|ELF(3)||Library Functions Manual||ELF(3)|
elf — API for
manipulating ELF objects
The library “libelf” 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
functions that comprise the library.
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
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
ELFDATA2MSB for big-endian
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.
elf library distinguishes between
“native” representations of ELF data structures and their
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.
elf library offers
elf64_xlatetom(3)) to and from
these representations. It also provides higher-level APIs
gelf_xlatetom(3)) that retrieve
and store data from the ELF object in a class-agnostic manner.
Conceptually, there are three version numbers associated with an application using the ELF library to manipulate ELF objects:
In order to facilitate working with ELF objects
of differing versions, the ELF library requires the application to call the
function 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
The ELF library uses the following prefixes:
In addition, the library uses symbols with prefixes
_libelf for its
Applications communicate with the library using descriptors. These are:
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
functions. A new Ehdr structure may be allocated using the
The “Program Header Table”
associated with an ELF descriptor may be allocated using the
functions. A new program header table may be allocated or an existing
table resized using the
The Elf structure is opaque and has no members visible to the application.
Elf_Data descriptors are usually used in conjunction with Elf_Scn descriptors.
The Elf_Scn descriptor
for a specific section in an ELF object can be retrieved using the
function. The sections contained in an ELF object can be traversed using
function. New sections are allocated using the
descriptors associated with a given section can be retrieved using the
function. New data descriptors can be added to a section descriptor
function. The untranslated “file” representation of data
in a section can be retrieved using the
The following ELF datatypes are supported by the library.
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|
||‘.dynamic’ section entries.|
||Symbols for dynamic linking.|
||Termination function pointers.|
||GNU hash sections.|
||List of libraries to be pre-linked.|
||Symbol version definitions.|
||Symbol versioning requirements.|
||Section group marker.|
||Initialization function pointers.|
||Empty sections. See elf(5).|
||ELF note records.|
||Pre-initialization function pointers.|
||ELF relocation records.|
||Relocation records with addends.|
||Used with extended section numbering.|
||Used by dtrace(1).|
||ELF move records.|
||Additional symbol flags.|
Section types in the range [
SHT_HIUSER] are otherwise considered to be of type
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.
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
flag on an ELF descriptor using
elf_flagelf(3), following which the
library will use the data offsets 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
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).
The library keeps track of all Elf_Scn and Elf_Data descriptors associated 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 applications responsibility to free this arena, though the library will reclaim the space used by the Elf_Data descriptor itself.
elf API was developed for
AT&T System V UNIX. The current
implementation of the API appeared in FreeBSD
The ELF library was written by Joseph Koshy <jkoshy@FreeBSD.org>.
|June 12, 2019||OpenBSD-current|