.\"	$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.
.\"
.\" %%%LICENSE_START(PERMISSIVE_MISC)
.\"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.
.\" %%%LICENSE_END
.\"
.\"	$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
.\" 2007-10-11, Mike Frysinger <vapier@gentoo.org>, various fixes
.\" 2007-12-08, mtk, Converted from mdoc to man macros
.\"
.TH ELF 5 2018-04-30 "Linux" "Linux Programmer's Manual"
.SH NAME
elf \- format of Executable and Linking Format (ELF) files
.SH SYNOPSIS
.nf
.\" .B #include <elf_abi.h>
.B #include <elf.h>
.fi
.SH DESCRIPTION
The header file
.I <elf.h>
defines the format of ELF executable binary files.
Amongst these files are
normal executable files, relocatable object files, core files, and shared
objects.
.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
.\" .I <elf_abi.h>
.\" in their source code.
.\" These applications should need to refer to
.\" all the types and structures by their generic names
.\" "Elf_xxx"
.\" and to the macros by
.\" 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
.\" "Elf32_xxx"
.\" or
.\" "Elf64_xxx"
.\" type and structure names.
.\" Likewise, the macros need to be identified by
.\" "ELF32_xxx"
.\" or
.\" "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.
.\"
.SS Basic types
The following types are used for N-bit architectures (N=32,64,
.I ElfN
stands for
.I Elf32
or
.IR Elf64 ,
.I uintN_t
stands for
.I uint32_t
or
.IR uint64_t ):
.PP
.in +4n
.EX
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
.EE
.in
.PP
(Note: the *BSD terminology is a bit different.
There,
.I Elf64_Half
is
twice as large as
.IR Elf32_Half ,
and
.I Elf64Quarter
is used for
.IR 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
"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, and so on.
.\"
.SS ELF header (Ehdr)
The ELF header is described by the type
.I Elf32_Ehdr
or
.IR Elf64_Ehdr :
.PP
.in +4n
.EX
#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;
.EE
.in
.PP
The fields have the following meanings:
.\"
.nr l1_indent 10
.\"
.TP \n[l1_indent]
.IR e_ident
This array of bytes specifies how 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
.BR EI_
and may contain values which start with the prefix
.BR ELF .
The following macros are defined:
.RS
.TP 9
.BR EI_MAG0
The first byte of the magic number.
It must be filled with
.BR ELFMAG0 .
(0: 0x7f)
.TP
.BR EI_MAG1
The second byte of the magic number.
It must be filled with
.BR ELFMAG1 .
(1: \(aqE\(aq)
.TP
.BR EI_MAG2
The third byte of the magic number.
It must be filled with
.BR ELFMAG2 .
(2: \(aqL\(aq)
.TP
.BR EI_MAG3
The fourth byte of the magic number.
It must be filled with
.BR ELFMAG3 .
(3: \(aqF\(aq)
.TP
.BR EI_CLASS
The fifth byte identifies the architecture for this binary:
.RS
.TP 14
.PD 0
.BR ELFCLASSNONE
This class is invalid.
.TP
.BR ELFCLASS32
This defines the 32-bit architecture.
It supports machines with files
and virtual address spaces up to 4 Gigabytes.
.TP
.BR ELFCLASS64
This defines the 64-bit architecture.
.PD
.RE
.TP
.BR EI_DATA
The sixth byte specifies the data encoding of the processor-specific
data in the file.
Currently, these encodings are supported:
.RS 9
.TP 14
.PD 0
.BR ELFDATANONE
Unknown data format.
.TP
.BR ELFDATA2LSB
Two's complement, little-endian.
.TP
.BR ELFDATA2MSB
Two's complement, big-endian.
.PD
.RE
.TP
.BR EI_VERSION
The seventh byte is the version number of the ELF specification:
.IP
.PD 0
.RS
.TP 14
.BR EV_NONE
Invalid version.
.TP
.BR EV_CURRENT
Current version.
.PD
.RE
.\".El
.TP
.BR EI_OSABI
The eighth byte identifies the operating system
and ABI to which the object is targeted.
Some fields in other ELF structures have flags
and values that have platform-specific meanings;
the interpretation of those fields is determined by the value of this byte.
For example:
.RS
.TP 21
.PD 0
.BR ELFOSABI_NONE
Same as ELFOSABI_SYSV
.\" 0
.TP
.BR ELFOSABI_SYSV
UNIX System V ABI
.\" 0
.\" synonym: ELFOSABI_NONE
.TP
.BR ELFOSABI_HPUX
HP-UX ABI
.\" 1
.TP
.BR ELFOSABI_NETBSD
NetBSD ABI
.\" 2
.TP
.BR ELFOSABI_LINUX
Linux ABI
.\" 3
.\" .TP
.\" .BR ELFOSABI_HURD
.\" Hurd ABI
.\" 4
.\" .TP
.\" .BR ELFOSABI_86OPEN
.\" 86Open Common IA32 ABI
.\" 5
.TP
.BR ELFOSABI_SOLARIS
Solaris ABI
.\" 6
.\" .TP
.\" .BR ELFOSABI_MONTEREY
.\" Monterey project ABI
.\" Now replaced by
.\" ELFOSABI_AIX
.\" 7
.TP
.BR ELFOSABI_IRIX
IRIX ABI
.\" 8
.TP
.BR ELFOSABI_FREEBSD
FreeBSD ABI
.\" 9
.TP
.BR ELFOSABI_TRU64
TRU64 UNIX ABI
.\" 10
.\" ELFOSABI_MODESTO
.\" 11
.\" ELFOSABI_OPENBSD
.\" 12
.TP
.BR ELFOSABI_ARM
ARM architecture ABI
.\" 97
.TP
.BR ELFOSABI_STANDALONE
Stand-alone (embedded) ABI
.\" 255
.PD
.RE
.TP
.BR EI_ABIVERSION
The ninth byte identifies the version of the ABI
to which the object is targeted.
This field is used to distinguish among incompatible versions of an ABI.
The interpretation of this version number
is dependent on the ABI identified by the
.B EI_OSABI
field.
Applications conforming to this specification use the value 0.
.TP
.BR EI_PAD
Start of padding.
These bytes are reserved and set to zero.
Programs
which read them should ignore them.
The value for
.B EI_PAD
will change in
the future if currently unused bytes are given meanings.
.\" As reported by Yuri Kozlov and confirmed by Mike Frysinger, EI_BRAND is
.\" not in GABI (http://www.sco.com/developers/gabi/latest/ch4.eheader.html)
.\" It looks to be a BSDism
.\" .TP
.\" .BR EI_BRAND
.\" Start of architecture identification.
.TP
.BR EI_NIDENT
The size of the
.I e_ident
array.
.RE
.TP
.IR e_type
This member of the structure identifies the object file type:
.RS
.TP 16
.PD 0
.BR ET_NONE
An unknown type.
.TP
.BR ET_REL
A relocatable file.
.TP
.BR ET_EXEC
An executable file.
.TP
.BR ET_DYN
A shared object.
.TP
.BR ET_CORE
A core file.
.PD
.RE
.TP
.IR e_machine
This member specifies the required architecture for an individual file.
For example:
.RS \n[l1_indent]
.TP 16
.PD 0
.BR EM_NONE
An unknown machine
.\" 0
.TP
.BR EM_M32
AT&T WE 32100
.\" 1
.TP
.BR EM_SPARC
Sun Microsystems SPARC
.\" 2
.TP
.BR EM_386
Intel 80386
.\" 3
.TP
.BR EM_68K
Motorola 68000
.\" 4
.TP
.BR EM_88K
Motorola 88000
.\" 5
.\" .TP
.\" .BR EM_486
.\" Intel 80486
.\" 6
.TP
.BR EM_860
Intel 80860
.\" 7
.TP
.BR EM_MIPS
MIPS RS3000 (big-endian only)
.\" 8
.\" EM_S370
.\" 9
.\" .TP
.\" .BR EM_MIPS_RS4_BE
.\" MIPS RS4000 (big-endian only). Deprecated
.\" 10
.\" EM_MIPS_RS3_LE (MIPS R3000 little-endian)
.\" 10
.TP
.BR EM_PARISC
HP/PA
.\" 15
.TP
.BR EM_SPARC32PLUS
SPARC with enhanced instruction set
.\" 18
.TP
.BR EM_PPC
PowerPC
.\" 20
.TP
.BR EM_PPC64
PowerPC 64-bit
.\" 21
.TP
.BR EM_S390
IBM S/390
.\" 22
.TP
.BR EM_ARM
Advanced RISC Machines
.\" 40
.TP
.BR EM_SH
Renesas SuperH
.\" 42
.TP
.BR EM_SPARCV9
SPARC v9 64-bit
.\" 43
.TP
.BR EM_IA_64
Intel Itanium
.\" 50
.TP
.BR EM_X86_64
AMD x86-64
.\" 62
.TP
.BR EM_VAX
DEC Vax
.\" 75
.\" EM_CRIS
.\" 76
.\" .TP
.\" .BR EM_ALPHA
.\" Compaq [DEC] Alpha
.\" .TP
.\" .BR EM_ALPHA_EXP
.\" Compaq [DEC] Alpha with enhanced instruction set
.PD
.RE
.TP
.IR e_version
This member identifies the file version:
.RS
.TP 16
.PD 0
.BR EV_NONE
Invalid version
.TP
.BR EV_CURRENT
Current version
.PD
.RE
.TP
.IR 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.
.TP
.IR 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.
.TP
.IR 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.
.TP
.IR 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.
.TP
.IR e_ehsize
This member holds the ELF header's size in bytes.
.TP
.IR 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.
.TP
.IR e_phnum
This member holds the number of entries in the program header
table.
Thus the product of
.IR e_phentsize
and
.IR e_phnum
gives the table's size
in bytes.
If a file has no program header,
.IR e_phnum
holds the value zero.
.IP
If the number of entries in the program header table is
larger than or equal to
.\" This is a Linux extension, added in Linux 2.6.34.
.BR PN_XNUM
(0xffff), this member holds
.BR PN_XNUM
(0xffff) and the real number of entries in the program header table is held
in the
.IR sh_info
member of the initial entry in section header table.
Otherwise, the
.IR sh_info
member of the initial entry contains the value zero.
.RS \n[l1_indent]
.TP 9
.BR PN_XNUM
This is defined as 0xffff, the largest number
.IR e_phnum
can have, specifying where the actual number of program headers is assigned.
.PD
.RE
.IP
.TP
.IR 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.
.TP
.IR e_shnum
This member holds the number of entries in the section header table.
Thus
the product of
.IR e_shentsize
and
.IR e_shnum
gives the section header table's size in bytes.
If a file has no section
header table,
.IR e_shnum
holds the value of zero.
.IP
If the number of entries in the section header table is
larger than or equal to
.BR SHN_LORESERVE
(0xff00),
.IR e_shnum
holds the value zero and the real number of entries in the section header
table is held in the
.IR sh_size
member of the initial entry in section header table.
Otherwise, the
.IR sh_size
member of the initial entry in the section header table holds
the value zero.
.TP
.IR 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
.BR SHN_UNDEF .
.IP
If the index of section name string table section is
larger than or equal to
.BR SHN_LORESERVE
(0xff00), this member holds
.BR SHN_XINDEX
(0xffff) and the real index of the section name string table section
is held in the
.IR sh_link
member of the initial entry in section header table.
Otherwise, the
.IR sh_link
member of the initial entry in section header table contains the value zero.
.\"
.SS Program header (Phdr)
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
.IR segment
contains one or more
.IR 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
.IR e_phentsize
and
.IR e_phnum
members.
The ELF program header is described by the type
.I Elf32_Phdr
or
.I Elf64_Phdr
depending on the architecture:
.PP
.in +4n
.EX
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;
.EE
.in
.PP
.in +4n
.EX
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;
.EE
.in
.PP
The main difference between the 32-bit and the 64-bit program header lies
in the location of the
.IR p_flags
member in the total struct.
.TP 10
.IR p_type
This member of the structure indicates what kind of segment this array
element describes or how to interpret the array element's information.
.RS 10
.TP 12
.BR PT_NULL
The array element is unused and the other members' values are undefined.
This lets the program header have ignored entries.
.TP
.BR PT_LOAD
The array element specifies a loadable segment, described by
.IR p_filesz
and
.IR p_memsz .
The bytes from the file are mapped to the beginning of the memory
segment.
If the segment's memory size
.IR p_memsz
is larger than the file size
.IR p_filesz ,
the
"extra"
bytes are defined to hold the value 0 and to follow the segment's
initialized area.
The file size may not be larger than the memory size.
Loadable segment entries in the program header table appear in ascending
order, sorted on the
.IR p_vaddr
member.
.TP
.BR PT_DYNAMIC
The array element specifies dynamic linking information.
.TP
.BR PT_INTERP
The array element specifies the location and size of a null-terminated
pathname to invoke as an interpreter.
This segment type is meaningful
only for executable files (though it may occur for shared objects).
However it may not occur more than once in a file.
If it is present, it must precede any loadable segment entry.
.TP
.BR PT_NOTE
The array element specifies the location of notes (ElfN_Nhdr).
.TP
.BR 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.
.TP
.BR PT_PHDR
The array element, if present,
specifies the location and size of the program header table itself,
both in the file and in the memory image of the program.
This segment type may not occur more than once in a file.
Moreover, it may
occur only if the program header table is part of the memory image of the
program.
If it is present, it must precede any loadable segment entry.
.TP
.BR PT_LOPROC ", " PT_HIPROC
Values in the inclusive range
.RB [ PT_LOPROC ", " PT_HIPROC ]
are reserved for processor-specific semantics.
.TP
.BR PT_GNU_STACK
GNU extension which is used by the Linux kernel to control the state of the
stack via the flags set in the
.IR p_flags
member.
.RE
.TP
.IR p_offset
This member holds the offset from the beginning of the file at which
the first byte of the segment resides.
.TP
.IR p_vaddr
This member holds the virtual address at which the first byte of the
segment resides in memory.
.TP
.IR 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.
.TP
.IR p_filesz
This member holds the number of bytes in the file image of the segment.
It may be zero.
.TP
.IR p_memsz
This member holds the number of bytes in the memory image of the segment.
It may be zero.
.TP
.IR p_flags
This member holds a bit mask of flags relevant to the segment:
.RS \n[l1_indent]
.TP
.PD 0
.BR PF_X
An executable segment.
.TP
.BR PF_W
A writable segment.
.TP
.BR PF_R
A readable segment.
.PD
.RE
.IP
A text segment commonly has the flags
.BR PF_X
and
.BR PF_R .
A data segment commonly has
.BR PF_X ,
.BR PF_W ,
and
.BR PF_R .
.TP
.IR 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
.IR p_vaddr
and
.IR p_offset ,
modulo the page size.
Values of zero and one mean no alignment is required.
Otherwise,
.IR p_align
should be a positive, integral power of two, and
.IR p_vaddr
should equal
.IR p_offset ,
modulo
.IR p_align .
.\"
.SS Section header (Shdr)
A file's section header table lets one locate all the file's sections.
The
section header table is an array of
.I Elf32_Shdr
or
.I Elf64_Shdr
structures.
The
ELF header's
.IR e_shoff
member gives the byte offset from the beginning of the file to the section
header table.
.IR e_shnum
holds the number of entries the section header table contains.
.IR 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:
the initial entry and the indices between
.B SHN_LORESERVE
and
.BR SHN_HIRESERVE .
The initial entry is used in ELF extensions for
.IR e_phnum ,
.IR e_shnum
and
.IR e_strndx ;
in other cases, each field in the initial entry is set to zero.
An object file does not have sections for
these special indices:
.TP
.BR SHN_UNDEF
This value marks an undefined, missing, irrelevant,
or otherwise meaningless section reference.
.TP
.BR SHN_LORESERVE
This value specifies the lower bound of the range of reserved indices.
.TP
.BR SHN_LOPROC ", " SHN_HIPROC
Values greater in the inclusive range
.RB [ SHN_LOPROC ", " SHN_HIPROC ]
are reserved for processor-specific semantics.
.TP
.BR SHN_ABS
This value specifies the absolute value for the corresponding reference.
For
example, a symbol defined relative to section number
.BR SHN_ABS
has an absolute value and is not affected by relocation.
.TP
.BR SHN_COMMON
Symbols defined relative to this section are common symbols,
such as FORTRAN COMMON or unallocated C external variables.
.TP
.BR SHN_HIRESERVE
This value specifies the upper bound of the range of reserved indices.
The
system reserves indices between
.BR SHN_LORESERVE
and
.BR SHN_HIRESERVE ,
inclusive.
The section header table does not contain entries for the
reserved indices.
.PP
The section header has the following structure:
.PP
.in +4n
.EX
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;
.EE
.in
.PP
.in +4n
.EX
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;
.EE
.in
.PP
No real differences exist between the 32-bit and 64-bit section headers.
.TP \n[l1_indent]
.IR 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.
.TP
.IR sh_type
This member categorizes the section's contents and semantics.
.RS \n[l1_indent]
.TP 15
.BR 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.
.TP
.BR SHT_PROGBITS
This section holds information defined by the program, whose
format and meaning are determined solely by the program.
.TP
.BR SHT_SYMTAB
This section holds a symbol table.
Typically,
.BR 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
.BR SHT_DYNSYM
section.
.TP
.BR SHT_STRTAB
This section holds a string table.
An object file may have multiple
string table sections.
.TP
.BR SHT_RELA
This section holds relocation entries with explicit addends, such
as type
.IR Elf32_Rela
for the 32-bit class of object files.
An object may have multiple
relocation sections.
.TP
.BR 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.
.TP
.BR SHT_DYNAMIC
This section holds information for dynamic linking.
An object file may
have only one dynamic section.
.TP
.BR SHT_NOTE
This section holds notes (ElfN_Nhdr).
.TP
.BR SHT_NOBITS
A section of this type occupies no space in the file but otherwise
resembles
.BR SHT_PROGBITS .
Although this section contains no bytes, the
.IR sh_offset
member contains the conceptual file offset.
.TP
.BR SHT_REL
This section holds relocation offsets without explicit addends, such
as type
.IR Elf32_Rel
for the 32-bit class of object files.
An object file may have multiple
relocation sections.
.TP
.BR SHT_SHLIB
This section is reserved but has unspecified semantics.
.TP
.BR SHT_DYNSYM
This section holds a minimal set of dynamic linking symbols.
An
object file can also contain a
.BR SHT_SYMTAB
section.
.TP
.BR SHT_LOPROC ", " SHT_HIPROC
Values in the inclusive range
.RB [ SHT_LOPROC ", " SHT_HIPROC ]
are reserved for processor-specific semantics.
.TP
.BR SHT_LOUSER
This value specifies the lower bound of the range of indices reserved for
application programs.
.TP
.BR SHT_HIUSER
This value specifies the upper bound of the range of indices reserved for
application programs.
Section types between
.BR SHT_LOUSER
and
.BR SHT_HIUSER
may be used by the application, without conflicting with current or future
system-defined section types.
.RE
.TP
.IR sh_flags
Sections support one-bit flags that describe miscellaneous attributes.
If a flag bit is set in
.IR sh_flags ,
the attribute is
"on"
for the section.
Otherwise, the attribute is
"off"
or does not apply.
Undefined attributes are set to zero.
.RS \n[l1_indent]
.TP 15
.BR SHF_WRITE
This section contains data that should be writable during process
execution.
.TP
.BR 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.
.TP
.BR SHF_EXECINSTR
This section contains executable machine instructions.
.TP
.BR SHF_MASKPROC
All bits included in this mask are reserved for processor-specific
semantics.
.RE
.TP
.IR 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.
.TP
.IR 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,
.BR SHT_NOBITS ,
occupies no space in the file, and its
.IR sh_offset
member locates the conceptual placement in the file.
.TP
.IR sh_size
This member holds the section's size in bytes.
Unless the section type
is
.BR SHT_NOBITS ,
the section occupies
.IR sh_size
bytes in the file.
A section of type
.BR SHT_NOBITS
may have a nonzero size, but it occupies no space in the file.
.TP
.IR sh_link
This member holds a section header table index link, whose interpretation
depends on the section type.
.TP
.IR sh_info
This member holds extra information, whose interpretation depends on the
section type.
.TP
.IR 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
.IR sh_addr
must be congruent to zero, modulo the value of
.IR sh_addralign .
Only zero and positive integral powers of two are allowed.
The value 0 or 1 means that the section has no alignment constraints.
.TP
.IR 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.
.PP
Various sections hold program and control information:
.TP \n[l1_indent]
.IR .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
.BR SHT_NOBITS .
The attribute types are
.BR SHF_ALLOC
and
.BR SHF_WRITE .
.TP
.IR .comment
This section holds version control information.
This section is of type
.BR SHT_PROGBITS .
No attribute types are used.
.TP
.IR .ctors
This section holds initialized pointers to the C++ constructor functions.
This section is of type
.BR SHT_PROGBITS .
The attribute types are
.BR SHF_ALLOC
and
.BR SHF_WRITE .
.TP
.IR .data
This section holds initialized data that contribute to the program's
memory image.
This section is of type
.BR SHT_PROGBITS .
The attribute types are
.BR SHF_ALLOC
and
.BR SHF_WRITE .
.TP
.IR .data1
This section holds initialized data that contribute to the program's
memory image.
This section is of type
.BR SHT_PROGBITS .
The attribute types are
.BR SHF_ALLOC
and
.BR SHF_WRITE .
.TP
.IR .debug
This section holds information for symbolic debugging.
The contents
are unspecified.
This section is of type
.BR SHT_PROGBITS .
No attribute types are used.
.TP
.IR .dtors
This section holds initialized pointers to the C++ destructor functions.
This section is of type
.BR SHT_PROGBITS .
The attribute types are
.BR SHF_ALLOC
and
.BR SHF_WRITE .
.TP
.IR .dynamic
This section holds dynamic linking information.
The section's attributes
will include the
.BR SHF_ALLOC
bit.
Whether the
.BR SHF_WRITE
bit is set is processor-specific.
This section is of type
.BR SHT_DYNAMIC .
See the attributes above.
.TP
.IR .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
.BR SHT_STRTAB .
The attribute type used is
.BR SHF_ALLOC .
.TP
.IR .dynsym
This section holds the dynamic linking symbol table.
This section is of type
.BR SHT_DYNSYM .
The attribute used is
.BR SHF_ALLOC .
.TP
.IR .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
.BR SHT_PROGBITS .
The attributes used are
.BR SHF_ALLOC
and
.BR SHF_EXECINSTR .
.TP
.IR .gnu.version
This section holds the version symbol table, an array of
.I ElfN_Half
elements.
This section is of type
.BR SHT_GNU_versym .
The attribute type used is
.BR SHF_ALLOC .
.TP
.IR .gnu.version_d
This section holds the version symbol definitions, a table of
.I ElfN_Verdef
structures.
This section is of type
.BR SHT_GNU_verdef .
The attribute type used is
.BR SHF_ALLOC .
.TP
.IR .gnu.version_r
This section holds the version symbol needed elements, a table of
.I ElfN_Verneed
structures.
This section is of
type
.BR SHT_GNU_versym .
The attribute type used is
.BR SHF_ALLOC .
.TP
.IR .got
This section holds the global offset table.
This section is of type
.BR SHT_PROGBITS .
The attributes are processor-specific.
.TP
.IR .hash
This section holds a symbol hash table.
This section is of type
.BR SHT_HASH .
The attribute used is
.BR SHF_ALLOC .
.TP
.IR .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
.BR SHT_PROGBITS .
The attributes used are
.BR SHF_ALLOC
and
.BR SHF_EXECINSTR .
.TP
.IR .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
.BR SHF_ALLOC
bit.
Otherwise, that bit will be off.
This section is of type
.BR SHT_PROGBITS .
.TP
.IR .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
.BR SHT_PROGBITS .
No attribute types are used.
.TP
.IR .note
This section holds various notes.
This section is of type
.BR SHT_NOTE .
No attribute types are used.
.TP
.IR .note.ABI-tag
This section is used to declare the expected run-time ABI of the ELF image.
It may include the operating system name and its run-time versions.
This section is of type
.BR SHT_NOTE .
The only attribute used is
.BR SHF_ALLOC .
.TP
.IR .note.gnu.build-id
This section is used to hold an ID that uniquely identifies
the contents of the ELF image.
Different files with the same build ID should contain the same executable
content.
See the
.BR \-\-build\-id
option to the GNU linker (\fBld\fR (1)) for more details.
This section is of type
.BR SHT_NOTE .
The only attribute used is
.BR SHF_ALLOC .
.TP
.IR .note.GNU-stack
This section is used in Linux object files for declaring stack attributes.
This section is of type
.BR SHT_PROGBITS .
The only attribute used is
.BR SHF_EXECINSTR .
This indicates to the GNU linker that the object file requires an
executable stack.
.TP
.IR .note.openbsd.ident
OpenBSD native executables usually contain this section
to identify themselves so the kernel can bypass any compatibility
ELF binary emulation tests when loading the file.
.TP
.IR .plt
This section holds the procedure linkage table.
This section is of type
.BR SHT_PROGBITS .
The attributes are processor-specific.
.TP
.IR .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
.BR 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
.BR .text
normally would have the name
.BR .rel.text .
This section is of type
.BR SHT_REL .
.TP
.IR .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
.BR 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
.BR .text
normally would have the name
.BR .rela.text .
This section is of type
.BR SHT_RELA .
.TP
.IR .rodata
This section holds read-only data that typically contributes to a
nonwritable segment in the process image.
This section is of type
.BR SHT_PROGBITS .
The attribute used is
.BR SHF_ALLOC .
.TP
.IR .rodata1
This section holds read-only data that typically contributes to a
nonwritable segment in the process image.
This section is of type
.BR SHT_PROGBITS .
The attribute used is
.BR SHF_ALLOC .
.TP
.IR .shstrtab
This section holds section names.
This section is of type
.BR SHT_STRTAB .
No attribute types are used.
.TP
.IR .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
.BR SHF_ALLOC
bit.
Otherwise, the bit will be off.
This section is of type
.BR SHT_STRTAB .
.TP
.IR .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
.BR SHF_ALLOC
bit.
Otherwise, the bit will be off.
This section is of type
.BR SHT_SYMTAB .
.TP
.IR .text
This section holds the
"text",
or executable instructions, of a program.
This section is of type
.BR SHT_PROGBITS .
The attributes used are
.BR SHF_ALLOC
and
.BR SHF_EXECINSTR .
.\"
.SS String and symbol tables
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 (\(aq\\0\(aq).
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.
.PP
.in +4n
.EX
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;
.EE
.in
.PP
.in +4n
.EX
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;
.EE
.in
.PP
The 32-bit and 64-bit versions have the same members, just in a different
order.
.TP \n[l1_indent]
.IR st_name
This member holds an index into the object file's symbol string table,
which holds character representations of the symbol names.
If the value
is nonzero, it represents a string table index that gives the symbol
name.
Otherwise, the symbol has no name.
.TP
.IR st_value
This member gives the value of the associated symbol.
.TP
.IR st_size
Many symbols have associated sizes.
This member holds zero if the symbol
has no size or an unknown size.
.TP
.IR st_info
This member specifies the symbol's type and binding attributes:
.RS \n[l1_indent]
.TP 12
.BR STT_NOTYPE
The symbol's type is not defined.
.TP
.BR STT_OBJECT
The symbol is associated with a data object.
.TP
.BR STT_FUNC
The symbol is associated with a function or other executable code.
.TP
.BR STT_SECTION
The symbol is associated with a section.
Symbol table entries of
this type exist primarily for relocation and normally have
.BR STB_LOCAL
bindings.
.TP
.BR STT_FILE
By convention, the symbol's name gives the name of the source file
associated with the object file.
A file symbol has
.BR STB_LOCAL
bindings, its section index is
.BR SHN_ABS ,
and it precedes the other
.BR STB_LOCAL
symbols of the file, if it is present.
.TP
.BR STT_LOPROC ", " STT_HIPROC
Values in the inclusive range
.RB [ STT_LOPROC ", " STT_HIPROC ]
are reserved for processor-specific semantics.
.TP
.BR 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.
.TP
.BR 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.
.TP
.BR STB_WEAK
Weak symbols resemble global symbols, but their definitions have lower
precedence.
.TP
.BR STB_LOPROC ", " STB_HIPROC
Values in the inclusive range
.RB [ STB_LOPROC ", " STB_HIPROC ]
are reserved for processor-specific semantics.
.RE
.IP
There are macros for packing and unpacking the binding and type fields:
.RS \n[l1_indent]
.TP
.BR ELF32_ST_BIND( \fIinfo\fP ) ", " ELF64_ST_BIND( \fIinfo\fP )
Extract a binding from an
.I st_info
value.
.TP
.BR ELF32_ST_TYPE( \fIinfo ) ", " ELF64_ST_TYPE( \fIinfo\fP )
Extract a type from an
.I st_info
value.
.TP
.BR ELF32_ST_INFO( \fIbind\fP ", " \fItype\fP ) ", " \
ELF64_ST_INFO( \fIbind\fP ", " \fItype\fP )
Convert a binding and a type into an
.I st_info
value.
.RE
.TP
.IR st_other
This member defines the symbol visibility.
.RS \n[l1_indent]
.TP 16
.PD 0
.BR STV_DEFAULT
Default symbol visibility rules.
Global and weak symbols are available to other modules;
references in the local module can be interposed
by definitions in other modules.
.TP
.BR STV_INTERNAL
Processor-specific hidden class.
.TP
.BR STV_HIDDEN
Symbol is unavailable to other modules;
references in the local module always resolve to the local symbol
(i.e., the symbol can't be interposed by definitions in other modules).
.TP
.BR STV_PROTECTED
Symbol is available to other modules,
but references in the local module always resolve to the local symbol.
.PD
.PP
There are macros for extracting the visibility type:
.PP
.BR ELF32_ST_VISIBILITY (other)
or
.BR ELF64_ST_VISIBILITY (other)
.RE
.TP
.IR st_shndx
Every symbol table entry is
"defined"
in relation to some section.
This member holds the relevant section
header table index.
.\"
.SS Relocation entries (Rel & Rela)
Relocation is the process of connecting symbolic references with
symbolic definitions.
Relocatable files must have information that
describes how to modify their section contents, thus allowing executable
and shared object files to hold the right information for a process's
program image.
Relocation entries are these data.
.PP
Relocation structures that do not need an addend:
.PP
.in +4n
.EX
typedef struct {
    Elf32_Addr r_offset;
    uint32_t   r_info;
} Elf32_Rel;
.EE
.in
.PP
.in +4n
.EX
typedef struct {
    Elf64_Addr r_offset;
    uint64_t   r_info;
} Elf64_Rel;
.EE
.in
.PP
Relocation structures that need an addend:
.PP
.in +4n
.EX
typedef struct {
    Elf32_Addr r_offset;
    uint32_t   r_info;
    int32_t    r_addend;
} Elf32_Rela;
.EE
.in
.PP
.in +4n
.EX
typedef struct {
    Elf64_Addr r_offset;
    uint64_t   r_info;
    int64_t    r_addend;
} Elf64_Rela;
.EE
.in
.TP \n[l1_indent]
.IR 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.
.TP
.IR 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
.BR ELF[32|64]_R_TYPE
or
.BR ELF[32|64]_R_SYM ,
respectively, to the entry's
.IR r_info
member.
.TP
.IR r_addend
This member specifies a constant addend used to compute the value to be
stored into the relocatable field.
.\"
.SS Dynamic tags (Dyn)
The
.I .dynamic
section contains a series of structures that hold relevant
dynamic linking information.
The
.I d_tag
member controls the interpretation
of
.IR d_un .
.PP
.in +4n
.EX
typedef struct {
    Elf32_Sword    d_tag;
    union {
        Elf32_Word d_val;
        Elf32_Addr d_ptr;
    } d_un;
} Elf32_Dyn;
extern Elf32_Dyn _DYNAMIC[];
.EE
.in
.PP
.in +4n
.EX
typedef struct {
    Elf64_Sxword    d_tag;
    union {
        Elf64_Xword d_val;
        Elf64_Addr  d_ptr;
    } d_un;
} Elf64_Dyn;
extern Elf64_Dyn _DYNAMIC[];
.EE
.in
.TP \n[l1_indent]
.IR d_tag
This member may have any of the following values:
.RS \n[l1_indent]
.TP 12
.BR DT_NULL
Marks end of dynamic section
.TP
.BR DT_NEEDED
String table offset to name of a needed library
.TP
.BR DT_PLTRELSZ
Size in bytes of PLT relocation entries
.TP
.BR DT_PLTGOT
Address of PLT and/or GOT
.TP
.BR DT_HASH
Address of symbol hash table
.TP
.BR DT_STRTAB
Address of string table
.TP
.BR DT_SYMTAB
Address of symbol table
.TP
.BR DT_RELA
Address of Rela relocation table
.TP
.BR DT_RELASZ
Size in bytes of the Rela relocation table
.TP
.BR DT_RELAENT
Size in bytes of a Rela relocation table entry
.TP
.BR DT_STRSZ
Size in bytes of string table
.TP
.BR DT_SYMENT
Size in bytes of a symbol table entry
.TP
.BR DT_INIT
Address of the initialization function
.TP
.BR DT_FINI
Address of the termination function
.TP
.BR DT_SONAME
String table offset to name of shared object
.TP
.BR DT_RPATH
String table offset to library search path (deprecated)
.TP
.BR DT_SYMBOLIC
Alert linker to search this shared object before the executable for symbols
.TP
.BR DT_REL
Address of Rel relocation table
.TP
.BR DT_RELSZ
Size in bytes of Rel relocation table
.TP
.BR DT_RELENT
Size in bytes of a Rel table entry
.TP
.BR DT_PLTREL
Type of relocation entry to which the PLT refers (Rela or Rel)
.TP
.BR DT_DEBUG
Undefined use for debugging
.TP
.BR DT_TEXTREL
Absence of this entry indicates that no relocation entries should
apply to a nonwritable segment
.TP
.BR DT_JMPREL
Address of relocation entries associated solely with the PLT
.TP
.BR DT_BIND_NOW
Instruct dynamic linker to process all relocations before
transferring control to the executable
.TP
.BR DT_RUNPATH
String table offset to library search path
.TP
.BR DT_LOPROC ", " DT_HIPROC
Values in the inclusive range
.RB [ DT_LOPROC ", " DT_HIPROC ]
are reserved for processor-specific semantics
.RE
.TP
.IR d_val
This member represents integer values with various interpretations.
.TP
.IR 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.
.TP
.I _DYNAMIC
Array containing all the dynamic structures in the
.I .dynamic
section.
This is automatically populated by the linker.
.\" GABI ELF Reference for Note Sections:
.\" http://www.sco.com/developers/gabi/latest/ch5.pheader.html#note_section
.\"
.\" Note that it implies the sizes and alignments of notes depend on the ELF
.\" size (e.g. 32-bit ELFs have three 4-byte words and use 4-byte alignment
.\" while 64-bit ELFs use 8-byte words & alignment), but that is not the case
.\" in the real world.  Notes always have three 4-byte words as can be seen
.\" in the source links below (remember that Elf64_Word is a 32-bit quantity).
.\" glibc:    https://sourceware.org/git/?p=glibc.git;a=blob;f=elf/elf.h;h=9e59b3275917549af0cebe1f2de9ded3b7b10bf2#l1173
.\" binutils: https://sourceware.org/git/?p=binutils-gdb.git;a=blob;f=binutils/readelf.c;h=274ddd17266aef6e4ad1f67af8a13a21500ff2af#l15943
.\" Linux:    https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/include/uapi/linux/elf.h?h=v4.8#n422
.\" Solaris:  https://docs.oracle.com/cd/E23824_01/html/819-0690/chapter6-18048.html
.\" FreeBSD:  https://svnweb.freebsd.org/base/head/sys/sys/elf_common.h?revision=303677&view=markup#l33
.\" NetBSD:   https://www.netbsd.org/docs/kernel/elf-notes.html
.\" OpenBSD:  https://github.com/openbsd/src/blob/master/sys/sys/exec_elf.h#L533
.\"
.SS Notes (Nhdr)
ELF notes allow for appending arbitrary information for the system to use.
They are largely used by core files
.RI ( e_type
of
.BR ET_CORE ),
but many projects define their own set of extensions.
For example,
the GNU tool chain uses ELF notes to pass information from
the linker to the C library.
.PP
Note sections contain a series of notes (see the
.I struct
definitions below).
Each note is followed by the name field (whose length is defined in
\fIn_namesz\fR) and then by the descriptor field (whose length is defined in
\fIn_descsz\fR) and whose starting address has a 4 byte alignment.
Neither field is defined in the note struct due to their arbitrary lengths.
.PP
An example for parsing out two consecutive notes should clarify their layout
in memory:
.PP
.in +4n
.EX
void *memory, *name, *desc;
Elf64_Nhdr *note, *next_note;

/* The buffer is pointing to the start of the section/segment */
note = memory;

/* If the name is defined, it follows the note */
name = note->n_namesz == 0 ? NULL : memory + sizeof(*note);

/* If the descriptor is defined, it follows the name
   (with alignment) */

desc = note->n_descsz == 0 ? NULL :
       memory + sizeof(*note) + ALIGN_UP(note->n_namesz, 4);

/* The next note follows both (with alignment) */
next_note = memory + sizeof(*note) +
                     ALIGN_UP(note->n_namesz, 4) +
                     ALIGN_UP(note->n_descsz, 4);
.EE
.in
.PP
Keep in mind that the interpretation of
.I n_type
depends on the namespace defined by the
.I n_namesz
field.
If the
.I n_namesz
field is not set (e.g., is 0), then there are two sets of notes:
one for core files and one for all other ELF types.
If the namespace is unknown, then tools will usually fallback to these sets
of notes as well.
.PP
.in +4n
.EX
typedef struct {
    Elf32_Word n_namesz;
    Elf32_Word n_descsz;
    Elf32_Word n_type;
} Elf32_Nhdr;
.EE
.in
.PP
.in +4n
.EX
typedef struct {
    Elf64_Word n_namesz;
    Elf64_Word n_descsz;
    Elf64_Word n_type;
} Elf64_Nhdr;
.EE
.in
.TP \n[l1_indent]
.IR n_namesz
The length of the name field in bytes.
The contents will immediately follow this note in memory.
The name is null terminated.
For example, if the name is "GNU", then
.I n_namesz
will be set to 4.
.TP
.IR n_descsz
The length of the descriptor field in bytes.
The contents will immediately follow the name field in memory.
.TP
.IR n_type
Depending on the value of the name field, this member may have any of the
following values:
.RS \n[l1_indent]
.TP 5
.B Core files (e_type = ET_CORE)
Notes used by all core files.
These are highly operating system or architecture specific and often require
close coordination with kernels, C libraries, and debuggers.
These are used when the namespace is the default (i.e.,
.I n_namesz
will be set to 0), or a fallback when the namespace is unknown.
.RS
.TP 21
.PD 0
.B NT_PRSTATUS
prstatus struct
.TP
.B NT_FPREGSET
fpregset struct
.TP
.B NT_PRPSINFO
prpsinfo struct
.TP
.B NT_PRXREG
prxregset struct
.TP
.B NT_TASKSTRUCT
task structure
.TP
.B NT_PLATFORM
String from sysinfo(SI_PLATFORM)
.TP
.B NT_AUXV
auxv array
.TP
.B NT_GWINDOWS
gwindows struct
.TP
.B NT_ASRS
asrset struct
.TP
.B NT_PSTATUS
pstatus struct
.TP
.B NT_PSINFO
psinfo struct
.TP
.B NT_PRCRED
prcred struct
.TP
.B NT_UTSNAME
utsname struct
.TP
.B NT_LWPSTATUS
lwpstatus struct
.TP
.B NT_LWPSINFO
lwpinfo struct
.TP
.B NT_PRFPXREG
fprxregset struct
.TP
.B NT_SIGINFO
siginfo_t (size might increase over time)
.TP
.B NT_FILE
Contains information about mapped files
.TP
.B NT_PRXFPREG
user_fxsr_struct
.TP
.B NT_PPC_VMX
PowerPC Altivec/VMX registers
.TP
.B NT_PPC_SPE
PowerPC SPE/EVR registers
.TP
.B NT_PPC_VSX
PowerPC VSX registers
.TP
.B NT_386_TLS
i386 TLS slots (struct user_desc)
.TP
.B NT_386_IOPERM
x86 io permission bitmap (1=deny)
.TP
.B NT_X86_XSTATE
x86 extended state using xsave
.TP
.B NT_S390_HIGH_GPRS
s390 upper register halves
.TP
.B NT_S390_TIMER
s390 timer register
.TP
.B NT_S390_TODCMP
s390 time-of-day (TOD) clock comparator register
.TP
.B NT_S390_TODPREG
s390 time-of-day (TOD) programmable register
.TP
.B NT_S390_CTRS
s390 control registers
.TP
.B NT_S390_PREFIX
s390 prefix register
.TP
.B NT_S390_LAST_BREAK
s390 breaking event address
.TP
.B NT_S390_SYSTEM_CALL
s390 system call restart data
.TP
.B NT_S390_TDB
s390 transaction diagnostic block
.TP
.B NT_ARM_VFP
ARM VFP/NEON registers
.TP
.B NT_ARM_TLS
ARM TLS register
.TP
.B NT_ARM_HW_BREAK
ARM hardware breakpoint registers
.TP
.B NT_ARM_HW_WATCH
ARM hardware watchpoint registers
.TP
.B NT_ARM_SYSTEM_CALL
ARM system call number
.PD
.RE
.TP
.B n_name = GNU
Extensions used by the GNU tool chain.
.RS
.TP
.B NT_GNU_ABI_TAG
Operating system (OS) ABI information.
The desc field will be 4 words:
.IP
.PD 0
.RS
.IP \(bu 2
word 0: OS descriptor
(\fBELF_NOTE_OS_LINUX\fR, \fBELF_NOTE_OS_GNU\fR, and so on)`
.IP \(bu
word 1: major version of the ABI
.IP \(bu
word 2: minor version of the ABI
.IP \(bu
word 3: subminor version of the ABI
.RE
.PD
.TP
.B NT_GNU_HWCAP
Synthetic hwcap information.
The desc field begins with two words:
.IP
.PD 0
.RS
.IP \(bu 2
word 0: number of entries
.IP \(bu
word 1: bit mask of enabled entries
.RE
.PD
.IP
Then follow variable-length entries, one byte followed by a null-terminated
hwcap name string.
The byte gives the bit number to test if enabled, (1U << bit) & bit mask.
.TP
.B NT_GNU_BUILD_ID
Unique build ID as generated by the GNU
.BR ld (1)
.BR \-\-build\-id
option.
The desc consists of any nonzero number of bytes.
.TP
.B NT_GNU_GOLD_VERSION
The desc contains the GNU Gold linker version used.
.RE
.TP
.B Default/unknown namespace (e_type != ET_CORE)
These are used when the namespace is the default (i.e.,
.I n_namesz
will be set to 0), or a fallback when the namespace is unknown.
.RS
.TP 21
.PD 0
.B NT_VERSION
A version string of some sort.
.TP
.B NT_ARCH
Architecture information.
.PD
.RE
.PP
.RE
.SH NOTES
.\" OpenBSD
.\" ELF support first appeared in
.\" OpenBSD 1.2,
.\" although not all supported platforms use it as the native
.\" binary file format.
ELF first appeared in
System V.
The ELF format is an adopted standard.
.PP
The extensions for
.IR e_phnum ,
.IR e_shnum
and
.IR e_strndx
respectively are
Linux extensions.
Sun, BSD and AMD64 also support them; for further information,
look under SEE ALSO.
.\" .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
.\" man page.
.SH SEE ALSO
.BR as (1),
.BR elfedit (1),
.BR gdb (1),
.BR ld (1),
.BR nm (1),
.BR objdump (1),
.BR patchelf (1),
.BR readelf (1),
.BR size (1),
.BR strings (1),
.BR strip (1),
.BR execve (2),
.BR dl_iterate_phdr (3),
.BR core (5)
.PP
Hewlett-Packard,
.IR "Elf-64 Object File Format" .
.PP
Santa Cruz Operation,
.IR "System V Application Binary Interface" .
.PP
UNIX System Laboratories,
"Object Files",
.IR "Executable and Linking Format (ELF)" .
.PP
Sun Microsystems,
.IR "Linker and Libraries Guide" .
.PP
AMD64 ABI Draft,
.IR "System V Application Binary Interface AMD64 Architecture Processor Supplement" .
.PP