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Man Pages


Manual Reference Pages  -  ELF (3)

NAME

elf - API for manipulating ELF objects

CONTENTS

Library
Synopsis
Description
     ELF Concepts
     Data Representation And Translation
     Library Working Version
     Namespace use
     Descriptors
     Supported Elf Types
     Functional Grouping
     Controlling ELF Object Layout
     Error Handling
     Memory Management Rules
See Also
History
Authors

LIBRARY


.Lb libelf

SYNOPSIS


.In libelf.h

DESCRIPTION

The
.Lb 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 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 and also provides higher-level APIs that retrieve and store data from the ELF object in a transparent manner.

    Library Working Version

Conceptually, there are three version numbers associated with an application 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 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 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
.Vt 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
.Vt Elf_Kind enumeration.
ELF_T_*
  These values are defined by the
.Vt Elf_Type enumeration, and denote the types of ELF data structures that can be present in an ELF object.

    Descriptors

Applications communicate with the library using descriptors. These are:
Vt Elf An
.Vt 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
.Vt Elf descriptor can be used to read and write data to an ELF file. An
.Vt Elf descriptor can be associated with zero or more
.Vt 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
.Vt Elf structure is opaque and has no members visible to the application.

Vt Elf_Data
  An
.Vt Elf_Data data structure describes an individual chunk of a ELF file as represented in memory. It has the following application visible members:
Vt uint64_t d_align The in-file alignment of the data buffer within its containing ELF section. This value must be a power of two.
Vt uint64_t d_off The offset with the containing section where this descriptors data would be placed. This field will be computed by the library unless the application requests full control of the ELF object’s layout.
Vt uint64_t d_size The number of bytes of data in this descriptor.
Vt void *d_buf A pointer to data in memory.
Vt Elf_Type d_type The ELF type (see below) of the data in this descriptor.
Vt unsigned int d_version
  The operating version for the data in this buffer.


.Vt Elf_Data descriptors are usually associated with
.Vt Elf_Scn descriptors. Existing data descriptors associated with an ELF section may be structures are retrieved using the elf_getdata function. The elf_newdata function may be used to attach new data descriptors to an ELF section.
Vt Elf_Scn
.Vt Elf_Scn descriptors represent a section in an ELF object.

They are retrieved using the elf_getscn function. An application may iterate through the existing sections of an ELF object using the elf_nextscn function. New sections may be allocated using the elf_newscn function.

The
.Vt Elf_Scn descriptor is opaque and contains no application modifiable fields.

    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_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      Xo
  '.dynamic' section entries.
 
SHT_DYNSYM      ELF_T_SYM      Symbols for dynamic linking.
SHT_FINI_ARRAY      ELF_T_ADDR      Termination function pointers.
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      Xo
  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_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_SUNW_move      ELF_T_MOVE      ELF move records.
SHT_SUNW_syminfo      ELF_T_SYMINFO      Additional symbol flags.
 

    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
.Vt 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 sectino.
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
.Vt 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 representation to its file representation.
elf32_xlatetom, elf64_xlatetom
  Translate an ELF data structure from its file representation 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 area.
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
.Vt 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
.Vt 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 underlying file.
Queries
 
elf32_checksum, elf64_checkum
  Compute checksum of an ELF object.
elf_getident
  Retrieve the identification bytes for an ELF object.
elf_getshnum
  Retrieve the number of sections in an ELF object.
elf_getshstrndx
  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 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 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 an 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
.Vt Elf_Scn and
.Vt 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
.Vt 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 area, though the library will reclaim the space used by the
.Vt Elf_Data descriptor itself.

SEE ALSO

gelf(3), elf(5)

HISTORY

The original ELF(3) API was developed for Unix System V. The current implementation of the ELF(3) API appeared in
.Fx 7.0 .

AUTHORS

The ELF library was written by
.An Joseph Koshy <jkoshy@FreeBSD.org>.
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