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LINK(5) 			     BSD File Formats Manual				  LINK(5)

NAME
     link -- dynamic loader and link editor interface

SYNOPSIS
     #include <link.h>

DESCRIPTION
     The include file <link.h> declares several structures that are present in dynamically linked
     programs and libraries.  The structures define the interface between several components of
     the link-editor and loader mechanism.  The layout of a number of these structures within the
     binaries resembles the a.out(5) format in many places as it serves such similar functions as
     symbol definitions (including the accompanying string table) and relocation records needed
     to resolve references to external entities.

     It also records a number of data structures unique to the dynamic loading and linking
     process.  These include references to other objects that are required to complete the link-
     editing process and indirection tables to facilitate Position Independent Code (PIC) to
     improve sharing of code pages among different processes.

     The collection of data structures described here will be referred to as the Run-time
     Relocation Section (RRS) and is embedded in the standard text and data segments of the
     dynamically linked program or shared object image as the existing a.out(5) format offers no
     room for it elsewhere.

     Several utilities cooperate to ensure that the task of getting a program ready to run can
     complete successfully in a way that optimizes the use of system resources.  The compiler
     emits PIC code from which shared libraries can be built by ld(1).	The compiler also
     includes size information of any initialized data items through the .size assembler direc-
     tive.

     PIC code differs from conventional code in that it accesses data variables through an indi-
     rection table, the Global Offset Table, by convention accessible by the reserved name
     _GLOBAL_OFFSET_TABLE_.  The exact mechanism used for this is machine dependent, usually a
     machine register is reserved for the purpose.  The rational behind this construct is to gen-
     erate code that is independent of the actual load address.  Only the values contained in the
     Global Offset Table may need updating at run-time depending on the load addresses of the
     various shared objects in the address space.

     Likewise, procedure calls to globally defined functions are redirected through the Procedure
     Linkage Table (PLT) residing in the data segment of the core image.  Again, this is done to
     avoid run-time modifications to the text segment.

     The linker-editor allocates the Global Offset Table and Procedure Linkage Table when combin-
     ing PIC object files into an image suitable for mapping into the process address space.  It
     also collects all symbols that may be needed by the run-time link-editor and stores these
     along with the image's text and data bits.  Another reserved symbol, _DYNAMIC is used to
     indicate the presence of the run-time linker structures.  Whenever _DYNAMIC is relocated to
     0, there is no need to invoke the run-time link-editor.  If this symbol is non-zero, it
     points at a data structure from which the location of the necessary relocation- and symbol
     information can be derived.  This is most notably used by the start-up module, crt0.  The
     _DYNAMIC structure is conventionally located at the start of the data segment of the image
     to which it pertains.

DATA STRUCTURES
     The data structures supporting dynamic linking and run-time relocation reside both in the
     text and data segments of the image they apply to.  The text segments contain read-only data
     such as symbols descriptions and names, while the data segments contain the tables that need
     to be modified by during the relocation process.

     The _DYNAMIC symbol references a _dynamic structure:

	   struct  _dynamic {
		   int	   d_version;
		   struct  so_debug *d_debug;
		   union {
			   struct section_dispatch_table *d_sdt;
		   } d_un;
		   struct  ld_entry *d_entry;
	   };

     d_version	This field provides for different versions of the dynamic linking implementation.
		The current version numbers understood by ld and ld.so are LD_VERSION_SUN (3),
		which is used by the SunOS 4.x releases, and LD_VERSION_BSD (8), which is cur-
		rently in use by NetBSD.

     d_un	Refers to a d_version dependent data structure.

     d_debug	this field provides debuggers with a hook to access symbol tables of shared
		objects loaded as a result of the actions of the run-time link-editor.

     d_entry	this field is obsoleted by CRT interface version CRT_VERSION_BSD4, and is
		replaced by the crt_ldentry in crt_ldso.

     The section_dispatch_table structure is the main ``dispatcher'' table, containing offsets
     into the image's segments where various symbol and relocation information is located.

	   struct section_dispatch_table {
		   struct  so_map *sdt_loaded;
		   long    sdt_sods;
		   long    sdt_paths;
		   long    sdt_got;
		   long    sdt_plt;
		   long    sdt_rel;
		   long    sdt_hash;
		   long    sdt_nzlist;
		   long    sdt_filler2;
		   long    sdt_buckets;
		   long    sdt_strings;
		   long    sdt_str_sz;
		   long    sdt_text_sz;
		   long    sdt_plt_sz;
	   };

     sdt_loaded  A pointer to the first link map loaded (see below).  This field is set by
		 ld.so(1) for the benefit of debuggers that may use it to load a shared object's
		 symbol table.

     sdt_sods	 The start of a (linked) list of shared object descriptors needed by this object.

     sdt_paths	 Library search rules.	A colon separated list of directories corresponding to
		 the -R option of ld(1).

     sdt_got	 The location of the Global Offset Table within this image.

     sdt_plt	 The location of the Procedure Linkage Table within this image.

     sdt_rel	 The location of an array of relocation_info structures (see a.out(5)) specifying
		 run-time relocations.

     sdt_hash	 The location of the hash table for fast symbol lookup in this object's symbol
		 table.

     sdt_nzlist  The location of the symbol table.

     sdt_filler2
		 Currently unused.

     sdt_buckets
		 The number of buckets in sdt_hash

     sdt_strings
		 The location of the symbol string table that goes with sdt_nzlist.

     sdt_str_sz  The size of the string table.

     sdt_text_sz
		 The size of the object's text segment.

     sdt_plt_sz  The size of the Procedure Linkage Table.

     A sod structure describes a shared object that is needed to complete the link edit process
     of the object containing it.  A list of such objects (chained through sod_next) is pointed
     at by the sdt_sods in the section_dispatch_table structure.

	   struct sod {
		   long    sod_name;
		   u_int   sod_library : 1,
			   sod_unused : 31;
		   short   sod_major;
		   short   sod_minor;
		   long    sod_next;
	   };

     sod_name	  The offset in the text segment of a string describing this link object.

     sod_library  If set, sod_name specifies a library that is to be searched for by ld.so.  The
		  path name is obtained by searching a set of directories (see also ldconfig(8))
		  for a shared object matching lib<sod_name>.so.n.m.  If not set, sod_name should
		  point at a full path name for the desired shared object.

     sod_major	  Specifies the major version number of the shared object to load.

     sod_minor	  Specifies the preferred minor version number of the shared object to load.

     The run-time link-editor maintains a list of structures called link maps to keep track of
     all shared objects loaded into a process' address space.  These structures are only used at
     run-time and do not occur within the text or data segment of an executable or shared
     library.

	   struct so_map {
		   void    *som_addr;
		   char    *som_path;
		   struct  so_map *som_next;
		   struct  sod *som_sod;
		   void *som_sodbase;
		   u_int   som_write : 1;
		   struct  _dynamic *som_dynamic;
		   void    *som_spd;
	   };

     som_addr	  The address at which the shared object associated with this link map has been
		  loaded.

     som_path	  The full path name of the loaded object.

     som_next	  Pointer to the next link map.

     som_sod	  The sod structure that was responsible for loading this shared object.

     som_sodbase  Tossed in later versions the run-time linker.

     som_write	  Set if (some portion of) this object's text segment is currently writable.

     som_dynamic  Pointer to this object's _dynamic structure.

     som_spd	  Hook for attaching private data maintained by the run-time link-editor.

     Symbol description with size.  This is simply an nlist structure with one field (nz_size)
     added.  Used to convey size information on items in the data segment of shared objects.  An
     array of these lives in the shared object's text segment and is addressed by the sdt_nzlist
     field of section_dispatch_table.

	   struct nzlist {
		   struct nlist    nlist;
		   u_long	   nz_size;
	   #define nz_un	   nlist.n_un
	   #define nz_strx	   nlist.n_un.n_strx
	   #define nz_name	   nlist.n_un.n_name
	   #define nz_type	   nlist.n_type
	   #define nz_value	   nlist.n_value
	   #define nz_desc	   nlist.n_desc
	   #define nz_other	   nlist.n_other
	   };

     nlist    (see nlist(3)).

     nz_size  The size of the data represented by this symbol.

     A hash table is included within the text segment of shared object to facilitate quick lookup
     of symbols during run-time link-editing.  The sdt_hash field of the section_dispatch_table
     structure points at an array of rrs_hash structures:

	   struct rrs_hash {
		   int	   rh_symbolnum;	   /* symbol number */
		   int	   rh_next;		   /* next hash entry */
	   };

     rh_symbolnum  The index of the symbol in the shared object's symbol table (as given by the
		   ld_symbols field).

     rh_next	   In case of collisions, this field is the offset of the next entry in this hash
		   table bucket.  It is zero for the last bucket element.
     The rt_symbol structure is used to keep track of run-time allocated commons and data items
     copied from shared objects.  These items are kept on linked list and is exported through the
     dd_cc field in the so_debug structure (see below) for use by debuggers.

	   struct rt_symbol {
		   struct nzlist	   *rt_sp;
		   struct rt_symbol	   *rt_next;
		   struct rt_symbol	   *rt_link;
		   void 		   *rt_srcaddr;
		   struct so_map	   *rt_smp;
	   };

     rt_sp	 The symbol description.

     rt_next	 Virtual address of next rt_symbol.

     rt_link	 Next in hash bucket.  Used by internally by ld.so.

     rt_srcaddr  Location of the source of initialized data within a shared object.

     rt_smp	 The shared object which is the original source of the data that this run-time
		 symbol describes.

     The so_debug structure is used by debuggers to gain knowledge of any shared objects that
     have been loaded in the process's address space as a result of run-time link-editing.  Since
     the run-time link-editor runs as a part of process initialization, a debugger that wishes to
     access symbols from shared objects can only do so after the link-editor has been called from
     crt0.  A dynamically linked binary contains a so_debug structure which can be located by
     means of the d_debug field in _dynamic.

	   struct  so_debug {
		   int	   dd_version;
		   int	   dd_in_debugger;
		   int	   dd_sym_loaded;
		   char    *dd_bpt_addr;
		   int	   dd_bpt_shadow;
		   struct rt_symbol *dd_cc;
	   };

     dd_version      Version number of this interface.

     dd_in_debugger  Set by the debugger to indicate to the run-time linker that the program is
		     run under control of a debugger.

     dd_sym_loaded   Set by the run-time linker whenever it adds symbols by loading shared
		     objects.

     dd_bpt_addr     The address were a breakpoint will be set by the run-time linker to divert
		     control to the debugger.  This address is determined by the start-up module,
		     crt0.o, to be some convenient place before the call to _main.

     dd_bpt_shadow   Contains the original instruction that was at dd_bpt_addr.  The debugger is
		     expected to put this instruction back before continuing the program.

     dd_cc	     A pointer to the linked list of run-time allocated symbols that the debugger
		     may be interested in.

     The ld_entry structure defines a set of service routines within ld.so.  See dlfcn(3) for
     more information.

	   struct ld_entry {
		   void    *(*dlopen)(char *, int);
		   int	   (*dlclose)(void *);
		   void    *(*dlsym)(void *, char *);
		   int	   (*dlctl)(void *, int, void *);
		   void    (*dlexit)(void);
	   };

     The crt_ldso structure defines the interface between ld.so and the start-up code in crt0.

	   struct crt_ldso {
		   int		   crt_ba;
		   int		   crt_dzfd;
		   int		   crt_ldfd;
		   struct _dynamic *crt_dp;
		   char 	   **crt_ep;
		   void 	   *crt_bp;
		   char 	   *crt_prog;
		   char 	   *crt_ldso;
		   char 	   *crt_ldentry;
	   };
	   #define CRT_VERSION_SUN	   1
	   #define CRT_VERSION_BSD2	   2
	   #define CRT_VERSION_BSD3	   3
	   #define CRT_VERSION_BSD4	   4

     crt_ba    The virtual address at which ld.so was loaded by crt0.

     crt_dzfd  On SunOS systems, this field contains an open file descriptor to ``/dev/zero''
	       used to get demand paged zeroed pages.  On NetBSD systems it contains -1.

     crt_ldfd  Contains an open file descriptor that was used by crt0 to load ld.so.

     crt_dp    A pointer to main's _dynamic structure.

     crt_ep    A pointer to the environment strings.

     crt_bp    The address at which a breakpoint will be placed by the run-time linker if the
	       main program is run by a debugger.  See so_debug

     crt_prog  The name of the main program as determined by crt0 (CRT_VERSION_BSD3 only).

     crt_ldso  The path of the run-time linker as mapped by crt0 (CRT_VERSION_BSD4 only).

     crt_ldentry
	       The dlfcn(3) entry points provided by the run-time linker (CRT_VERSION_BSD4 only).

     The hints_header and hints_bucket structures define the layout of the library hints, nor-
     mally found in ``/var/run/ld.so.hints'', which is used by ld.so to quickly locate the shared
     object images in the file system.	The organization of the hints file is not unlike that of
     an a.out(5) object file, in that it contains a header determining the offset and size of a
     table of fixed sized hash buckets and a common string pool.

	   struct hints_header {
		   long 	   hh_magic;
	   #define HH_MAGIC	   011421044151
		   long 	   hh_version;
	   #define LD_HINTS_VERSION_1	   1
	   #define LD_HINTS_VERSION_2	   2
		   long 	   hh_hashtab;
		   long 	   hh_nbucket;
		   long 	   hh_strtab;
		   long 	   hh_strtab_sz;
		   long 	   hh_ehints;
		   long 	   hh_dirlist;
	   };

     hh_magic	   Hints file magic number.

     hh_version    Interface version number.

     hh_hashtab    Offset of hash table.

     hh_strtab	   Offset of string table.

     hh_strtab_sz  Size of strings.

     hh_ehints	   Maximum usable offset in hints file.

     hh_dirlist    Offset in string table of a colon-separated list of directories that was used
		   in constructing the hints file.  See also ldconfig(8).  This field is only
		   available with interface version number LD_HINTS_VERSION_2 and higher.

	   /*
	    * Hash table element in hints file.
	    */
	   struct hints_bucket {
		   int		   hi_namex;
		   int		   hi_pathx;
		   int		   hi_dewey[MAXDEWEY];
		   int		   hi_ndewey;
	   #define hi_major hi_dewey[0]
	   #define hi_minor hi_dewey[1]
		   int		   hi_next;
	   };

     hi_namex	Index of the string identifying the library.

     hi_pathx	Index of the string representing the full path name of the library.

     hi_dewey	The version numbers of the shared library.

     hi_ndewey	The number of valid entries in hi_dewey.

     hi_next	Next bucket in case of hashing collisions.

BSD					 October 23, 1993				      BSD
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