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ELF(3)                   BSD 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) library 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 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
                                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 arena, 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.  This
     implementation first appeared in FreeBSD 7.0 and NetBSD 6.0.

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

BSD                            October 21, 2007                            BSD

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