elf(3)
- NetBSD Manual Pages
ELF(3) NetBSD Library Functions Manual ELF(3)
NAME
elf -- API for manipulating ELF objects
LIBRARY
ELF Object Manipulation Library (libelf, -lelf)
SYNOPSIS
#include <libelf.h>
DESCRIPTION
The ELF Object Manipulation Library (libelf, -lelf) provides functions
that allow an application to read and manipulate ELF object files, and to
read ar(1) archives. The library allows the manipulation of ELF objects
in a byte ordering and word-size independent way, allowing an application
to read and create ELF objects for 32 and 64 bit architectures and for
little- and big-endian machines. The library is capable of processing
ELF objects that use extended section numbering.
This manual page serves to provide an overview of the functionality in
the ELF library. Further information may found in the manual pages for
individual ELF(3) functions that comprise the library.
ELF Concepts
As described in elf(5), ELF files contain several data structures that
are laid out in a specific way. ELF files begin with an ``Executable
Header'', and may contain an optional ``Program Header Table'', and
optional data in the form of ELF ``sections''. A ``Section Header
Table'' describes the content of the data in these sections.
ELF objects have an associated ``ELF class'' which denotes the natural
machine word size for the architecture the object is associated with.
Objects for 32 bit architectures have an ELF class of ELFCLASS32.
Objects for 64 bit architectures have an ELF class of ELFCLASS64.
ELF objects also have an associated ``endianness'' which denotes the
endianness of the machine architecture associated with the object. This
may be ELFDATA2LSB for little-endian architectures and ELFDATA2MSB for
big-endian architectures.
ELF objects are also associated with an API version number. This version
number determines the layout of the individual components of an ELF file
and the semantics associated with these.
Data Representation And Translation
The ELF(3) library distinguishes between ``native'' representations of
ELF data structures and their ``file'' representations.
An application would work with ELF data in its ``native'' representation,
i.e., using the native byteorder and alignment mandated by the processor
the application is running on. The ``file'' representation of the same
data could use a different byte ordering and follow different constraints
on object alignment than these native constraints.
Accordingly, the ELF(3) library offers translation facilities
(elf32_xlatetof(3), elf32_xlatetom(3), elf64_xlatetof(3) and
elf64_xlatetom(3)) to and from these representations. It also provides
higher-level APIs (gelf_xlatetof(3), gelf_xlatetom(3)) that retrieve and
store data from the ELF object in a class-agnostic manner.
Library Working Version
Conceptually, there are three version numbers associated with an applica-
tion using the ELF library to manipulate ELF objects:
· The ELF version that the application was compiled against.
This version determines the ABI expected by the application.
· The ELF version of the ELF object being manipulated by the
application through the ELF library.
· The ELF version (or set of versions) supported by the ELF
library itself.
In order to facilitate working with ELF objects of differing versions,
the ELF library requires the application to call the elf_version() func-
tion before invoking many of its operations, in order to inform the
library of the application's desired working version.
In the current implementation, all three versions have to be EV_CURRENT.
Namespace use
The ELF library uses the following prefixes:
elf_ Used for class-independent functions.
elf32_ Used for functions working with 32 bit ELF objects.
elf64_ Used for functions working with 64 bit ELF objects.
Elf_ Used for class-independent data types.
ELF_C_ Used for command values used in a few functions. These symbols
are defined as members of the Elf_Cmd enumeration.
ELF_E_ Used for error numbers.
ELF_F_ Used for flags.
ELF_K_ These constants define the kind of file associated with an ELF
descriptor. See elf_kind(3). The symbols are defined by the
Elf_Kind enumeration.
ELF_T_ These values are defined by the Elf_Type enumeration, and denote
the types of ELF data structures that can be present in an ELF
object.
In addition, the library uses symbols with prefixes _ELF and _libelf for
its internal use.
Descriptors
Applications communicate with the library using descriptors. These are:
Elf An Elf descriptor represents an ELF object or an ar(1) archive.
It is allocated using one of the elf_begin() or elf_memory()
functions. An Elf descriptor can be used to read and write
data to an ELF file. An Elf descriptor can be associated with
zero or more Elf_Scn section descriptors.
Given an ELF descriptor, the application may retrieve the ELF
object's class-dependent ``Executable Header'' structures using
the elf32_getehdr() or elf64_getehdr() functions. A new Ehdr
structure may be allocated using the elf64_newehdr() or
elf64_newehdr() functions.
The ``Program Header Table'' associated with an ELF descriptor
may be allocated using the elf32_getphdr() or elf64_getphdr()
functions. A new program header table may be allocated or an
existing table resized using the elf32_newphdr() or
elf64_newphdr() functions.
The Elf structure is opaque and has no members visible to the
application.
Elf_Data An Elf_Data data structure describes an individual chunk of a
ELF file as represented in memory. It has the following appli-
cation-visible members:
uint64_t d_align The in-file alignment of the data
buffer within its containing ELF sec-
tion. This value must be non-zero and
a power of two.
void *d_buf A pointer to data in memory.
uint64_t d_off The offset within the containing sec-
tion where this descriptor's data would
be placed. This field will be computed
by the library unless the application
requests full control of the ELF
object's layout.
uint64_t d_size The number of bytes of data in this
descriptor.
Elf_Type d_type The ELF type (see below) of the data in
this descriptor.
unsigned int d_version The operating version for the data in
this buffer.
Elf_Data descriptors are usually used in conjunction with
Elf_Scn descriptors.
Elf_Scn Elf_Scn descriptors represent sections in an ELF object. These
descriptors are opaque and contain no application modifiable
fields.
The Elf_Scn descriptor for a specific section in an ELF object
can be retrieved using the elf_getscn() function. The sections
contained in an ELF object can be traversed using the
elf_nextscn() function. New sections are allocated using the
elf_newscn() function.
The Elf_Data descriptors associated with a given section can be
retrieved using the elf_getdata() function. New data descrip-
tors can be added to a section descriptor using the
elf_newdata() function. The untranslated ``file'' representa-
tion of data in a section can be retrieved using the
elf_rawdata() function.
Supported Elf Types
The following ELF datatypes are supported by the library.
ELF_T_ADDR Machine addresses.
ELF_T_BYTE Byte data. The library will not attempt to translate byte
data.
ELF_T_CAP Software and hardware capability records.
ELF_T_DYN Records used in a section of type SHT_DYNAMIC.
ELF_T_EHDR ELF executable header.
ELF_T_GNUHASH GNU-style hash tables.
ELF_T_HALF 16-bit unsigned words.
ELF_T_LWORD 64 bit unsigned words.
ELF_T_MOVE ELF Move records.
ELF_T_NOTE ELF Note structures.
ELF_T_OFF File offsets.
ELF_T_PHDR ELF program header table entries.
ELF_T_REL ELF relocation entries.
ELF_T_RELA ELF relocation entries with addends.
ELF_T_SHDR ELF section header entries.
ELF_T_SWORD Signed 32-bit words.
ELF_T_SXWORD Signed 64-bit words.
ELF_T_SYMINFO ELF symbol information.
ELF_T_SYM ELF symbol table entries.
ELF_T_VDEF Symbol version definition records.
ELF_T_VNEED Symbol version requirement records.
ELF_T_WORD Unsigned 32-bit words.
ELF_T_XWORD Unsigned 64-bit words.
The symbol ELF_T_NUM denotes the number of Elf types known to the
library.
The following table shows the mapping between ELF section types defined
in elf(5) and the types supported by the library.
Section Type Library Type Description
SHT_DYNAMIC ELF_T_DYN `.dynamic' section entries.
SHT_DYNSYM ELF_T_SYM Symbols for dynamic linking.
SHT_FINI_ARRAY ELF_T_ADDR Termination function pointers.
SHT_GNU_HASH ELF_T_GNUHASH GNU hash sections.
SHT_GNU_LIBLIST ELF_T_WORD List of libraries to be pre-linked.
SHT_GNU_verdef ELF_T_VDEF Symbol version definitions.
SHT_GNU_verneed ELF_T_VNEED Symbol versioning requirements.
SHT_GNU_versym ELF_T_HALF Version symbols.
SHT_GROUP ELF_T_WORD Section group marker.
SHT_HASH ELF_T_HASH Symbol hashes.
SHT_INIT_ARRAY ELF_T_ADDR Initialization function pointers.
SHT_NOBITS ELF_T_BYTE Empty sections. See elf(5).
SHT_NOTE ELF_T_NOTE ELF note records.
SHT_PREINIT_ARRAY ELF_T_ADDR Pre-initialization function
pointers.
SHT_PROGBITS ELF_T_BYTE Machine code.
SHT_REL ELF_T_REL ELF relocation records.
SHT_RELA ELF_T_RELA Relocation records with addends.
SHT_STRTAB ELF_T_BYTE String tables.
SHT_SYMTAB ELF_T_SYM Symbol tables.
SHT_SYMTAB_SHNDX ELF_T_WORD Used with extended section
numbering.
SHT_SUNW_dof ELF_T_BYTE Used by dtrace(1).
SHT_SUNW_move ELF_T_MOVE ELF move records.
SHT_SUNW_syminfo ELF_T_SYMINFO Additional symbol flags.
SHT_SUNW_verdef ELF_T_VDEF Same as SHT_GNU_verdef.
SHT_SUNW_verneed ELF_T_VNEED Same as SHT_GNU_verneed.
SHT_SUNW_versym ELF_T_HALF Same as SHT_GNU_versym.
Section types in the range [SHT_LOOS, SHT_HIUSER] are otherwise consid-
ered to be of type ELF_T_BYTE.
Functional Grouping
This section contains a brief overview of the available functionality in
the ELF library. Each function listed here is described further in its
own manual page.
Archive Access
elf_getarsym()
Retrieve the archive symbol table.
elf_getarhdr()
Retrieve the archive header for an object.
elf_getbase()
Retrieve the offset of a member inside an archive.
elf_next()
Iterate through an ar(1) archive.
elf_rand()
Random access inside an ar(1) archive.
Data Structures
elf_getdata()
Retrieve translated data for an ELF section.
elf_getscn()
Retrieve the section descriptor for a named section.
elf_ndxscn()
Retrieve the index for a section.
elf_newdata()
Add a new Elf_Data descriptor to an ELF section.
elf_newscn()
Add a new section descriptor to an ELF descriptor.
elf_nextscn()
Iterate through the sections in an ELF object.
elf_rawdata()
Retrieve untranslated data for an ELF section.
elf_rawfile()
Return a pointer to the untranslated file contents for an
ELF object.
elf32_getehdr(), elf64_getehdr()
Retrieve the Executable Header in an ELF object.
elf32_getphdr(), elf64_getphdr()
Retrieve the Program Header Table in an ELF object.
elf32_getshdr(), elf64_getshdr()
Retrieve the ELF section header associated with an
Elf_Scn descriptor.
elf32_newehdr(), elf64_newehdr()
Allocate an Executable Header in an ELF object.
elf32_newphdr(), elf64_newphdr()
Allocate or resize the Program Header Table in an ELF
object.
Data Translation
elf32_xlatetof(), elf64_xlatetof()
Translate an ELF data structure from its native represen-
tation to its file representation.
elf32_xlatetom(), elf64_xlatetom()
Translate an ELF data structure from its file representa-
tion to a native representation.
Error Reporting
elf_errno()
Retrieve the current error.
elf_errmsg()
Retrieve a human readable description of the current
error.
Initialization
elf_begin()
Opens an ar(1) archive or ELF object given a file
descriptor.
elf_end()
Close an ELF descriptor and release all its resources.
elf_memory()
Opens an ar(1) archive or ELF object present in a memory
arena.
elf_version()
Sets the operating version.
IO Control
elf_cntl() Manage the association between and ELF
descriptor and its underlying file.
elf_flagdata() Mark an Elf_Data descriptor as dirty.
elf_flagehdr() Mark the ELF Executable Header in an ELF
descriptor as dirty.
elf_flagphdr() Mark the ELF Program Header Table in an ELF
descriptor as dirty.
elf_flagscn() Mark an Elf_Scn descriptor as dirty.
elf_flagshdr() Mark an ELF Section Header as dirty.
elf_setshstrndx() Set the index of the section name string table
for the ELF object.
elf_update() Recompute ELF object layout and optionally
write the modified object back to the underly-
ing file.
Queries
elf32_checksum(), elf64_checkum()
Compute checksum of an ELF object.
elf_getident() Retrieve the identification bytes for an ELF
object.
elf_getphdrnum() Retrieve the number of program headers in an
ELF object.
elf_getshdrnum() Retrieve the number of sections in an ELF
object.
elf_getshdrstrndx()
Retrieve the section index of the section name
string table in an ELF object.
elf_hash() Compute the ELF hash value of a string.
elf_kind() Query the kind of object associated with an
ELF descriptor.
elf32_fsize(), elf64_fsize()
Return the size of the file representation of
an ELF type.
Controlling ELF Object Layout
In the usual mode of operation, library will compute section offsets and
alignments based on the contents of an ELF descriptor's sections without
need for further intervention by the application.
However, if the application wishes to take complete charge of the layout
of the ELF file, it may set the ELF_F_LAYOUT flag on an ELF descriptor
using elf_flagelf(3), following which the library will use the data off-
sets and alignments specified by the application when laying out the
file. Application control of file layout is described further in the
elf_update(3) manual page.
Gaps in between sections will be filled with the fill character set by
function elf_fill().
Error Handling
In case an error is encountered, these library functions set an internal
error number and signal the presence of the error by returning a special
return value. The application can check the current error number by
calling elf_errno(3). A human readable description of the recorded error
is available by calling elf_errmsg(3).
Memory Management Rules
The library keeps track of all Elf_Scn and Elf_Data descriptors associ-
ated with an ELF descriptor and recovers them when the descriptor is
closed using elf_end(3). Thus the application must not call free(3) on
data structures allocated by the ELF library.
Conversely the library will not free data that it has not allocated. As
an example, an application may call elf_newdata(3) to allocate a new
Elf_Data descriptor and can set the d_off member of the descriptor to
point to a region of memory allocated using malloc(3). It is the appli-
cations responsibility to free this arena, though the library will
reclaim the space used by the Elf_Data descriptor itself.
SEE ALSO
gelf(3), ar(5), elf(5)
HISTORY
The original elf API was developed for AT&T System V UNIX. The current
implementation of the API appeared in FreeBSD 7.0 and NetBSD 6.0.
AUTHORS
The ELF library was written by Joseph Koshy <jkoshy@FreeBSD.org>.
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