xref: /freebsd/share/man/man5/elf.5 (revision 202f6d830dbf0a67484b19b324da1f1858c0eddc)

.\" 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.
.\"
.\" $FreeBSD$
.\"
.Dd March 11, 2020
.Dt ELF 5
.Os
.Sh NAME
.Nm elf
.Nd format of ELF executable binary files
.Sh SYNOPSIS
.In elf.h
.Sh DESCRIPTION
The header file
.In 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
.In elf.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 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 include both
.In sys/elf32.h
and
.In sys/elf64.h
instead of
.In elf.h .
Furthermore, all types and structures need to be identified by either
.Dq Elf32_xxx
or
.Dq Elf64_xxx .
The macros need to be identified by
.Dq ELF32_xxx
or
.Dq ELF64_xxx .
.Pp
Whatever the system's architecture is, it will always include
.In sys/elf_common.h
as well as
.In sys/elf_generic.h .
.Pp
These header files describe the above mentioned headers as C structures
and also include structures for dynamic sections, relocation sections and
symbol tables.
.Pp
The following types are being used for 32-bit architectures:
.Bd -literal -offset indent
Elf32_Addr	Unsigned 32-bit program address
Elf32_Half	Unsigned 16-bit field
Elf32_Lword	Unsigned 64-bit field
Elf32_Off	Unsigned 32-bit file offset
Elf32_Sword	Signed 32-bit field or integer
Elf32_Word	Unsigned 32-bit field or integer
.Ed
.Pp
For 64-bit architectures we have the following types:
.Bd -literal -offset indent
Elf64_Addr	Unsigned 64-bit program address
Elf64_Half	Unsigned 16-bit field
Elf64_Lword	Unsigned 64-bit field
Elf64_Off	Unsigned 64-bit file offset
Elf64_Sword	Signed 32-bit field
Elf64_Sxword	Signed 64-bit field or integer
Elf64_Word	Unsigned 32-bit field
Elf64_Xword	Unsigned 64-bit field or integer
.Ed
.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
typedef struct {
        unsigned char   e_ident[EI_NIDENT];
        Elf32_Half      e_type;
        Elf32_Half      e_machine;
        Elf32_Word      e_version;
        Elf32_Addr      e_entry;
        Elf32_Off       e_phoff;
        Elf32_Off       e_shoff;
        Elf32_Word      e_flags;
        Elf32_Half      e_ehsize;
        Elf32_Half      e_phentsize;
        Elf32_Half      e_phnum;
        Elf32_Half      e_shentsize;
        Elf32_Half      e_shnum;
        Elf32_Half      e_shstrndx;
} Elf32_Ehdr;
.Ed
.Bd -literal -offset indent
typedef struct {
	unsigned char   e_ident[EI_NIDENT];
	Elf64_Half      e_type;
	Elf64_Half      e_machine;
	Elf64_Word      e_version;
	Elf64_Addr      e_entry;
	Elf64_Off       e_phoff;
	Elf64_Off       e_shoff;
	Elf64_Word      e_flags;
	Elf64_Half      e_ehsize;
	Elf64_Half      e_phentsize;
	Elf64_Half      e_phnum;
	Elf64_Half      e_shentsize;
	Elf64_Half      e_shnum;
	Elf64_Half      e_shstrndx;
} Elf64_Ehdr;
.Ed
.Pp
The fields have the following meanings:
.Pp
.Bl -tag -width "e_phentsize" -compact -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:
.Pp
.Bl -tag -width "EI_ABIVERSION" -compact
.It Dv EI_MAG0
The first byte of the magic number.
It must be filled with
.Sy ELFMAG0 .
.It Dv EI_MAG1
The second byte of the magic number.
It must be filled with
.Sy ELFMAG1 .
.It Dv EI_MAG2
The third byte of the magic number.
It must be filled with
.Sy ELFMAG2 .
.It Dv EI_MAG3
The fourth byte of the magic number.
It must be filled with
.Sy ELFMAG3 .
.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.
The following values are currently defined:
.Pp
.Bl -tag -width "ELFOSABI_STANDALONE" -compact
.It Dv ELFOSABI_SYSV
UNIX System V ABI.
.It Dv ELFOSABI_HPUX
HP-UX operating system ABI.
.It Dv ELFOSABI_NETBSD
.Nx
operating system ABI.
.It Dv ELFOSABI_LINUX
GNU/Linux operating system ABI.
.It Dv ELFOSABI_HURD
GNU/Hurd operating system ABI.
.It Dv ELFOSABI_86OPEN
86Open Common IA32 ABI.
.It Dv ELFOSABI_SOLARIS
Solaris operating system ABI.
.It Dv ELFOSABI_MONTEREY
Monterey project ABI.
.It Dv ELFOSABI_IRIX
IRIX operating system ABI.
.It Dv ELFOSABI_FREEBSD
.Fx
operating system ABI.
.It Dv ELFOSABI_TRU64
TRU64 UNIX operating system ABI.
.It Dv ELFOSABI_ARM
ARM architecture ABI.
.It Dv ELFOSABI_STANDALONE
Standalone (embedded) ABI.
.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:
.Pp
.Bl -tag -width "EM_MIPS_RS4_BE" -compact
.It Dv EM_NONE
An unknown machine.
.It Dv EM_M32
AT&T WE 32100.
.It Dv EM_SPARC
Sun Microsystems SPARC.
.It Dv EM_386
Intel 80386.
.It Dv EM_68K
Motorola 68000.
.It Dv EM_88K
Motorola 88000.
.It Dv EM_486
Intel 80486.
.It Dv EM_860
Intel 80860.
.It Dv EM_MIPS
MIPS RS3000 (big-endian only).
.It Dv EM_MIPS_RS4_BE
MIPS RS4000 (big-endian only).
.It Dv EM_SPARC64
SPARC v9 64-bit unofficial.
.It Dv EM_PARISC
HPPA.
.It Dv EM_PPC
PowerPC.
.It Dv EM_ALPHA
Compaq [DEC] Alpha.
.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.
If the file is using extended program header numbering, then the
.Sy e_phnum
member will contain the value
.Dv PN_XNUM
and the actual number of program header table entries will be stored
in the
.Sy sh_info
member of the section header at index
.Dv SHN_UNDEF .
The product of
.Sy e_phentsize
and the number of program header table entries gives the program
header 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.
If the file is using extended section numbering, then the
.Sy e_shnum
member will be zero and the actual section number will be stored in the
.Sy sh_size
member of the section header at index
.Dv SHN_UNDEF .
If a file has no section header table, both the
.Sy e_shnum
and the
.Sy e_shoff
fields of the ELF header will be zero.
The product of
.Sy e_shentsize
and the number of sections in the file gives the section header
table's size in bytes.
.It Dv e_shstrndx
This member holds the section header table index of the entry associated
with the section name string table.
If extended section numbering is being used, this field will hold the
value
.Sy SHN_XINDEX ,
and the actual section header table index will be present in the
.Sy sh_link
field of the section header entry at index
.Dv SHN_UNDEF .
If the file has no section name string
table, this member holds the value
.Sy SHN_UNDEF .
.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.
As with the Elf executable header, the program header
also has different versions depending on the architecture:
.Bd -literal -offset indent
typedef struct {
        Elf32_Word      p_type;
        Elf32_Off       p_offset;
        Elf32_Addr      p_vaddr;
        Elf32_Addr      p_paddr;
        Elf32_Word      p_filesz;
        Elf32_Word      p_memsz;
        Elf32_Word      p_flags;
        Elf32_Word      p_align;
} Elf32_Phdr;
.Ed
.Bd -literal -offset indent
typedef struct {
        Elf64_Word      p_type;
        Elf64_Word      p_flags;
        Elf64_Off       p_offset;
        Elf64_Addr      p_vaddr;
        Elf64_Addr      p_paddr;
        Elf64_Xword     p_filesz;
        Elf64_Xword     p_memsz;
        Elf64_Xword     p_align;
} Elf64_Phdr;
.Ed
.Pp
The main difference between the 32-bit and the 64-bit program header lies
only in the location of a
.Sy p_flags
member in the total struct.
.Pp
.Bl -tag -width "p_offset" -compact -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.
.Pp
.Bl -tag -width "PT_DYNAMIC" -compact
.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
This value up to and including
.Sy PT_HIPROC
are reserved for processor-specific semantics.
.It Dv PT_HIPROC
This value down to and including
.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
An 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:
.Pp
.Bl -tag -width "SHN_LORESERVE" -compact
.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
This value up to and including
.Sy SHN_HIPROC
are reserved for processor-specific semantics.
.It Dv SHN_HIPROC
This value down to and including
.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.
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 {
	Elf32_Word      sh_name;
	Elf32_Word      sh_type;
	Elf32_Word      sh_flags;
	Elf32_Addr      sh_addr;
	Elf32_Off       sh_offset;
	Elf32_Word      sh_size;
	Elf32_Word      sh_link;
	Elf32_Word      sh_info;
	Elf32_Word      sh_addralign;
	Elf32_Word      sh_entsize;
} Elf32_Shdr;
.Ed
.Bd -literal -offset indent
typedef struct {
	Elf64_Word      sh_name;
	Elf64_Word      sh_type;
	Elf64_Xword     sh_flags;
	Elf64_Addr      sh_addr;
	Elf64_Off       sh_offset;
	Elf64_Xword     sh_size;
	Elf64_Word      sh_link;
	Elf64_Word      sh_info;
	Elf64_Xword     sh_addralign;
	Elf64_Xword     sh_entsize;
} Elf64_Shdr;
.Ed
.Pp
.Bl -tag -width "sh_addralign" -compact
.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.
.Pp
.Bl -tag -width "SHT_PROGBITS" -compact
.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
The 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 SHN_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.
All 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 SHN_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 SHN_SYMTAB
section.
.It Dv SHT_LOPROC
This value up to and including
.Sy SHT_HIPROC
are reserved for processor-specific semantics.
.It Dv SHT_HIPROC
This value down to and including
.Sy SHT_LOPROC
are 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
The 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
The 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 the section will appear 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" -compact
.It .bss
(Block Started by Symbol)
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 attributes 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 .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 .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.
.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 hold 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 .
.It .jcr
This section holds information about Java classes that must
be registered.
.It .eh_frame
This section holds information used for C++ exception-handling.
.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 character.
Similarly, a string table's last byte is defined to
hold a null character, 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 {
	Elf32_Word      st_name;
	Elf32_Addr      st_value;
	Elf32_Word      st_size;
	unsigned char   st_info;
	unsigned char   st_other;
	Elf32_Half      st_shndx;
} Elf32_Sym;
.Ed
.Bd -literal -offset indent
typedef struct {
	Elf64_Word      st_name;
	unsigned char   st_info;
	unsigned char   st_other;
	Elf64_Half      st_shndx;
	Elf64_Addr      st_value;
	Elf64_Xword     st_size;
} Elf64_Sym;
.Ed
.Pp
.Bl -tag -width "st_value" -compact
.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:
.Pp
.Bl -tag -width "STT_SECTION" -compact
.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
are reserved for processor-specific semantics.
.It Dv STT_HIPROC
This value down to and including
.Sy STT_LOPROC
are reserved for processor-specific semantics.
.El
.Pp
.Bl -tag -width "STB_GLOBAL" -compact
.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 file
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
are reserved for processor-specific semantics.
.It Dv STB_HIPROC
This value down to and including
.Sy STB_LOPROC
are reserved for processor-specific semantics.
.Pp
There are macros for packing and unpacking the binding and type fields:
.Pp
.Bl -tag -width "ELF32_ST_INFO(bind, type)" -compact
.It Xo
.Fn ELF32_ST_BIND info
.Xc
or
.Fn ELF64_ST_BIND info
extract a binding from an st_info value.
.It Xo
.Fn ELF64_ST_TYPE info
.Xc
or
.Fn ELF32_ST_TYPE info
extract a type from an st_info value.
.It Xo
.Fn ELF32_ST_INFO bind type
.Xc
or
.Fn ELF64_ST_INFO bind type
convert a binding and a type into an st_info value.
.El
.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;
	Elf32_Word      r_info;
} Elf32_Rel;
.Ed
.Bd -literal -offset indent
typedef struct {
	Elf64_Addr      r_offset;
	Elf64_Xword     r_info;
} Elf64_Rel;
.Ed
.Pp
Relocation structures that need an addend:
.Bd -literal -offset indent
typedef struct {
	Elf32_Addr      r_offset;
	Elf32_Word      r_info;
	Elf32_Sword     r_addend;
} Elf32_Rela;
.Ed
.Bd -literal -offset indent
typedef struct {
	Elf64_Addr      r_offset;
	Elf64_Xword     r_info;
	Elf64_Sxword    r_addend;
} Elf64_Rela;
.Ed
.Pp
.Bl -tag -width "r_offset" -compact
.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
.Ss Note Section
ELF note sections consist of entries with the following format:
.Bl -column -offset indent "namesz" "32 bits" "Null-terminated originator name"
.Sy Field       Ta Sy Size    Ta Sy Description
.It Va namesz   Ta 32 bits    Ta Size of "name"
.It Va descsz   Ta 32 bits    Ta Size of "desc"
.It Va type     Ta 32 bits    Ta OS-dependent note type
.It Va name     Ta Va namesz  Ta Null-terminated originator name
.It Va desc     Ta Va descsz  Ta OS-dependent note data
.El
.Pp
The
.Va name
and
.Va desc
fields are padded to ensure 4-byte alignemnt.
.Va namesz
and
.Va descsz
specify the unpadded length.
.Sh SEE ALSO
.Xr as 1 ,
.Xr gdb 1 ,
.Xr ld 1 ,
.Xr objdump 1 ,
.Xr readelf 1 ,
.Xr execve 2 ,
.Xr ar 5 ,
.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
The ELF header files made their appearance in
.Fx 2.2.6 .
ELF in itself first appeared in
.At V .
The ELF format is an adopted standard.
.Sh AUTHORS
This manual page was written by
.An Jeroen Ruigrok van der Werven Aq Mt asmodai@FreeBSD.org
with inspiration from BSDi's
.Bsx
.Nm
manpage.