ELF(5) FreeBSD File Formats Manual ELF(5)
NAME
elf - format of ELF executable binary files
SYNOPSIS
#include <elf.h>
DESCRIPTION
The header file <elf.h> defines the format of ELF executable binary
files. Amongst these files are normal executable files, relocatable
object files, core files and shared libraries.
An executable file using the ELF file format consists of an ELF header,
followed by a program header table or a section header table, or both.
The ELF header is always at offset zero of the file. The program header
table and the section header table's offset in the file are defined in
the ELF header. The two tables describe the rest of the particularities
of the file.
Applications which wish to process ELF binary files for their native
architecture only should include <elf.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 whether the host is 32-bit or 64-bit.
Should an application need to process ELF files of an unknown
architecture then the application needs to include both <sys/elf32.h> and
<sys/elf64.h> instead of <elf.h>. Furthermore, all types and structures
need to be identified by either "Elf32_xxx" or "Elf64_xxx". The macros
need to be identified by "ELF32_xxx" or "ELF64_xxx".
Whatever the system's architecture is, it will always include
<sys/elf_common.h> as well as <sys/elf_generic.h>.
These header files describe the above mentioned headers as C structures
and also include structures for dynamic sections, relocation sections and
symbol tables.
The following types are being used for 32-bit architectures:
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
For 64-bit architectures we have the following types:
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
All data structures that the file format defines follow the "natural"
size and alignment guidelines for the relevant class. If necessary, data
structures contain explicit padding to ensure 4-byte alignment for 4-byte
objects, to force structure sizes to a multiple of 4, etc.
The ELF header is described by the type Elf32_Ehdr or Elf64_Ehdr:
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;
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;
The fields have the following meanings:
e_ident This array of bytes specifies to interpret the file,
independent of the processor or the file's remaining
contents. Within this array everything is named by
macros, which start with the prefix EI_ and may
contain values which start with the prefix ELF. The
following macros are defined:
EI_MAG0 The first byte of the magic number. It
must be filled with ELFMAG0.
EI_MAG1 The second byte of the magic number.
It must be filled with ELFMAG1.
EI_MAG2 The third byte of the magic number. It
must be filled with ELFMAG2.
EI_MAG3 The fourth byte of the magic number.
It must be filled with ELFMAG3.
EI_CLASS The fifth byte identifies the
architecture for this binary:
ELFCLASSNONE This class is invalid.
ELFCLASS32 This defines the 32-bit
architecture. It
supports machines with
files and virtual address
spaces up to 4 Gigabytes.
ELFCLASS64 This defines the 64-bit
architecture.
EI_DATA The sixth byte specifies the data
encoding of the processor-specific data
in the file. Currently these encodings
are supported:
ELFDATANONE Unknown data format.
ELFDATA2LSB Two's complement, little-
endian.
ELFDATA2MSB Two's complement, big-
endian.
EI_VERSION The version number of the ELF
specification:
EV_NONE Invalid version.
EV_CURRENT Current version.
EI_OSABI This byte identifies the operating
system and ABI to which the object is
targeted. Some fields in other ELF
structures have flags and values that
have platform specific meanings; the
interpretation of those fields is
determined by the value of this byte.
The following values are currently
defined:
ELFOSABI_SYSV UNIX System V ABI.
ELFOSABI_HPUX HP-UX operating
system ABI.
ELFOSABI_NETBSD NetBSD operating
system ABI.
ELFOSABI_LINUX GNU/Linux
operating system
ABI.
ELFOSABI_HURD GNU/Hurd operating
system ABI.
ELFOSABI_86OPEN 86Open Common IA32
ABI.
ELFOSABI_SOLARIS Solaris operating
system ABI.
ELFOSABI_MONTEREY Monterey project
ABI.
ELFOSABI_IRIX IRIX operating
system ABI.
ELFOSABI_FREEBSD FreeBSD operating
system ABI.
ELFOSABI_TRU64 TRU64 UNIX
operating system
ABI.
ELFOSABI_ARM ARM architecture
ABI.
ELFOSABI_STANDALONE Standalone
(embedded) ABI.
EI_ABIVERSION This byte identifies the version of the
ABI to which the object is targeted.
This field is used to distinguish among
incompatible versions of an ABI. The
interpretation of this version number
is dependent on the ABI identified by
the EI_OSABI field. Applications
conforming to this specification use
the value 0.
EI_PAD Start of padding. These bytes are
reserved and set to zero. Programs
which read them should ignore them.
The value for EI_PAD will change in the
future if currently unused bytes are
given meanings.
EI_BRAND Start of architecture identification.
EI_NIDENT The size of the e_ident array.
e_type This member of the structure identifies the object
file type:
ET_NONE An unknown type.
ET_REL A relocatable file.
ET_EXEC An executable file.
ET_DYN A shared object.
ET_CORE A core file.
e_machine This member specifies the required architecture for an
individual file:
EM_NONE An unknown machine.
EM_M32 AT&T WE 32100.
EM_SPARC Sun Microsystems SPARC.
EM_386 Intel 80386.
EM_68K Motorola 68000.
EM_88K Motorola 88000.
EM_486 Intel 80486.
EM_860 Intel 80860.
EM_MIPS MIPS RS3000 (big-endian only).
EM_MIPS_RS4_BE MIPS RS4000 (big-endian only).
EM_SPARC64 SPARC v9 64-bit unofficial.
EM_PARISC HPPA.
EM_PPC PowerPC.
EM_ALPHA Compaq [DEC] Alpha.
e_version This member identifies the file version:
EV_NONE Invalid version
EV_CURRENT Current version
e_entry This member gives the virtual address to which the
system first transfers control, thus starting the
process. If the file has no associated entry point,
this member holds zero.
e_phoff This member holds the program header table's file
offset in bytes. If the file has no program header
table, this member holds zero.
e_shoff This member holds the section header table's file
offset in bytes. If the file has no section header
table this member holds zero.
e_flags This member holds processor-specific flags associated
with the file. Flag names take the form
EF_`machine_flag'. Currently no flags have been
defined.
e_ehsize This member holds the ELF header's size in bytes.
e_phentsize This member holds the size in bytes of one entry in
the file's program header table; all entries are the
same size.
e_phnum This member holds the number of entries in the program
header table. If the file is using extended program
header numbering, then the e_phnum member will contain
the value PN_XNUM and the actual number of program
header table entries will be stored in the sh_info
member of the section header at index SHN_UNDEF. The
product of 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,
e_phnum holds the value zero.
e_shentsize This member holds a sections header's size in bytes.
A section header is one entry in the section header
table; all entries are the same size.
e_shnum This member holds the number of entries in the section
header table. If the file is using extended section
numbering, then the e_shnum member will be zero and
the actual section number will be stored in the
sh_size member of the section header at index
SHN_UNDEF. If a file has no section header table,
both the e_shnum and the e_shoff fields of the ELF
header will be zero. The product of e_shentsize and
the number of sections in the file gives the section
header table's size in bytes.
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 SHN_XINDEX, and the
actual section header table index will be present in
the sh_link field of the section header entry at index
SHN_UNDEF. If the file has no section name string
table, this member holds the value SHN_UNDEF.
An executable or shared object file's program header table is an array of
structures, each describing a segment or other information the system
needs to prepare the program for execution. An object file segment
contains one or more sections. Program headers are meaningful only for
executable and shared object files. A file specifies its own program
header size with the ELF header's e_phentsize and e_phnum members. As
with the Elf executable header, the program header also has different
versions depending on the architecture:
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;
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;
The main difference between the 32-bit and the 64-bit program header lies
only in the location of a p_flags member in the total struct.
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.
PT_NULL The array element is unused and the other
members' values are undefined. This lets the
program header have ignored entries.
PT_LOAD The array element specifies a loadable
segment, described by p_filesz and p_memsz.
The bytes from the file are mapped to the
beginning of the memory segment. If the
segment's memory size (p_memsz) is larger
than the file size (p_filesz), the "extra"
bytes are defined to hold the value 0 and to
follow the segment's initialized area. The
file size may not be larger than the memory
size. Loadable segment entries in the
program header table appear in ascending
order, sorted on the p_vaddr member.
PT_DYNAMIC The array element specifies dynamic linking
information.
PT_INTERP The array element specifies the location and
size of a null-terminated path name to invoke
as an interpreter. This segment type is
meaningful only for executable files (though
it may occur for shared objects). However it
may not occur more than once in a file. If
it is present it must precede any loadable
segment entry.
PT_NOTE The array element specifies the location and
size for auxiliary information.
PT_SHLIB This segment type is reserved but has
unspecified semantics. Programs that contain
an array element of this type do not conform
to the ABI.
PT_PHDR The array element, if present, specifies the
location and size of the program header table
itself, both in the file and in the memory
image of the program. This segment type may
not occur more than once in a file.
Moreover, it may only occur if the program
header table is part of the memory image of
the program. If it is present it must
precede any loadable segment entry.
PT_LOPROC This value up to and including PT_HIPROC are
reserved for processor-specific semantics.
PT_HIPROC This value down to and including PT_LOPROC
are reserved for processor-specific
semantics.
p_offset This member holds the offset from the beginning of the
file at which the first byte of the segment resides.
p_vaddr This member holds the virtual address at which the first
byte of the segment resides in memory.
p_paddr On systems for which physical addressing is relevant,
this member is reserved for the segment's physical
address. Under BSD this member is not used and must be
zero.
p_filesz This member holds the number of bytes in the file image
of the segment. It may be zero.
p_memsz This member holds the number of bytes in the memory image
of the segment. It may be zero.
p_flags This member holds flags relevant to the segment:
PF_X An executable segment.
PF_W A writable segment.
PF_R A readable segment.
A text segment commonly has the flags PF_X and PF_R. A
data segment commonly has PF_X, PF_W and PF_R.
p_align This member holds the value to which the segments are
aligned in memory and in the file. Loadable process
segments must have congruent values for p_vaddr and
p_offset, modulo the page size. Values of zero and one
mean no alignment is required. Otherwise, p_align should
be a positive, integral power of two, and p_vaddr should
equal p_offset, modulo p_align.
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 e_shoff member gives the byte offset from
the beginning of the file to the section header table. e_shnum holds the
number of entries the section header table contains. e_shentsize holds
the size in bytes of each entry.
A section header table index is a subscript into this array. Some
section header table indices are reserved. An object file does not have
sections for these special indices:
SHN_UNDEF This value marks an undefined, missing, irrelevant, or
otherwise meaningless section reference. For example, a
symbol "defined" relative to section number SHN_UNDEF is
an undefined symbol.
SHN_LORESERVE This value specifies the lower bound of the range of
reserved indices.
SHN_LOPROC This value up to and including SHN_HIPROC are reserved for
processor-specific semantics.
SHN_HIPROC This value down to and including SHN_LOPROC are reserved
for processor-specific semantics.
SHN_ABS This value specifies absolute values for the corresponding
reference. For example, symbols defined relative to
section number SHN_ABS have absolute values and are not
affected by relocation.
SHN_COMMON Symbols defined relative to this section are common
symbols, such as FORTRAN COMMON or unallocated C external
variables.
SHN_HIRESERVE This value specifies the upper bound of the range of
reserved indices. The system reserves indices between
SHN_LORESERVE and SHN_HIRESERVE, inclusive. The section
header table does not contain entries for the reserved
indices.
The section header has the following structure:
typedef struct {
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;
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;
sh_name This member specifies the name of the section. Its value
is an index into the section header string table section,
giving the location of a null-terminated string.
sh_type This member categorizes the section's contents and
semantics.
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.
SHT_PROGBITS The section holds information defined by the
program, whose format and meaning are
determined solely by the program.
SHT_SYMTAB This section holds a symbol table.
Typically, 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 SHN_DYNSYM section.
SHT_STRTAB This section holds a string table. An object
file may have multiple string table sections.
SHT_RELA This section holds relocation entries with
explicit addends, such as type Elf32_Rela for
the 32-bit class of object files. An object
may have multiple relocation sections.
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.
SHT_DYNAMIC This section holds information for dynamic
linking. An object file may have only one
dynamic section.
SHT_NOTE This section holds information that marks the
file in some way.
SHT_NOBITS A section of this type occupies no space in
the file but otherwise resembles
SHN_PROGBITS. Although this section contains
no bytes, the sh_offset member contains the
conceptual file offset.
SHT_REL This section holds relocation offsets without
explicit addends, such as type Elf32_Rel for
the 32-bit class of object files. An object
file may have multiple relocation sections.
SHT_SHLIB This section is reserved but has unspecified
semantics.
SHT_DYNSYM This section holds a minimal set of dynamic
linking symbols. An object file can also
contain a SHN_SYMTAB section.
SHT_LOPROC This value up to and including SHT_HIPROC are
reserved for processor-specific semantics.
SHT_HIPROC This value down to and including SHT_LOPROC
are reserved for processor-specific
semantics.
SHT_LOUSER This value specifies the lower bound of the
range of indices reserved for application
programs.
SHT_HIUSER This value specifies the upper bound of the
range of indices reserved for application
programs. Section types between SHT_LOUSER
and SHT_HIUSER may be used by the
application, without conflicting with current
or future system-defined section types.
sh_flags Sections support one-bit flags that describe miscellaneous
attributes. If a flag bit is set in sh_flags, the
attribute is "on" for the section. Otherwise, the
attribute is "off" or does not apply. Undefined attributes
are set to zero.
SHF_WRITE This section contains data that should be
writable during process execution.
SHF_ALLOC 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.
SHF_EXECINSTR The section contains executable machine
instructions.
SHF_MASKPROC All bits included in this mask are reserved
for processor-specific semantics.
SHF_COMPRESSED
The section data is compressed.
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.
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, SHT_NOBITS, occupies no space in the
file, and its sh_offset member locates the conceptual
placement in the file.
sh_size This member holds the section's size in bytes. Unless the
section type is SHT_NOBITS, the section occupies sh_size
bytes in the file. A section of type SHT_NOBITS may have a
non-zero size, but it occupies no space in the file.
sh_link This member holds a section header table index link, whose
interpretation depends on the section type.
sh_info This member holds extra information, whose interpretation
depends on the section type.
sh_addralign Some sections have address alignment constraints. If a
section holds a doubleword, the system must ensure
doubleword alignment for the entire section. That is, the
value of sh_addr must be congruent to zero, modulo the
value of sh_addralign. Only zero and positive integral
powers of two are allowed. Values of zero or one mean the
section has no alignment constraints.
sh_entsize Some sections hold a table of fixed-sized entries, such as
a symbol table. For such a section, this member gives the
size in bytes for each entry. This member contains zero if
the section does not hold a table of fixed-size entries.
Various sections hold program and control information:
.bss (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
SHT_NOBITS. The attributes types are SHF_ALLOC and SHF_WRITE.
.comment This section holds version control information. This section
is of type SHT_PROGBITS. No attribute types are used.
.data This section holds initialized data that contribute to the
program's memory image. This section is of type SHT_PROGBITS.
The attribute types are SHF_ALLOC and SHF_WRITE.
.data1 This section holds initialized data that contribute to the
program's memory image. This section is of type SHT_PROGBITS.
The attribute types are SHF_ALLOC and SHF_WRITE.
.debug This section holds information for symbolic debugging. The
contents are unspecified. This section is of type
SHT_PROGBITS. No attribute types are used.
.dynamic This section holds dynamic linking information. The section's
attributes will include the SHF_ALLOC bit. Whether the
SHF_WRITE bit is set is processor-specific. This section is
of type SHT_DYNAMIC. See the attributes above.
.dynstr This section holds strings needed for dynamic linking, most
commonly the strings that represent the names associated with
symbol table entries. This section is of type SHT_STRTAB.
The attribute type used is SHF_ALLOC.
.dynsym This section holds the dynamic linking symbol table. This
section is of type SHT_DYNSYM. The attribute used is
SHF_ALLOC.
.fini This section holds executable instructions that contribute to
the process termination code. When a program exits normally
the system arranges to execute the code in this section. This
section is of type SHT_PROGBITS. The attributes used are
SHF_ALLOC and SHF_EXECINSTR.
.got This section holds the global offset table. This section is
of type SHT_PROGBITS. The attributes are processor-specific.
.hash This section holds a symbol hash table. This section is of
type SHT_HASH. The attribute used is SHF_ALLOC.
.init This section holds executable instructions that contribute to
the process initialization code. When a program starts to run
the system arranges to execute the code in this section before
calling the main program entry point. This section is of type
SHT_PROGBITS. The attributes used are SHF_ALLOC and
SHF_EXECINSTR.
.interp This section holds the pathname of a program interpreter. If
the file has a loadable segment that includes the section, the
section's attributes will include the SHF_ALLOC bit.
Otherwise, that bit will be off. This section is of type
SHT_PROGBITS.
.line This section holds line number information for symbolic
debugging, which describes the correspondence between the
program source and the machine code. The contents are
unspecified. This section is of type SHT_PROGBITS. No
attribute types are used.
.note This section holds information in the "Note Section" format
described below. This section is of type SHT_NOTE. No
attribute types are used.
.plt This section holds the procedure linkage table. This section
is of type SHT_PROGBITS. The attributes are processor-
specific.
.relNAME This section holds relocation information as described below.
If the file has a loadable segment that includes relocation,
the section's attributes will include the SHF_ALLOC bit.
Otherwise the bit will be off. By convention, "NAME" is
supplied by the section to which the relocations apply. Thus
a relocation section for .text normally would have the name
.rel.text. This section is of type SHT_REL.
.relaNAME This section holds relocation information as described below.
If the file has a loadable segment that includes relocation,
the section's attributes will include the SHF_ALLOC bit.
Otherwise the bit will be off. By convention, "NAME" is
supplied by the section to which the relocations apply. Thus
a relocation section for .text normally would have the name
.rela.text. This section is of type SHT_RELA.
.rodata This section holds read-only data that typically contributes
to a non-writable segment in the process image. This section
is of type SHT_PROGBITS. The attribute used is SHF_ALLOC.
.rodata1 This section hold read-only data that typically contributes to
a non-writable segment in the process image. This section is
of type SHT_PROGBITS. The attribute used is SHF_ALLOC.
.shstrtab This section holds section names. This section is of type
SHT_STRTAB. No attribute types are used.
.strtab This section holds strings, most commonly the strings that
represent the names associated with symbol table entries. If
the file has a loadable segment that includes the symbol
string table, the section's attributes will include the
SHF_ALLOC bit. Otherwise the bit will be off. This section
is of type SHT_STRTAB.
.symtab This section holds a symbol table. If the file has a loadable
segment that includes the symbol table, the section's
attributes will include the SHF_ALLOC bit. Otherwise the bit
will be off. This section is of type SHT_SYMTAB.
.text This section holds the "text", or executable instructions, of
a program. This section is of type SHT_PROGBITS. The
attributes used are SHF_ALLOC and SHF_EXECINSTR.
.jcr This section holds information about Java classes that must be
registered.
.eh_frame This section holds information used for C++ exception-
handling.
A section with the SHF_COMPRESSED flag set contains a compressed copy of
the section data. Compressed section data begins with an Elf64_Chdr or
Elf32_Chdr structure which encodes the compression algorithm and some
characteristics of the uncompressed data.
typedef struct {
Elf32_Word ch_type;
Elf32_Word ch_size;
Elf32_Word ch_addralign;
} Elf32_Chdr;
typedef struct {
Elf64_Word ch_type;
Elf64_Word ch_reserved;
Elf64_Xword ch_size;
Elf64_Xword ch_addralign;
} Elf64_Chdr;
ch_type The compression algorithm used. A value of
ELFCOMPRESS_ZLIB indicates that the data is compressed
using zlib(3).
ch_size The size, in bytes, of the uncompressed section data. This
corresponds to the sh_size field of a section header
containing uncompressed data.
ch_addralign The address alignment of the uncompressed section data.
This corresponds to the sh_addralign field of a section
header containing uncompressed data.
String table sections hold null-terminated character sequences, commonly
called strings. The object file uses these strings to represent symbol
and section names. One references a string as an index into the string
table section. The first byte, which is index zero, is defined to hold a
null character. Similarly, a string table's last byte is defined to hold
a null character, ensuring null termination for all strings.
An object file's symbol table holds information needed to locate and
relocate a program's symbolic definitions and references. A symbol table
index is a subscript into this array.
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;
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;
st_name This member holds an index into the object file's symbol string
table, which holds character representations of the symbol
names. If the value is non-zero, it represents a string table
index that gives the symbol name. Otherwise, the symbol table
has no name.
st_value This member gives the value of the associated symbol.
st_size Many symbols have associated sizes. This member holds zero if
the symbol has no size or an unknown size.
st_info This member specifies the symbol's type and binding attributes:
STT_NOTYPE The symbol's type is not defined.
STT_OBJECT The symbol is associated with a data object.
STT_FUNC The symbol is associated with a function or other
executable code.
STT_SECTION The symbol is associated with a section. Symbol
table entries of this type exist primarily for
relocation and normally have STB_LOCAL bindings.
STT_FILE By convention the symbol's name gives the name of
the source file associated with the object file.
A file symbol has STB_LOCAL bindings, its section
index is SHN_ABS, and it precedes the other
STB_LOCAL symbols of the file, if it is present.
STT_LOPROC This value up to and including STT_HIPROC are
reserved for processor-specific semantics.
STT_HIPROC This value down to and including STT_LOPROC are
reserved for processor-specific semantics.
STB_LOCAL Local symbols are not visible outside the object
file containing their definition. Local symbols of
the same name may exist in multiple file without
interfering with each other.
STB_GLOBAL Global symbols are visible to all object files
being combined. One file's definition of a global
symbol will satisfy another file's undefined
reference to the same symbol.
STB_WEAK Weak symbols resemble global symbols, but their
definitions have lower precedence.
STB_LOPROC This value up to and including STB_HIPROC are
reserved for processor-specific semantics.
STB_HIPROC This value down to and including STB_LOPROC are
reserved for processor-specific semantics.
There are macros for packing and unpacking the
binding and type fields:
ELF32_ST_BIND(info) or ELF64_ST_BIND(info)
extract a binding from
an st_info value.
ELF64_ST_TYPE(info) or ELF32_ST_TYPE(info)
extract a type from an
st_info value.
ELF32_ST_INFO(bind, type) or ELF64_ST_INFO(bind,
type) convert a binding
and a type into an
st_info value.
st_other This member currently holds zero and has no defined meaning.
st_shndx Every symbol table entry is "defined" in relation to some
section. This member holds the relevant section header table
index.
Relocation is the process of connecting symbolic references with symbolic
definitions. Relocatable files must have information that describes how
to modify their section contents, thus allowing executable and shared
object files to hold the right information for a process' program image.
Relocation entries are these data.
Relocation structures that do not need an addend:
typedef struct {
Elf32_Addr r_offset;
Elf32_Word r_info;
} Elf32_Rel;
typedef struct {
Elf64_Addr r_offset;
Elf64_Xword r_info;
} Elf64_Rel;
Relocation structures that need an addend:
typedef struct {
Elf32_Addr r_offset;
Elf32_Word r_info;
Elf32_Sword r_addend;
} Elf32_Rela;
typedef struct {
Elf64_Addr r_offset;
Elf64_Xword r_info;
Elf64_Sxword r_addend;
} Elf64_Rela;
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.
r_info This member gives both the symbol table index with respect to
which the relocation must be made and the type of relocation to
apply. Relocation types are processor-specific. When the text
refers to a relocation entry's relocation type or symbol table
index, it means the result of applying ELF_[32|64]_R_TYPE or
ELF[32|64]_R_SYM, respectively to the entry's r_info member.
r_addend This member specifies a constant addend used to compute the
value to be stored into the relocatable field.
Note Section
ELF note sections consist of entries with the following format:
Field Size Description
namesz 32 bits Size of name
descsz 32 bits Size of desc
type 32 bits OS-dependent note type
name namesz Null-terminated originator name
desc descsz OS-dependent note data
The name and desc fields are padded to ensure 4-byte alignemnt. namesz
and descsz specify the unpadded length.
FreeBSD defines the following ELF note types (with corresponding
interpretation of desc):
NT_FREEBSD_ABI_TAG (Value: 1)
Indicates the OS ABI version in a form of a 32-bit integer
containing expected ABI version (i.e., __FreeBSD_version).
NT_FREEBSD_NOINIT_TAG (Value: 2)
Indicates that the C startup does not call initialization routines,
and thus rtld(1) must do so. desc is ignored.
NT_FREEBSD_ARCH_TAG (Value: 3)
Contains the MACHINE_ARCH that the executable was built for.
NT_FREEBSD_FEATURE_CTL (Value: 4)
Contains a bitmask of mitigations and features to enable:
NT_FREEBSD_FCTL_ASLR_DISABLE (Value: 0x01)
Request that address randomization (ASLR) not be performed.
See security(7).
NT_FREEBSD_FCTL_PROTMAX_DISABLE (Value: 0x02)
Request that mmap(2) calls not set PROT_MAX to the initial
value of the prot argument.
NT_FREEBSD_FCTL_STKGAP_DISABLE (Value: 0x04)
Disable stack gap.
NT_FREEBSD_FCTL_WXNEEDED (Value: 0x08)
Indicate that the binary requires mappings that are
simultaneously writeable and executable.
SEE ALSO
as(1), gdb(1), ld(1), objdump(1), readelf(1), execve(2), zlib(3), ar(5),
core(5)
Hewlett Packard, Elf-64 Object File Format.
Santa Cruz Operation, System V Application Binary Interface.
Unix System Laboratories, "Object Files", Executable and Linking Format
(ELF).
HISTORY
The ELF header files made their appearance in FreeBSD 2.2.6. ELF in
itself first appeared in AT&T System V UNIX. The ELF format is an
adopted standard.
AUTHORS
This manual page was written by Jeroen Ruigrok van der Werven
<asmodai@FreeBSD.org> with inspiration from BSDi's BSD/OS elf manpage.
FreeBSD 13.1-RELEASE-p6 November 27, 2021 FreeBSD 13.1-RELEASE-p6
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