ELF(5) BSD Reference Manual ELF(5)NAMEelf - format of ELF executable binary files
SYNOPSIS
#include <elf_abi.h>
DESCRIPTION
The header file <elf_abi.h> defines the format of ELF executable binary
files. Amongst these files are normal executable files, relocatable ob-
ject 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 ar-
chitecture 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". Applica-
tions 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 architec-
ture, 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 con-
tain values which start with the prefix ELF. The fol-
lowing 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 ar-
chitecture. It supports
machines with files and vir-
tual address spaces up to 4
Gigabytes.
ELFCLASS64 This defines the 64-bit ar-
chitecture.
EI_DATA The sixth byte specifies the data encoding
of the processor-specific data in the
file. Currently these encodings are sup-
ported:
ELFDATANONE Unknown data format.
ELFDATA2LSB Two's complement, little-
endian.
ELFDATA2MSB Two's complement, big-endian.
EI_VERSION The version number of the ELF specifica-
tion:
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 in-
struction 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 pro-
cess. 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 de-
fined.
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 sym-
bol "defined" relative to section
number SHN_UNDEF is an undefined sym-
bol.
SHN_LORESERVE This value specifies the lower bound of
the range of reserved indices.
SHN_LOPROC This value up to and including
SHN_HIPROC is reserved for processor-
specific semantics.
SHN_HIPROC This value down to and including
SHN_LOPROC is 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 reloca-
tion.
SHN_COMMON Symbols defined relative to this sec-
tion are common symbols, such as For-
tran 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, in-
clusive; 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 con-
tains one or more sections. Program headers are meaningful only for exe-
cutable 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 ar-
ray 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 seg-
ment, described by p_filesz and p_memsz. The
bytes from the file are mapped to the begin-
ning of the memory segment. If the segment's
memory size (p_memsz) is larger than the file
size (p_filesz), the "extra" bytes are de-
fined 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 mean-
ingful 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 un-
specified 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 load-
able segment entry.
PT_LOOS This value up to and including PT_HIOS is
reserverd for operating system-specific se-
mantics.
PT_HIOS This value down to and including PT_LOOS is
reserved for operating system-specific seman-
tics.
PT_LOPROC This value up to and including PT_HIPROC is
reserved for processor-specific semantics.
PT_HIPROC This value down to and including PT_LOPROC is
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 ad-
dress. 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 seg-
ments 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 struc-
tures. The ELF header's e_shoff member gives the byte offset from the be-
ginning 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 oth-
erwise meaningless section reference. For example, a sym-
bol "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 is reserved for
processor-specific semantics.
SHN_HIPROC This value down to and including SHN_LOPROC is reserved
for processor-specific semantics.
SHN_ABS This value specifies the absolute value for the
corresponding reference. For example, a symbol defined re-
lative 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 sym-
bols, 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 in-
dices.
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, giv-
ing the location of a null-terminated string.
sh_type This member categorizes the section's contents and seman-
tics.
SHT_NULL This value marks the section header as inac-
tive. It does not have an associated section.
Other members of the section header have un-
defined values.
SHT_PROGBITS This section holds information defined by the
program, whose format and meaning are deter-
mined 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 link-
ing. As a complete symbol table, it may con-
tain 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 con-
tain 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 SHT_LOPROC
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 SHT_LOUSER
and SHT_HIUSER may be used by the applica-
tion, without conflicting with current or fu-
ture 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 re-
side in the memory image of an object file.
This attribute is off for those sections.
SHF_EXECINSTR This section contains executable machine in-
structions.
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 begin-
ning of the file to the first byte in the section. One sec-
tion 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 sec-
tion 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 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 sec-
tion 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 sec-
tion 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. Other-
wise, that bit will be off. This section is of type
SHT_PROGBITS.
.line This section holds line number information for symbolic debug-
ging, 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 attri-
bute types are used. OpenBSD native executables usually con-
tain 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. Oth-
erwise the bit will be off. By convention, "NAME" is supplied
by the section to which the relocations apply. Thus a reloca-
tion 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. Oth-
erwise the bit will be off. By convention, "NAME" is supplied
by the section to which the relocations apply. Thus a reloca-
tion 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 attri-
butes 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 attri-
butes 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 re-
locate 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 re-
location 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 be-
ing combined. One file's definition of a global
symbol will satisfy another file's undefined refer-
ence to the same symbol.
STB_WEAK Weak symbols resemble global symbols, but their de-
finitions 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 sec-
tion. This member holds the relevant section header table in-
dex.
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 ob-
ject 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.
SEE ALSOas(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).
HISTORY
OpenBSD ELF support first appeared in OpenBSD 1.2, although not all sup-
ported 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 stan-
dard.
AUTHORS
This manual page was written by Jeroen Ruigrok van der Werven
<asmodai@FreeBSD.org> with inspiration from BSDi's elf manpage.
MirOS BSD #10-current July 31, 1999 13
