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

NAME
     link — dynamic loader and link editor interface

SYNOPSIS
     #include <sys/types.h>
     #include <nlist.h>
     #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 mecha‐
     nism.  The layout of a number of these structures within the binaries
     resembles the a.out format in many places as it serves such similar func‐
     tions 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 for short) to improve sharing
     of code pages among different processes.  The collection of data struc‐
     tures 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 infor‐
     mation 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 indirection table, the Global Offset Table, by con‐
     vention 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
     generate 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 combining 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 modi‐
     fied 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 under‐
		stood by ld(1) and ld.so(1) are LD_VERSION_SUN (3), which is
		used by the SunOS 4.x releases, and LD_VERSION_BSD (8), which
		has been in use since FreeBSD 1.1.

     d_un	Refers to a d_version dependent data structure.

     so_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.

     The section_dispatch_table structure is the main “dispatcher” table, con‐
     taining offsets into the image's segments where various symbol and relo‐
     cation information is located.

	   struct section_dispatch_table {
		   struct  so_map *sdt_loaded;
		   long	   sdt_sods;
		   long	   sdt_filler1;
		   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

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

     sdt_filler1  Deprecated (used by SunOS to specify library search rules).

     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 sec‐
     tion_dispatch_table structure.

	   struct sod {
		   long	   sod_name;
		   u_int   sod_library : 1,
			   sod_reserved : 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 {
		   caddr_t som_addr;
		   char	   *som_path;
		   struct  so_map *som_next;
		   struct  sod *som_sod;
		   caddr_t som_sodbase;
		   u_int   som_write : 1;
		   struct  _dynamic *som_dynamic;
		   caddr_t 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 out in later versions of the run-time linker.

     som_write	  Set if (some portion of) this object's text segment is cur‐
		  rently 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 com‐
     mons 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;
		   caddr_t		   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 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 sym‐
     bols from shared objects can only do so after the link-editor has been
     called from crt0.	A dynamically linked binary contains a so_debug struc‐
     ture 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 where 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 sym‐
		     bols that the debugger may be interested in.

     The ld_entry structure defines a set of service routines within ld.so.

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

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

	   struct crt_ldso {
		   int		   crt_ba;
		   int		   crt_dzfd;
		   int		   crt_ldfd;
		   struct _dynamic *crt_dp;
		   char		   **crt_ep;
		   caddr_t	   crt_bp;
		   char		   *crt_prog;
		   char		   *crt_ldso;
		   struct ld_entry *crt_ldentry;
	   };
	   #define CRT_VERSION_SUN	   1
	   #define CRT_VERSION_BSD_2	   2
	   #define CRT_VERSION_BSD_3	   3
	   #define CRT_VERSION_BSD_4	   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
	       FreeBSD 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_VER‐
	       SION_BSD3 only).

     crt_ldso  The path of the run-time linker as mapped by crt0 (CRT_VER‐
	       SION_BSD4 only).

     The hints_header and hints_bucket structures define the layout of the
     library hints, normally 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” 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
		   long		   hh_hashtab;
		   long		   hh_nbucket;
		   long		   hh_strtab;
		   long		   hh_strtab_sz;
		   long		   hh_ehints;
	   };

     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.

	   /*
	    * 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.

CAVEATS
     Only the (GNU) C compiler currently supports the creation of shared
     libraries.	 Other programming languages cannot be used.

BSD			       October 23, 1993				   BSD
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