.It Dv ELFCLASSNONE
This class is invalid.
.It Dv ELFCLASS32
This defines the 32-bit architecture.
It supports machines with files
and virtual address spaces up to 4 Gigabytes.
.It Dv ELFCLASS64
This defines the 64-bit architecture.
.El
.It Dv EI_DATA
The sixth byte specifies the data encoding of the processor-specific
data in the file.
Currently these encodings are supported:
.Pp
.Bl -tag -width "ELFDATA2LSB" -compact
.It Dv ELFDATANONE
Unknown data format.
.It Dv ELFDATA2LSB
Two's complement, little-endian.
.It Dv ELFDATA2MSB
Two's complement, big-endian.
.El
.It Dv EI_VERSION
The version number of the ELF specification:
.Pp
.Bl -tag -width "EV_CURRENT" -compact
.It Dv EV_NONE
Invalid version.
.It Dv EV_CURRENT
Current version.
.El
.It Dv 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 specific meanings;
the interpretation of those fields is determined by the value of this byte.
E.g.:
.Pp
.Bl -tag -width "ELFOSABI_STANDALONE" -compact
.It Dv ELFOSABI_NONE
Same as ELFOSABI_SYSV
.\" 0
.It Dv ELFOSABI_SYSV
UNIX System V ABI.
.\" 0
.\" synonym: ELFOSABI_NONE
.It Dv ELFOSABI_HPUX
HP-UX ABI.
.\" 1
.It Dv ELFOSABI_NETBSD
NetBSD ABI.
.\" 2
.It Dv ELFOSABI_LINUX
Linux ABI.
.\" 3
.\" .It Dv ELFOSABI_HURD
.\" Hurd ABI.
.\" 4
.\" .It Dv ELFOSABI_86OPEN
.\" 86Open Common IA32 ABI.
.\" 5
.It Dv ELFOSABI_SOLARIS
Solaris ABI.
.\" 6
.\" .It Dv ELFOSABI_MONTEREY
.\" Monterey project ABI. Now replaced by
.\" ELFOSABI_AIX
.\" 7
.It Dv ELFOSABI_IRIX
IRIX ABI.
.\" 8
.It Dv ELFOSABI_FREEBSD
FreeBSD ABI.
.\" 9
.It Dv ELFOSABI_TRU64
TRU64 UNIX ABI.
.\" 10
.\" ELFOSABI_MODESTO
.\" 11
.\" ELFOSABI_OPENBSD
.\" 12
.It Dv ELFOSABI_ARM
ARM architecture ABI.
.\" 97
.It Dv ELFOSABI_STANDALONE
Stand-alone (embedded) ABI.
.\" 255
.El
.It Dv 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.
.It Dv 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.
.It Dv EI_BRAND
Start of architecture identification.
.It Dv EI_NIDENT
The size of the e_ident array.
.El
.Pp
.It Dv e_type
This member of the structure identifies the object file type:
.Pp
.Bl -tag -width "ET_NONE" -compact
.It Dv ET_NONE
An unknown type.
.It Dv ET_REL
A relocatable file.
.It Dv ET_EXEC
An executable file.
.It Dv ET_DYN
A shared object.
.It Dv ET_CORE
A core file.
.El
.Pp
.It Dv e_machine
This member specifies the required architecture for an individual file.
E.g.:
.Pp
.Bl -tag -width "EM_MIPS_RS4_BE" -compact
.It Dv EM_NONE
An unknown machine.
.\" 0
.It Dv EM_M32
AT&T WE 32100.
.\" 1
.It Dv EM_SPARC
Sun Microsystems SPARC.
.\" 2
.It Dv EM_386
Intel 80386.
.\" 3
.It Dv EM_68K
Motorola 68000.
.\" 4
.It Dv EM_88K
Motorola 88000.
.\" 5
.\" .It Dv EM_486
.\" Intel 80486.
.\" 6
.It Dv EM_860
Intel 80860.
.\" 7
.It Dv EM_MIPS
MIPS RS3000 (big-endian only).
.\" 8
.\" EM_S370
.\" 9
.\" .It Dv EM_MIPS_RS4_BE
.\" MIPS RS4000 (big-endian only). Deprecated.
.\" 10
.\" EM_MIPS_RS3_LE (MIPS R3000 little-endian)
.\" 10
.It Dv EM_PARISC
HP/PA.
.\" 15
.It Dv EM_SPARC32PLUS
SPARC with enhanced instruction set.
.\" 18
.It Dv EM_PPC
PowerPC.
.\" 20
.It Dv EM_PPC64
PowerPC 64-bit.
.\" 21
.It Dv EM_S390
IBM S/390
.\" 22
.It Dv EM_ARM
Advanced RISC Machines
.\" 40
.It Dv EM_SH
Renesas SuperH
.\" 42
.It Dv EM_SPARCV9
SPARC v9 64-bit.
.\" 43
.It Dv EM_IA_64
Intel Itanium
.\" 50
.It Dv EM_X86_64
AMD x86-64
.\" 62
.It Dv EM_VAX
DEC Vax.
.\" 75
.\" EM_CRIS
.\" 76
.\" .It Dv EM_ALPHA
.\" Compaq [DEC] Alpha.
.\" .It Dv EM_ALPHA_EXP
.\" Compaq [DEC] Alpha with enhanced instruction set.
.El
.Pp
.It Dv e_version
This member identifies the file version:
.Pp
.Bl -tag -width "EV_CURRENT" -compact
.It Dv EV_NONE
Invalid version.
.It Dv EV_CURRENT
Current version.
.El
.It Dv 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.
.It Dv 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.
.It Dv 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.
.It Dv 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.
.It Dv e_ehsize
This member holds the ELF header's size in bytes.
.It Dv 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.
.It Dv e_phnum
This member holds the number of entries in the program header
table.
Thus the product of
.Sy e_phentsize
and
.Sy e_phnum
gives the table's size
in bytes.
If a file has no program header,
.Sy e_phnum
holds the value zero.
.It Dv 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.
.It Dv e_shnum
This member holds the number of entries in the section header table.
Thus
the product of
.Sy e_shentsize
and
.Sy e_shnum
gives the section header table's size in bytes.
If a file has no section
header table,
.Sy e_shnum
holds the value of zero.
.It Dv 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
.Sy SHN_UNDEF .
.Bl -tag -width "SHN_LORESERVE"
.It Dv SHN_UNDEF
This value marks an undefined, missing, irrelevant, or otherwise meaningless
section reference.
For example, a symbol
.Dq defined
relative to section number
.Sy SHN_UNDEF
is an undefined symbol.
.It Dv SHN_LORESERVE
This value specifies the lower bound of the range of reserved indices.
.It Dv SHN_LOPROC
Values greater than or equal to
.Sy SHN_HIPROC
are reserved for processor-specific semantics.
.It Dv SHN_HIPROC
Values less than or equal to
.Sy SHN_LOPROC
are reserved for processor-specific semantics.
.It Dv SHN_ABS
This value specifies absolute values for the corresponding reference.
For
example, symbols defined relative to section number
.Sy SHN_ABS
have absolute values and are not affected by relocation.
.It Dv SHN_COMMON
Symbols defined relative to this section are common symbols, such as Fortran
COMMON or unallocated C external variables.
.It Dv SHN_HIRESERVE
This value specifies the upper bound of the range of reserved
indices between
.Sy SHN_LORESERVE
and
.Sy SHN_HIRESERVE ,
inclusive; the values do
not reference the section header table.
That is, the section header table
does
.Em not
contain entries for the reserved indices.
.El
.El
.Pp
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
.Em segment
contains one or more
.Em 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
.Sy e_phentsize
and
.Sy e_phnum
members.
The ELF program header is described by the type Elf32_Phdr or Elf64_Phdr
depending on the architecture:
.Bd -literal -offset indent
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;
.Ed
.Bd -literal -offset indent
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;
.Ed
.Pp
The main difference between the 32-bit and the 64-bit program header lies
in the location of the
.Sy p_flags
member in the total struct.
.Bl -tag -width "p_offset" -offset indent
.It Dv 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.
.Bl -tag -width "PT_DYNAMIC"
.It Dv PT_NULL
The array element is unused and the other members' values are undefined.
This lets the program header have ignored entries.
.It Dv PT_LOAD
The array element specifies a loadable segment, described by
.Sy p_filesz
and
.Sy p_memsz .
The bytes from the file are mapped to the beginning of the memory
segment.
If the segment's memory size
.Pq Sy p_memsz
is larger than the file size
.Pq Sy p_filesz ,
the
.Dq 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
.Sy p_vaddr
member.
.It Dv PT_DYNAMIC
The array element specifies dynamic linking information.
.It Dv 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.
.It Dv PT_NOTE
The array element specifies the location and size for auxiliary information.
.It Dv 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.
.It Dv 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
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.
.It Dv PT_LOPROC
Values greater than or equal to
.Sy PT_HIPROC
are reserved for processor-specific semantics.
.It Dv PT_HIPROC
Values less than or equal to
.Sy PT_LOPROC
are reserved for processor-specific semantics.
.El
.Pp
.It Dv p_offset
This member holds the offset from the beginning of the file at which
the first byte of the segment resides.
.It Dv p_vaddr
This member holds the virtual address at which the first byte of the
segment resides in memory.
.It Dv p_paddr
On systems for which physical addressing is relevant, this member is
reserved for the segment's physical address.
Under
.Bx
this member is
not used and must be zero.
.It Dv p_filesz
This member holds the number of bytes in the file image of the segment.
It may be zero.
.It Dv p_memsz
This member holds the number of bytes in the memory image of the segment.
It may be zero.
.It Dv p_flags
This member holds flags relevant to the segment:
.Pp
.Bl -tag -width "PF_X" -compact
.It Dv PF_X
An executable segment.
.It Dv PF_W
A writable segment.
.It Dv PF_R
A readable segment.
.El
.Pp
A text segment commonly has the flags
.Sy PF_X
and
.Sy PF_R .
A data segment commonly has
.Sy PF_X ,
.Sy PF_W
and
.Sy PF_R .
.It Dv 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
.Sy p_vaddr
and
.Sy p_offset ,
modulo the page size.
Values of zero and one mean no alignment is required.
Otherwise,
.Sy p_align
should be a positive, integral power of two, and
.Sy p_vaddr
should equal
.Sy p_offset ,
modulo
.Sy p_align .
.El
.Pp
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
.Sy e_shoff
member gives the byte offset from the beginning of the file to the section
header table.
.Sy e_shnum
holds the number of entries the section header table contains.
.Sy e_shentsize
holds the size in bytes of each entry.
.Pp
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:
.Bl -tag -width "SHN_LORESERVE"
.It Dv SHN_UNDEF
This value marks an undefined, missing, irrelevant or otherwise meaningless
section reference.
.It Dv SHN_LORESERVE
This value specifies the lower bound of the range of reserved indices.
.It Dv SHN_LOPROC
Values greater than or equal to
.Sy SHN_HIPROC
are reserved for processor-specific semantics.
.It Dv SHN_HIPROC
Values less than or equal to
.Sy SHN_LOPROC
are reserved for processor-specific semantics.
.It Dv SHN_ABS
This value specifies the absolute value for the corresponding reference.
For
example, a symbol defined relative to section number
.Sy SHN_ABS
has an absolute value and is not affected by relocation.
.It Dv SHN_COMMON
Symbols defined relative to this section are common symbols, such as FORTRAN
COMMON or unallocated C external variables.
.It Dv SHN_HIRESERVE
This value specifies the upper bound of the range of reserved indices.
The
system reserves indices between
.Sy SHN_LORESERVE
and
.Sy SHN_HIRESERVE ,
inclusive.
The section header table does not contain entries for the
reserved indices.
.El
.Pp
The section header has the following structure:
.Bd -literal -offset indent
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;
.Ed
.Bd -literal -offset indent
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;
.Ed
.Pp
No real differences exist between the 32-bit and 64-bit section headers.
.Bl -tag -width "sh_addralign" -offset indent
.It Dv 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.
.It Dv sh_type
This member categorizes the section's contents and semantics.
.Bl -tag -width "SHT_PROGBITS"
.It Dv 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.
.It Dv SHT_PROGBITS
This section holds information defined by the program, whose
format and meaning are determined solely by the program.
.It Dv SHT_SYMTAB
This section holds a symbol table.
Typically,
.Sy 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
.Sy SHT_DYNSYM
section.
.It Dv SHT_STRTAB
This section holds a string table.
An object file may have multiple
string table sections.
.It Dv SHT_RELA
This section holds relocation entries with explicit addends, such
as type
.Sy Elf32_Rela
for the 32-bit class of object files.
An object may have multiple
relocation sections.
.It Dv 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.
.It Dv SHT_DYNAMIC
This section holds information for dynamic linking.
An object file may
have only one dynamic section.
.It Dv SHT_NOTE
This section holds information that marks the file in some way.
.It Dv SHT_NOBITS
A section of this type occupies no space in the file but otherwise
resembles
.Sy SHT_PROGBITS .
Although this section contains no bytes, the
.Sy sh_offset
member contains the conceptual file offset.
.It Dv SHT_REL
This section holds relocation offsets without explicit addends, such
as type
.Sy Elf32_Rel
for the 32-bit class of object files.
An object file may have multiple
relocation sections.
.It Dv SHT_SHLIB
This section is reserved but has unspecified semantics.
.It Dv SHT_DYNSYM
This section holds a minimal set of dynamic linking symbols.
An
object file can also contain a
.Sy SHT_SYMTAB
section.
.It Dv SHT_LOPROC
This value up to and including
.Sy SHT_HIPROC
is reserved for processor-specific semantics.
.It Dv SHT_HIPROC
This value down to and including
.Sy SHT_LOPROC
is reserved for processor-specific semantics.
.It Dv SHT_LOUSER
This value specifies the lower bound of the range of indices reserved for
application programs.
.It Dv SHT_HIUSER
This value specifies the upper bound of the range of indices reserved for
application programs.
Section types between
.Sy SHT_LOUSER
and
.Sy SHT_HIUSER
may be used by the application, without conflicting with current or future
system-defined section types.
.El
.Pp
.It Dv sh_flags
Sections support one-bit flags that describe miscellaneous attributes.
If a flag bit is set in
.Sy sh_flags ,
the attribute is
.Dq on
for the section.
Otherwise, the attribute is
.Dq off
or does not apply.
Undefined attributes are set to zero.
.Pp
.Bl -tag -width "SHF_EXECINSTR" -compact
.It Dv SHF_WRITE
This section contains data that should be writable during process
execution.
.It Dv 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.
.It Dv SHF_EXECINSTR
This section contains executable machine instructions.
.It Dv SHF_MASKPROC
All bits included in this mask are reserved for processor-specific
semantics.
.El
.Pp
.It Dv 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.
.It Dv 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,
.Sy SHT_NOBITS ,
occupies no space in the file, and its
.Sy sh_offset
member locates the conceptual placement in the file.
.It Dv sh_size
This member holds the section's size in bytes.
Unless the section type
is
.Sy SHT_NOBITS ,
the section occupies
.Sy sh_size
bytes in the file.
A section of type
.Sy SHT_NOBITS
may have a non-zero size, but it occupies no space in the file.
.It Dv sh_link
This member holds a section header table index link, whose interpretation
depends on the section type.
.It Dv sh_info
This member holds extra information, whose interpretation depends on the
section type.
.It Dv 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
.Sy sh_addr
must be congruent to zero, modulo the value of
.Sy sh_addralign .
Only zero and positive integral powers of two are allowed.
Values of zero
or one mean the section has no alignment constraints.
.It Dv 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.
.El
.Pp
Various sections hold program and control information:
.Bl -tag -width ".shstrtab" -offset indent
.It .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
.Sy SHT_NOBITS .
The attribute types are
.Sy SHF_ALLOC
and
.Sy SHF_WRITE .
.It .comment
This section holds version control information.
This section is of type
.Sy SHT_PROGBITS .
No attribute types are used.
.It .ctors
This section holds initialized pointers to the C++ constructor functions.
This section is of type
.Sy SHT_PROGBITS .
The attribute types are
.Sy SHF_ALLOC
and
.Sy SHF_WRITE .
.It .data
This section holds initialized data that contribute to the program's
memory image.
This section is of type
.Sy SHT_PROGBITS .
The attribute types are
.Sy SHF_ALLOC
and
.Sy SHF_WRITE .
.It .data1
This section holds initialized data that contribute to the program's
memory image.
This section is of type
.Sy SHT_PROGBITS .
The attribute types are
.Sy SHF_ALLOC
and
.Sy SHF_WRITE .
.It .debug
This section holds information for symbolic debugging.
The contents
are unspecified.
This section is of type
.Sy SHT_PROGBITS .
No attribute types are used.
.It .dtors
This section holds initialized pointers to the C++ destructor functions.
This section is of type
.Sy SHT_PROGBITS .
The attribute types are
.Sy SHF_ALLOC
and
.Sy SHF_WRITE .
.It .dynamic
This section holds dynamic linking information.
The section's attributes
will include the
.Sy SHF_ALLOC
bit.
Whether the
.Sy SHF_WRITE
bit is set is processor-specific.
This section is of type
.Sy SHT_DYNAMIC .
See the attributes above.
.It .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
.Sy SHT_STRTAB .
The attribute type used is
.Sy SHF_ALLOC .
.It .dynsym
This section holds the dynamic linking symbol table.
This section is of type
.Sy SHT_DYNSYM .
The attribute used is
.Sy SHF_ALLOC .
.It .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
.Sy SHT_PROGBITS .
The attributes used are
.Sy SHF_ALLOC
and
.Sy SHF_EXECINSTR .
.It .got
This section holds the global offset table.
This section is of type
.Sy SHT_PROGBITS .
The attributes are processor-specific.
.It .hash
This section holds a symbol hash table.
This section is of type
.Sy SHT_HASH .
The attribute used is
.Sy SHF_ALLOC .
.It .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
.Sy SHT_PROGBITS .
The attributes used are
.Sy SHF_ALLOC
and
.Sy SHF_EXECINSTR .
.It .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
.Sy SHF_ALLOC
bit.
Otherwise, that bit will be off.
This section is of type
.Sy SHT_PROGBITS .
.It .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
.Sy SHT_PROGBITS .
No attribute types are used.
.It .note
This section holds information in the
.Dq Note Section
format described below.
This section is of type
.Sy SHT_NOTE .
No attribute types are used.
.Ox
native executables usually contain a
.Sy .note.openbsd.ident
section to identify themselves, for the kernel to bypass any compatibility
ELF binary emulation tests when loading the file.
.It .plt
This section holds the procedure linkage table.
This section is of type
.Sy SHT_PROGBITS .
The attributes are processor-specific.
.It .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
.Sy SHF_ALLOC
bit.
Otherwise the bit will be off.
By convention,
.Dq NAME
is supplied by the section to which the relocations apply.
Thus a relocation
section for
.Sy .text
normally would have the name
.Sy .rel.text .
This section is of type
.Sy SHT_REL .
.It .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
.Sy SHF_ALLOC
bit.
Otherwise the bit will be off.
By convention,
.Dq NAME
is supplied by the section to which the relocations apply.
Thus a relocation
section for
.Sy .text
normally would have the name
.Sy .rela.text .
This section is of type
.Sy SHT_RELA .
.It .rodata
This section holds read-only data that typically contributes to a
non-writable segment in the process image.
This section is of type
.Sy SHT_PROGBITS .
The attribute used is
.Sy SHF_ALLOC .
.It .rodata1
This section holds read-only data that typically contributes to a
non-writable segment in the process image.
This section is of type
.Sy SHT_PROGBITS .
The attribute used is
.Sy SHF_ALLOC .
.It .shstrtab
This section holds section names.
This section is of type
.Sy SHT_STRTAB .
No attribute types are used.
.It .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
.Sy SHF_ALLOC
bit.
Otherwise the bit will be off.
This section is of type
.Sy SHT_STRTAB .
.It .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
.Sy SHF_ALLOC
bit.
Otherwise the bit will be off.
This section is of type
.Sy SHT_SYMTAB .
.It .text
This section holds the
.Dq text ,
or executable instructions, of a program.
This section is of type
.Sy SHT_PROGBITS .
The attributes used are
.Sy SHF_ALLOC
and
.Sy SHF_EXECINSTR .
.El
.Pp
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.
.Pp
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.
.Bd -literal -offset indent
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;
.Ed
.Bd -literal -offset indent
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;
.Ed
.Pp
The 32-bit and 64-bit versions have the same members, just in a different 
order.
.Bl -tag -width "st_value" -offset indent
.It Dv 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.
.It Dv st_value
This member gives the value of the associated symbol.
.It Dv st_size
Many symbols have associated sizes.
This member holds zero if the symbol
has no size or an unknown size.
.It Dv st_info
This member specifies the symbol's type and binding attributes:
.Bl -tag -width "STT_SECTION"
.It Dv STT_NOTYPE
The symbol's type is not defined.
.It Dv STT_OBJECT
The symbol is associated with a data object.
.It Dv STT_FUNC
The symbol is associated with a function or other executable code.
.It Dv STT_SECTION
The symbol is associated with a section.
Symbol table entries of
this type exist primarily for relocation and normally have
.Sy STB_LOCAL
bindings.
.It Dv STT_FILE
By convention, the symbol's name gives the name of the source file
associated with the object file.
A file symbol has
.Sy STB_LOCAL
bindings, its section index is
.Sy SHN_ABS ,
and it precedes the other
.Sy STB_LOCAL
symbols of the file, if it is present.
.It Dv STT_LOPROC
This value up to and including
.Sy STT_HIPROC
is reserved for processor-specific semantics.
.It Dv STT_HIPROC
This value down to and including
.Sy STT_LOPROC
is reserved for processor-specific semantics.
.El
.Bl -tag -width "STB_GLOBAL"
.It Dv 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.
.It Dv 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.
.It Dv STB_WEAK
Weak symbols resemble global symbols, but their definitions have lower
precedence.
.It Dv STB_LOPROC
This value up to and including
.Sy STB_HIPROC
is reserved for processor-specific semantics.
.It Dv STB_HIPROC
This value down to and including
.Sy STB_LOPROC
is reserved for processor-specific semantics.
.Pp
There are macros for packing and unpacking the binding and type fields:
.Pp
.Fn ELF32_ST_BIND info
or
.Fn ELF64_ST_BIND info
extract a binding from an st_info value.
.Pp
.Fn ELF32_ST_TYPE info
or
.Fn ELF64_ST_TYPE info
.br
extract a type from an st_info value.
.Pp
.Fn ELF32_ST_INFO bind type
or
.Fn ELF64_ST_INFO bind type
.br
convert a binding and a type into an st_info value.
.El
.Pp
.It Dv st_other
This member currently holds zero and has no defined meaning.
.It Dv st_shndx
Every symbol table entry is
.Dq defined
in relation to some section.
This member holds the relevant section
header table index.
.El
.Pp
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.
.Pp
Relocation structures that do not need an addend:
.Bd -literal -offset indent
typedef struct {
	Elf32_Addr     r_offset;
	uint32_t       r_info;
} Elf32_Rel;
.Ed
.Bd -literal -offset indent
typedef struct {
	Elf64_Addr     r_offset;
	uint64_t       r_info;
} Elf64_Rel;
.Ed
.Pp
Relocation structures that need an addend:
.Bd -literal -offset indent
typedef struct {
	Elf32_Addr      r_offset;
	uint32_t        r_info;
	int32_t         r_addend;
} Elf32_Rela;
.Ed
.Bd -literal -offset indent
typedef struct {
	Elf64_Addr      r_offset;
	uint64_t        r_info;
	int64_t         r_addend;
} Elf64_Rela;
.Ed
.Bl -tag -width "r_offset" -offset indent
.It Dv 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.
.It Dv 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
.Sy ELF_[32|64]_R_TYPE
or
.Sy ELF[32|64]_R_SYM ,
respectively, to the entry's
.Sy r_info
member.
.It Dv r_addend
This member specifies a constant addend used to compute the value to be
stored into the relocatable field.
.El
.Pp
The .dynamic section contains a series of structures that hold relevant 
dynamic linking information.  The d_tag member controls the interpretation 
of d_un.
.Bd -literal -offset indent
typedef struct {
	Elf32_Sword     d_tag;
	union {
	   Elf32_Word   d_val;
	   Elf32_Addr   d_ptr;
	} d_un;
} Elf32_Dyn;
extern Elf32_Dyn _DYNAMIC[];
.Ed
.Bd -literal -offset indent
typedef struct {
	Elf64_Sxword    d_tag;
	union {
	   Elf64_Xword  d_val;
	   Elf64_Addr   d_ptr;
	} d_un;
} Elf64_Dyn;
extern Elf64_Dyn _DYNAMIC[];
.Ed
.Bl -tag -width "d_tag" -offset indent
.It Dv d_tag
This member may have any of the following values:
.Bl -tag -width "DT_SYMBOLIC"
.It Dv DT_NULL
Marks end of dynamic section
.It Dv DT_NEEDED
String table offset to name of a needed library
.It Dv DT_PLTRELSZ
Size in bytes of PLT relocs
.It Dv DT_PLTGOT
Address of PLT and/or GOT
.It Dv DT_HASH
Address of symbol hash table
.It Dv DT_STRTAB
Address of string table
.It Dv DT_SYMTAB
Address of symbol table
.It Dv DT_RELA
Address of Rela relocs table
.It Dv DT_RELASZ
Size in bytes of Rela table
.It Dv DT_RELAENT
Size in bytes of a Rela table entry
.It Dv DT_STRSZ
Size in bytes of string table
.It Dv DT_SYMENT
Size in bytes of a symbol table entry
.It Dv DT_INIT
Address of the initialization function
.It Dv DT_FINI
Address of the termination function
.It Dv DT_SONAME
String table offset to name of shared object
.It Dv DT_RPATH
String table offset to library search path (deprecated)
.It Dv DT_SYMBOLIC
Alert linker to search this shared object before the executable for symbols
.It Dv DT_REL
Address of Rel relocs table
.It Dv DT_RELSZ
Size in bytes of Rel table
.It Dv DT_RELENT
Size in bytes of a Rel table entry
.It Dv DT_PLTREL
Type of reloc the PLT refers (Rela or Rel)
.It Dv DT_DEBUG
Undefined use for debugging
.It Dv DT_TEXTREL
Absence of this indicates no relocs should apply to a non-writable segment
.It Dv DT_JMPREL
Address of reloc entries solely for the PLT
.It Dv DT_BIND_NOW
Instruct dynamic linker to process all relocs before transferring control to 
the executable
.It Dv DT_RUNPATH
String table offset to library search path
.It Dv DT_LOPROC
Start of processor-specific semantics
.It Dv DT_HIPROC
End of processor-specific semantics
.El
.It Dv d_val
This member represents integer values with various interpretations.
.It Dv 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.
.It Dv _DYNAMIC
Array containing all the dynamic structures in the .dynamic section.  
This is automatically populated by the linker.
.El
.Sh SEE ALSO
.Xr as 1 ,
.Xr gdb 1 ,
.Xr ld 1 ,
.Xr objdump 1 ,
.Xr execve 2 ,
.Xr core 5
.Rs
.%A Hewlett-Packard
.%B Elf-64 Object File Format
.Re
.Rs
.%A Santa Cruz Operation
.%B System V Application Binary Interface
.Re
.Rs
.%A Unix System Laboratories
.%T Object Files
.%B "Executable and Linking Format (ELF)"
.Re
.Sh HISTORY
.Ox
ELF support first appeared in
.Ox 1.2 ,
although not all supported platforms use it as the native
binary file format.
ELF in itself first appeared in
.At V .
The ELF format is an adopted standard.
.Sh AUTHORS
The original version of this manual page was written by
.An Jeroen Ruigrok van der Werven
.Aq asmodai@FreeBSD.org
with inspiration from BSDi's
.Bsx
.Nm elf
manpage.