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1 .\" Copyright (c) 1993 Paul Kranenburg
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29 .\" $FreeBSD: src/share/man/man5/link.5,v 1.14.2.10 2004/05/17 11:38:55 brueffer Exp $
30 .\"
31 .Dd October 23, 1993
32 .Dt LINK 5
33 .Os
34 .Sh NAME
35 .Nm link
36 .Nd dynamic loader and link editor interface
37 .Sh SYNOPSIS
38 .In sys/types.h
39 .In nlist.h
40 .In link.h
41 .Sh DESCRIPTION
42 The include file
43 .In link.h
44 declares several structures that are present in dynamically linked
45 programs and libraries.
46 The structures define the interface between several components of the
47 link-editor and loader mechanism.
48 The layout of a number of these
49 structures within the binaries resembles the a.out format in many places
50 as it serves such similar functions as symbol definitions (including the
51 accompanying string table) and relocation records needed to resolve
52 references to external entities.
53 It also records a number of data structures
54 unique to the dynamic loading and linking process.
55 These include references
56 to other objects that are required to complete the link-editing process and
57 indirection tables to facilitate
58 .Em Position Independent Code
59 (PIC for short) to improve sharing of code pages among different processes.
60 The collection of data structures described here will be referred to as the
61 .Em Run-time Relocation Section (RRS)
62 and is embedded in the standard text and data segments of the dynamically
63 linked program or shared object image as the existing
64 .Xr a.out 5
65 format offers no room for it elsewhere.
66 .Pp
67 Several utilities cooperate to ensure that the task of getting a program
68 ready to run can complete successfully in a way that optimizes the use
69 of system resources.
70 The compiler emits PIC code from which shared libraries
71 can be built by
72 .Xr ld 1 .
73 The compiler also includes size information of any initialized data items
74 through the .size assembler directive.
75 PIC code differs from conventional code
76 in that it accesses data variables through an indirection table, the
77 Global Offset Table, by convention accessible by the reserved name
78 .Dv _GLOBAL_OFFSET_TABLE_ .
79 The exact mechanism used for this is machine dependent, usually a machine
80 register is reserved for the purpose.
81 The rationale behind this construct
82 is to generate code that is independent of the actual load address.
83 Only
84 the values contained in the Global Offset Table may need updating at run-time
85 depending on the load addresses of the various shared objects in the address
86 space.
87 .Pp
88 Likewise, procedure calls to globally defined functions are redirected through
89 the Procedure Linkage Table (PLT) residing in the data segment of the core
90 image.
91 Again, this is done to avoid run-time modifications to the text segment.
92 .Pp
93 The linker-editor allocates the Global Offset Table and Procedure Linkage Table
94 when combining PIC object files into an image suitable for mapping into the
95 process address space.
96 It also collects all symbols that may be needed by the
97 run-time link-editor and stores these along with the image's text and data bits.
98 Another reserved symbol,
99 .Em _DYNAMIC
100 is used to indicate the presence of the run-time linker structures.
101 Whenever
102 _DYNAMIC is relocated to 0, there is no need to invoke the run-time
103 link-editor.
104 If this symbol is non-zero, it points at a data structure from
105 which the location of the necessary relocation- and symbol information can
106 be derived.
107 This is most notably used by the start-up module,
108 .Em crt0 .
109 The _DYNAMIC structure is conventionally located at the start of the data
110 segment of the image to which it pertains.
111 .Sh DATA STRUCTURES
112 The data structures supporting dynamic linking and run-time relocation
113 reside both in the text and data segments of the image they apply to.
114 The text segments contain read-only data such as symbols descriptions and
115 names, while the data segments contain the tables that need to be modified by
116 during the relocation process.
117 .Pp
118 The _DYNAMIC symbol references a
119 .Fa _dynamic
120 structure:
121 .Bd -literal -offset indent
122 struct _dynamic {
123 int d_version;
124 struct so_debug *d_debug;
125 union {
126 struct section_dispatch_table *d_sdt;
127 } d_un;
128 struct ld_entry *d_entry;
129 };
130 .Ed
131 .Bl -tag -width d_version
132 .It Fa d_version
133 This field provides for different versions of the dynamic linking
134 implementation.
135 The current version numbers understood by
136 .Xr ld 1
137 and
138 .Xr rtld 1
139 are
140 .Em LD_VERSION_SUN (3) ,
141 which is used by the
142 .Tn SunOS
143 4.x releases, and
144 .Em LD_VERSION_BSD (8) ,
145 which has been in use since
146 .Fx 1.1 .
147 .It Fa d_un
148 Refers to a
149 .Em d_version
150 dependent data structure.
151 .It Fa so_debug
152 this field provides debuggers with a hook to access symbol tables of shared
153 objects loaded as a result of the actions of the run-time link-editor.
154 .El
155 .Pp
156 The
157 .Fa section_dispatch_table
158 structure is the main
159 .Dq dispatcher
160 table, containing offsets into the image's segments where various symbol
161 and relocation information is located.
162 .Bd -literal -offset indent
163 struct section_dispatch_table {
164 struct so_map *sdt_loaded;
165 long sdt_sods;
166 long sdt_filler1;
167 long sdt_got;
168 long sdt_plt;
169 long sdt_rel;
170 long sdt_hash;
171 long sdt_nzlist;
172 long sdt_filler2;
173 long sdt_buckets;
174 long sdt_strings;
175 long sdt_str_sz;
176 long sdt_text_sz;
177 long sdt_plt_sz;
178 };
179 .Ed
180 .Bl -tag -width sdt_filler1
181 .It Fa sdt_loaded
182 A pointer to the first link map loaded (see below). This field is set by
183 .Xr rtld 1
184 .It Fa sdt_sods
185 The start of a (linked) list of shared object descriptors needed by
186 .Em this
187 object.
188 .It Fa sdt_filler1
189 Deprecated (used by SunOS to specify library search rules).
190 .It Fa sdt_got
191 The location of the Global Offset Table within this image.
192 .It Fa sdt_plt
193 The location of the Procedure Linkage Table within this image.
194 .It Fa sdt_rel
195 The location of an array of
196 .Fa relocation_info
197 structures
198 (see
199 .Xr a.out 5 )
200 specifying run-time relocations.
201 .It Fa sdt_hash
202 The location of the hash table for fast symbol lookup in this object's
203 symbol table.
204 .It Fa sdt_nzlist
205 The location of the symbol table.
206 .It Fa sdt_filler2
207 Currently unused.
208 .It Fa sdt_buckets
209 The number of buckets in
210 .Fa sdt_hash
211 .It Fa sdt_strings
212 The location of the symbol string table that goes with
213 .Fa sdt_nzlist .
214 .It Fa sdt_str_sz
215 The size of the string table.
216 .It Fa sdt_text_sz
217 The size of the object's text segment.
218 .It Fa sdt_plt_sz
219 The size of the Procedure Linkage Table.
220 .El
221 .Pp
222 A
223 .Fa sod
224 structure describes a shared object that is needed
225 to complete the link edit process of the object containing it.
226 A list of such objects
227 (chained through
228 .Fa sod_next )
229 is pointed at
230 by the
231 .Fa sdt_sods
232 in the section_dispatch_table structure.
233 .Bd -literal -offset indent
234 struct sod {
235 long sod_name;
236 u_int sod_library : 1,
237 sod_reserved : 31;
238 short sod_major;
239 short sod_minor;
240 long sod_next;
241 };
242 .Ed
243 .Bl -tag -width sod_library
244 .It Fa sod_name
245 The offset in the text segment of a string describing this link object.
246 .It Fa sod_library
247 If set,
248 .Fa sod_name
249 specifies a library that is to be searched for by
250 .Xr rtld 1 .
251 The path name
252 is obtained by searching a set of directories
253 (see also
254 .Xr ldconfig 8 )
255 for a shared object matching
256 .Em lib\&<sod_name>\&.so.n.m .
257 If not set,
258 .Fa sod_name
259 should point at a full path name for the desired shared object.
260 .It Fa sod_major
261 Specifies the major version number of the shared object to load.
262 .It Fa sod_minor
263 Specifies the preferred minor version number of the shared object to load.
264 .El
265 .Pp
266 The run-time link-editor maintains a list of structures called
267 .Em link maps
268 to keep track of all shared objects loaded into a process' address space.
269 These structures are only used at run-time and do not occur within
270 the text or data segment of an executable or shared library.
271 .Bd -literal -offset indent
272 struct so_map {
273 caddr_t som_addr;
274 char *som_path;
275 struct so_map *som_next;
276 struct sod *som_sod;
277 caddr_t som_sodbase;
278 u_int som_write : 1;
279 struct _dynamic *som_dynamic;
280 caddr_t som_spd;
281 };
282 .Ed
283 .Bl -tag -width som_dynamic
284 .It Fa som_addr
285 The address at which the shared object associated with this link map has
286 been loaded.
287 .It Fa som_path
288 The full path name of the loaded object.
289 .It Fa som_next
290 Pointer to the next link map.
291 .It Fa som_sod
292 The
293 .Fa sod
294 structure that was responsible for loading this shared object.
295 .It Fa som_sodbase
296 Tossed out in later versions of the run-time linker.
297 .It Fa som_write
298 Set if (some portion of) this object's text segment is currently writable.
299 .It Fa som_dynamic
300 Pointer to this object's
301 .Fa _dynamic
302 structure.
303 .It Fa som_spd
304 Hook for attaching private data maintained by the run-time link-editor.
305 .El
306 .Pp
307 Symbol description with size.
308 This is simply an
309 .Fa nlist
310 structure with one field
311 .Pq Fa nz_size
312 added.
313 Used to convey size information on items in the data segment
314 of shared objects.
315 An array of these lives in the shared object's
316 text segment and is addressed by the
317 .Fa sdt_nzlist
318 field of
319 .Fa section_dispatch_table .
320 .Bd -literal -offset indent
321 struct nzlist {
322 struct nlist nlist;
323 u_long nz_size;
324 #define nz_un nlist.n_un
325 #define nz_strx nlist.n_un.n_strx
326 #define nz_name nlist.n_un.n_name
327 #define nz_type nlist.n_type
328 #define nz_value nlist.n_value
329 #define nz_desc nlist.n_desc
330 #define nz_other nlist.n_other
331 };
332 .Ed
333 .Bl -tag -width nz_size
334 .It Fa nlist
335 (see
336 .Xr nlist 3 ) .
337 .It Fa nz_size
338 The size of the data represented by this symbol.
339 .El
340 .Pp
341 A hash table is included within the text segment of shared object
342 to facilitate quick lookup of symbols during run-time link-editing.
343 The
344 .Fa sdt_hash
345 field of the
346 .Fa section_dispatch_table
347 structure points at an array of
348 .Fa rrs_hash
349 structures:
350 .Bd -literal -offset indent
351 struct rrs_hash {
352 int rh_symbolnum; /* symbol number */
353 int rh_next; /* next hash entry */
354 };
355 .Ed
356 .Bl -tag -width rh_symbolnum
357 .It Fa rh_symbolnum
358 The index of the symbol in the shared object's symbol table (as given by the
359 .Fa ld_symbols
360 field).
361 .It Fa rh_next
362 In case of collisions, this field is the offset of the next entry in this
363 hash table bucket.
364 It is zero for the last bucket element.
365 .El
366 The
367 .Fa rt_symbol
368 structure is used to keep track of run-time allocated commons
369 and data items copied from shared objects.
370 These items are kept on linked list
371 and is exported through the
372 .Fa dd_cc
373 field in the
374 .Fa so_debug
375 structure (see below) for use by debuggers.
376 .Bd -literal -offset indent
377 struct rt_symbol {
378 struct nzlist *rt_sp;
379 struct rt_symbol *rt_next;
380 struct rt_symbol *rt_link;
381 caddr_t rt_srcaddr;
382 struct so_map *rt_smp;
383 };
384 .Ed
385 .Bl -tag -width rt_scraddr
386 .It Fa rt_sp
387 The symbol description.
388 .It Fa rt_next
389 Virtual address of next rt_symbol.
390 .It Fa rt_link
391 Next in hash bucket.
392 Used internally by
393 .Xr rtld 1 .
394 .It Fa rt_srcaddr
395 Location of the source of initialized data within a shared object.
396 .It Fa rt_smp
397 The shared object which is the original source of the data that this
398 run-time symbol describes.
399 .El
400 .Pp
401 The
402 .Fa so_debug
403 structure is used by debuggers to gain knowledge of any shared objects
404 that have been loaded in the process's address space as a result of run-time
405 link-editing.
406 Since the run-time link-editor runs as a part of process
407 initialization, a debugger that wishes to access symbols from shared objects
408 can only do so after the link-editor has been called from crt0.
409 A dynamically linked binary contains a
410 .Fa so_debug
411 structure which can be located by means of the
412 .Fa d_debug
413 field in
414 .Fa _dynamic .
415 .Bd -literal -offset indent
416 struct so_debug {
417 int dd_version;
418 int dd_in_debugger;
419 int dd_sym_loaded;
420 char *dd_bpt_addr;
421 int dd_bpt_shadow;
422 struct rt_symbol *dd_cc;
423 };
424 .Ed
425 .Bl -tag -width dd_in_debugger
426 .It Fa dd_version
427 Version number of this interface.
428 .It Fa dd_in_debugger
429 Set by the debugger to indicate to the run-time linker that the program is
430 run under control of a debugger.
431 .It Fa dd_sym_loaded
432 Set by the run-time linker whenever it adds symbols by loading shared objects.
433 .It Fa dd_bpt_addr
434 The address where a breakpoint will be set by the run-time linker to
435 divert control to the debugger.
436 This address is determined by the start-up
437 module,
438 .Pa crt0.o ,
439 to be some convenient place before the call to _main.
440 .It Fa dd_bpt_shadow
441 Contains the original instruction that was at
442 .Fa dd_bpt_addr .
443 The debugger is expected to put this instruction back before continuing the
444 program.
445 .It Fa dd_cc
446 A pointer to the linked list of run-time allocated symbols that the debugger
447 may be interested in.
448 .El
449 .Pp
450 The
451 .Em ld_entry
452 structure defines a set of service routines within
453 .Xr rtld 1 .
454 .\" See
455 .\" .Xr libdl.a
456 .\" for more information.
457 .Bd -literal -offset indent
458 struct ld_entry {
459 void *(*dlopen)(char *, int);
460 int (*dlclose)(void *);
461 void *(*dlsym)(void *, char *);
462 char *(*dlerror)(void);
463 };
464 .Ed
465 .Pp
466 The
467 .Fa crt_ldso
468 structure defines the interface between the start-up code in crt0 and
469 .Xr rtld 1 .
470 .Bd -literal -offset indent
471 struct crt_ldso {
472 int crt_ba;
473 int crt_dzfd;
474 int crt_ldfd;
475 struct _dynamic *crt_dp;
476 char **crt_ep;
477 caddr_t crt_bp;
478 char *crt_prog;
479 char *crt_ldso;
480 struct ld_entry *crt_ldentry;
481 };
482 #define CRT_VERSION_SUN 1
483 #define CRT_VERSION_BSD_2 2
484 #define CRT_VERSION_BSD_3 3
485 #define CRT_VERSION_BSD_4 4
486 .Ed
487 .Bl -tag -width crt_dzfd
488 .It Fa crt_ba
489 The virtual address at which
490 .Xr rtld 1
491 was loaded by crt0.
492 .It Fa crt_dzfd
493 On SunOS systems, this field contains an open file descriptor to
494 .Dq Pa /dev/zero
495 used to get demand paged zeroed pages.
496 On
497 .Dx
498 systems it contains -1.
499 .It Fa crt_ldfd
500 Contains an open file descriptor that was used by crt0 to load
501 .Xr rtld 1 .
502 .It Fa crt_dp
503 A pointer to main's
504 .Fa _dynamic
505 structure.
506 .It Fa crt_ep
507 A pointer to the environment strings.
508 .It Fa crt_bp
509 The address at which a breakpoint will be placed by the run-time linker
510 if the main program is run by a debugger.
511 See
512 .Fa so_debug
513 .It Fa crt_prog
514 The name of the main program as determined by crt0 (CRT_VERSION_BSD3 only).
515 .It Fa crt_ldso
516 The path of the run-time linker as mapped by crt0 (CRT_VERSION_BSD4 only).
517 .El
518 .Pp
519 The
520 .Fa hints_header
521 and
522 .Fa hints_bucket
523 structures define the layout of the library hints, normally found in
524 .Dq Pa /var/run/ld-elf.so.hints ,
525 which is used by
526 .Xr rtld 1
527 to quickly locate the shared object images in the
528 filesystem.
529 The organization of the hints file is not unlike that of an
530 .Dq a.out
531 object file, in that it contains a header determining the offset and size
532 of a table of fixed sized hash buckets and a common string pool.
533 .Bd -literal -offset indent
534 struct hints_header {
535 long hh_magic;
536 #define HH_MAGIC 011421044151
537 long hh_version;
538 #define LD_HINTS_VERSION_1 1
539 long hh_hashtab;
540 long hh_nbucket;
541 long hh_strtab;
542 long hh_strtab_sz;
543 long hh_ehints;
544 };
545 .Ed
546 .Bl -tag -width hh_strtab_sz
547 .It Fa hh_magic
548 Hints file magic number.
549 .It Fa hh_version
550 Interface version number.
551 .It Fa hh_hashtab
552 Offset of hash table.
553 .It Fa hh_strtab
554 Offset of string table.
555 .It Fa hh_strtab_sz
556 Size of strings.
557 .It Fa hh_ehints
558 Maximum usable offset in hints file.
559 .El
560 .Bd -literal -offset indent
561 /*
562 * Hash table element in hints file.
563 */
564 struct hints_bucket {
565 int hi_namex;
566 int hi_pathx;
567 int hi_dewey[MAXDEWEY];
568 int hi_ndewey;
569 #define hi_major hi_dewey[0]
570 #define hi_minor hi_dewey[1]
571 int hi_next;
572 };
573 .Ed
574 .Bl -tag -width hi_ndewey
575 .It Fa hi_namex
576 Index of the string identifying the library.
577 .It Fa hi_pathx
578 Index of the string representing the full path name of the library.
579 .It Fa hi_dewey
580 The version numbers of the shared library.
581 .It Fa hi_ndewey
582 The number of valid entries in
583 .Fa hi_dewey .
584 .It Fa hi_next
585 Next bucket in case of hashing collisions.
586 .El
587 .Sh CAVEATS
588 Only the (GNU) C compiler currently supports the creation of shared libraries.
589 Other programming languages cannot be used.