elf - format of ELF executable binary files
#include <elf_abi.h>
The header file <elf_abi.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_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.
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''.
This header file describes the above mentioned headers as C
structures
and also includes structures for dynamic sections, relocation sections
and symbol tables.
The following types are used for 32-bit architectures:
Elf32_Addr Unsigned program address
Elf32_Half Unsigned halfword field
Elf32_Off Unsigned file offset
Elf32_Sword Signed large integer
Elf32_Word Field or unsigned large integer
And the following types are used for 64-bit architectures:
Elf64_Addr Unsigned program address
Elf64_Shalf Signed halfword field
Elf64_Half Unsigned halfword field
Elf64_Off Unsigned file offset
Elf64_Sword Signed large integer
Elf64_Word Field or unsigned large integer
Elf64_Xword Unsigned object size or alignment
Elf64_Sxword Signed object size or alignment
Elf64_Quarter Unsigned quarterword field
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_Quarter e_type;
Elf64_Quarter e_machine;
Elf64_Half e_version;
Elf64_Addr e_entry;
Elf64_Off e_phoff;
Elf64_Off e_shoff;
Elf64_Half e_flags;
Elf64_Quarter e_ehsize;
Elf64_Quarter e_phentsize;
Elf64_Quarter e_phnum;
Elf64_Quarter e_shentsize;
Elf64_Quarter e_shnum;
Elf64_Quarter 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_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_SPARC32PLUS SPARC with enhanced instruction set.
EM_PPC PowerPC.
EM_ALPHA Compaq [DEC] Alpha.
EM_SPARCV9 SPARC v9 64-bit.
EM_ALPHA_EXP Compaq [DEC] Alpha with
enhanced instruction
set.
EM_VAX DEC Vax.
e_version This member identifies the file version:
EV_NONE Invalid version.
EV_CURRENT Current version.
e_entry This member gives the virtual address to
which the
system first transfers control, thus
starting the process.
If the file has no associated entry point, this
member holds zero.
e_phoff This member holds the program header
table's file offset
in bytes. If the file has no program
header
table, this member holds zero.
e_shoff This member holds the section header
table's file offset
in bytes. If the file has no section
header table
this member holds zero.
e_flags This member holds processor-specific
flags associated
with the file. Flag names take the form
EF_`machine_flag'.
Currently no flags have
been defined.
e_ehsize This member holds the ELF header's size
in bytes.
e_phentsize This member holds the size in bytes of
one entry in
the file's program header table; all entries are the
same size.
e_phnum This member holds the number of entries
in the program
header table. Thus the product of
e_phentsize and
e_phnum gives the table's size in bytes.
If a file
has no program header, e_phnum holds the
value zero.
e_shentsize This member holds a sections header's
size in bytes.
A section header is one entry in the section header
table; all entries are the same size.
e_shnum This member holds the number of entries
in the section
header table. Thus the product of
e_shentsize and
e_shnum gives the section header table's
size in
bytes. If a file has no section header
table, e_shnum
holds the value of zero.
e_shstrndx This member holds the section header
table index of
the entry associated with the section
name string
table. If the file has no section name
string table,
this member holds the value SHN_UNDEF.
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 Values greater than or
equal to
SHN_HIPROC are reserved
for processorspecific
semantics.
SHN_HIPROC Values less than or equal
to SHN_LOPROC
are reserved for processor-specific semantics.
SHN_ABS This value specifies absolute values
for the corresponding reference. For
example, symbols defined
relative to
section number SHN_ABS
have absolute
values and are not affected by 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 between
SHN_LORESERVE and
SHN_HIRESERVE, inclusive;
the values do not
reference the
section header table.
That is, the
section header table does
not contain
entries for the reserved
indices.
An executable or shared object file's program header table
is an array of
structures, each describing a segment or other information
the system
needs to prepare the program for execution. An object file
segment 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_Half p_type;
Elf64_Half 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 Values greater than or equal to
PT_HIPROC are
reserved for processor-specific
semantics.
PT_HIPROC Values less than or equal to
PT_LOPROC are
reserved for processor-specific
semantics.
p_offset This member holds the offset from the beginning of the
file at which the first byte of the segment
resides.
p_vaddr This member holds the virtual address at
which the first
byte of the segment resides in memory.
p_paddr On systems for which physical addressing is
relevant,
this member is reserved for the segment's
physical address.
Under BSD this member is not used
and must be zero.
p_filesz This member holds the number of bytes in the
file image
of the segment. It may be zero.
p_memsz This member holds the number of bytes in the
memory image
of the segment. It may be zero.
p_flags This member holds flags relevant to the segment:
PF_X An executable segment.
PF_W A writable segment.
PF_R A readable segment.
A text segment commonly has the flags PF_X
and PF_R. A
data segment commonly has PF_X, PF_W and
PF_R.
p_align This member holds the value to which the
segments are
aligned in memory and in the file. Loadable
process segments
must have congruent values for p_vaddr
and
p_offset, modulo the page size. Values of
zero and one
mean no alignment is required. Otherwise,
p_align should
be a positive, integral power of two, and
p_vaddr should
equal p_offset, modulo p_align.
A file's section header table lets one locate all the file's
sections.
The section header table is an array of Elf32_Shdr or
Elf64_Shdr 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.
SHN_LORESERVE This value specifies the lower bound of the
range of reserved
indices.
SHN_LOPROC Values greater than or equal to SHN_HIPROC
are reserved
for processor-specific semantics.
SHN_HIPROC Values less than or equal to SHN_LOPROC are
reserved for
processor-specific semantics.
SHN_ABS This value specifies the absolute value for
the corresponding
reference. For example, a symbol
defined relative
to section number SHN_ABS has an absolute value and
is not affected by relocation.
SHN_COMMON Symbols defined relative to this section are
common 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_Half sh_name;
Elf64_Half sh_type;
Elf64_Xword sh_flags;
Elf64_Addr sh_addr;
Elf64_Off sh_offset;
Elf64_Xword sh_size;
Elf64_Half sh_link;
Elf64_Half 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 This section holds information
defined by the
program, whose format and meaning are determined
solely by the program.
SHT_SYMTAB This section holds a symbol
table. 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. An
object participating in dynamic
linking must
contain a symbol hash table. An
object file
may have only one hash table.
SHT_DYNAMIC This section holds information
for dynamic
linking. An object file may
have only one
dynamic section.
SHT_NOTE This section holds information
that marks the
file in some way.
SHT_NOBITS A section of this type occupies
no space in
the file but otherwise resembles
SHN_PROGBITS. Although this
section 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 is
reserved for processor-specific
semantics.
SHT_HIPROC This value down to and including is 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 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 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.
SHF_EXECINSTR This section contains executable machine instructions.
SHF_MASKPROC All bits included in this mask
are reserved
for processor-specific semantics.
sh_addr If this section appears in the memory image of
a process,
this member holds the address at which the
section's first
byte should reside. Otherwise, the member
contains zero.
sh_offset This member's value holds the byte offset from
the 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 fixedsize entries.
Various sections hold program and control information:
.bss This section holds uninitialized data that contributes to the
program's memory image. By definition, the system initializes
the data with zeros when the program begins to
run. This section
is of type SHT_NOBITS. The attribute types
are SHF_ALLOC
and SHF_WRITE.
.comment This section holds version control information.
This section
is of type SHT_PROGBITS. No attribute types are
used.
.ctors This section holds initialized pointers to the
C++ constructor
functions. This section is of type SHT_PROGBITS.
The attribute
types are SHF_ALLOC and SHF_WRITE.
.data This section holds initialized data that contribute to the
program's memory image. This section is of type
SHT_PROGBITS.
The attribute types are SHF_ALLOC and SHF_WRITE.
.data1 This section holds initialized data that contribute to the
program's memory image. This section is of type
SHT_PROGBITS.
The attribute types are SHF_ALLOC and SHF_WRITE.
.debug This section holds information for symbolic debugging. The
contents are unspecified. This section is of
type
SHT_PROGBITS. No attribute types are used.
.dtors This section holds initialized pointers to the
C++ destructor
functions. This section is of type SHT_PROGBITS.
The attribute
types are SHF_ALLOC and SHF_WRITE.
.dynamic This section holds dynamic linking information.
The 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. OpenBSD native executables usually
contain a .note.openbsd.ident section to identify
themselves,
for the kernel to bypass any compatibility ELF
binary emulation
tests when loading the file.
.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 holds read-only data that typically
contributes
to a non-writable segment in the process image.
This section
is of type SHT_PROGBITS. The attribute used is
SHF_ALLOC.
.shstrtab This section holds section names. This section
is of type
SHT_STRTAB. No attribute types are used.
.strtab This section holds strings, most commonly the
strings that
represent the names associated with symbol table
entries. If
the file has a loadable segment that includes the
symbol
string table, the section's attributes will include the
SHF_ALLOC bit. Otherwise the bit will be off.
This section
is of type SHT_STRTAB.
.symtab This section holds a symbol table. If the file
has a 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.
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_Half st_name;
Elf_Byte st_info;
Elf_Byte st_other;
Elf64_Quarter st_shndx;
Elf64_Xword 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 is reserved
for processor-specific semantics.
STT_HIPROC This value down to and including
STT_LOPROC is reserved
for processor-specific semantics.
STB_LOCAL Local symbols are not visible outside
the object
file containing their definition. Local symbols of
the same name may exist in multiple
files 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 is reserved
for processor-specific semantics.
STB_HIPROC This value down to and including
STB_LOPROC is reserved
for processor-specific semantics.
There are macros for packing and unpacking the
binding and type fields:
ELF32_ST_BIND(info) or
ELF64_ST_BIND(info)
extract a
binding from
an st_info
value.
ELF64_ST_TYPE(info) or
ELF32_ST_TYPE(info)
extract a
type from an
st_info
value.
ELF32_ST_INFO(bind, type) or
ELF64_ST_INFO(bind,
type) convert a binding
and a type
into an
st_info
value.
st_other This member currently holds zero and has no defined meaning.
st_shndx Every symbol table entry is ``defined'' in relation to some
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_Xword 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_Xword 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.
as(1), gdb(1), ld(1), objdump(1), execve(2), core(5)
Hewlett-Packard, Elf-64 Object File Format.
Santa Cruz Operation, System V Application Binary Interface.
Unix System Laboratories, "Object Files", Executable and
Linking Format
(ELF).
OpenBSD ELF support first appeared in OpenBSD 1.2, although
not all
supported platforms use it as the native binary file format.
ELF in
itself first appeared in AT&T System V UNIX. The ELF format
is an
adopted standard.
This manual page was written by Jeroen Ruigrok van der Werven
<asmodai@FreeBSD.org> with inspiration from BSDi's BSDI
BSD/OS elf manpage.
OpenBSD 3.6 July 31, 1999
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