Skip site navigation (1)Skip section navigation (2)

FreeBSD Man Pages

Man Page or Keyword Search:
Man Architecture
Apropos Keyword Search (all sections) Output format
home | help
ELF(3)                 FreeBSD Library Functions Manual                 ELF(3)

NAME
     elf - API for manipulating ELF objects

LIBRARY
     ELF Access Library (libelf, -lelf)

SYNOPSIS
     #include <libelf.h>

DESCRIPTION
     The ELF Access 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 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:
           +o   The ELF version that the application was compiled against.
               This version determines the ABI expected by the application.
           +o   The ELF version of the ELF object being manipulated by the
               application through the ELF library.
           +o   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 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.

   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
                   application visible members:
                   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.
                   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.
                   uint64_t d_size             The number of bytes of data in
                                               this descriptor.
                   void *d_buf                 A pointer to data in memory.
                   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 associated with 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.

     Elf_Scn       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 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            `.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           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 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
                            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 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 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 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 area, though the library will
     reclaim the space used by the 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 FreeBSD 7.0.

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

FreeBSD 11.0-PRERELEASE        October 21, 2007        FreeBSD 11.0-PRERELEASE

NAME | LIBRARY | SYNOPSIS | DESCRIPTION | SEE ALSO | HISTORY | AUTHORS

Want to link to this manual page? Use this URL:
<https://www.freebsd.org/cgi/man.cgi?query=elf&sektion=3&manpath=FreeBSD+10.3-RELEASE+and+Ports>

home | help