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NetBSD 6.1.5 - man page for elf (netbsd section 3)

ELF(3)				   BSD Library Functions Manual 			   ELF(3)

     elf -- API for manipulating ELF objects

     library ``libelf''

     #include <libelf.h>

     The library ``libelf'' library provides functions that allow an application to read and
     manipulate ELF object files, and to read ar(1) archives.  The library allows the manipula-
     tion of ELF objects in a byte ordering and word-size independent way, allowing an applica-
     tion 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

     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 com-
     prise 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

   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 dif-
     ferent 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 representa-
     tions 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 deter-
	       mines 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 mem-
	      bers 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.

     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 ta-
	       ble 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 repre-
	       sented in memory.  It has the following application visible members:
	       uint64_t d_align        The in-file alignment of the data buffer within its con-
				       taining ELF section.  This value must be a power of two.
	       uint64_t d_off	       The offset with the containing section where this descrip-
				       tors 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
	     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
	     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.

	     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 under-
				lying 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
	     elf_update()	Recompute ELF object layout and optionally write the modified
				object back to the underlying file.

	     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

     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.

     gelf(3), elf(5)

     The original ELF(3) API was developed for Unix System V.  This implementation first appeared
     in FreeBSD 7.0 and NetBSD 6.0.

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

BSD					 October 21, 2007				      BSD

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