1.\" Copyright (c) 1993 Paul Kranenburg 2.\" All rights reserved. 3.\" 4.\" Redistribution and use in source and binary forms, with or without 5.\" modification, are permitted provided that the following conditions 6.\" are met: 7.\" 1. Redistributions of source code must retain the above copyright 8.\" notice, this list of conditions and the following disclaimer. 9.\" 2. Redistributions in binary form must reproduce the above copyright 10.\" notice, this list of conditions and the following disclaimer in the 11.\" documentation and/or other materials provided with the distribution. 12.\" 3. All advertising materials mentioning features or use of this software 13.\" must display the following acknowledgement: 14.\" This product includes software developed by Paul Kranenburg. 15.\" 3. The name of the author may not be used to endorse or promote products 16.\" derived from this software without specific prior written permission 17.\" 18.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR 19.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES 20.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. 21.\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, 22.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT 23.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, 24.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY 25.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT 26.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF 27.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. 28.\" 29.\" $FreeBSD$ 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 42The include file 43.In link.h 44declares several structures that are present in dynamically linked 45programs and libraries. 46The structures define the interface between several components of the 47link-editor and loader mechanism. 48The layout of a number of these 49structures within the binaries resembles the a.out format in many places 50as it serves such similar functions as symbol definitions (including the 51accompanying string table) and relocation records needed to resolve 52references to external entities. 53It also records a number of data structures 54unique to the dynamic loading and linking process. 55These include references 56to other objects that are required to complete the link-editing process and 57indirection tables to facilitate 58.Em Position Independent Code 59(PIC for short) to improve sharing of code pages among different processes. 60The collection of data structures described here will be referred to as the 61.Em Run-time Relocation Section (RRS) 62and is embedded in the standard text and data segments of the dynamically 63linked program or shared object image as the existing 64.Xr a.out 5 65format offers no room for it elsewhere. 66.Pp 67Several utilities cooperate to ensure that the task of getting a program 68ready to run can complete successfully in a way that optimizes the use 69of system resources. 70The compiler emits PIC code from which shared libraries 71can be built by 72.Xr ld 1 . 73The compiler also includes size information of any initialized data items 74through the .size assembler directive. 75PIC code differs from conventional code 76in that it accesses data variables through an indirection table, the 77Global Offset Table, by convention accessible by the reserved name 78.Dv _GLOBAL_OFFSET_TABLE_ . 79The exact mechanism used for this is machine dependent, usually a machine 80register is reserved for the purpose. 81The rational behind this construct 82is to generate code that is independent of the actual load address. 83Only 84the values contained in the Global Offset Table may need updating at run-time 85depending on the load addresses of the various shared objects in the address 86space. 87.Pp 88Likewise, procedure calls to globally defined functions are redirected through 89the Procedure Linkage Table (PLT) residing in the data segment of the core 90image. 91Again, this is done to avoid run-time modifications to the text segment. 92.Pp 93The linker-editor allocates the Global Offset Table and Procedure Linkage Table 94when combining PIC object files into an image suitable for mapping into the 95process address space. 96It also collects all symbols that may be needed by the 97run-time link-editor and stores these along with the image's text and data bits. 98Another reserved symbol, 99.Em _DYNAMIC 100is used to indicate the presence of the run-time linker structures. 101Whenever 102_DYNAMIC is relocated to 0, there is no need to invoke the run-time 103link-editor. 104If this symbol is non-zero, it points at a data structure from 105which the location of the necessary relocation- and symbol information can 106be derived. 107This is most notably used by the start-up module, 108.Em crt0 . 109The _DYNAMIC structure is conventionally located at the start of the data 110segment of the image to which it pertains. 111.Sh DATA STRUCTURES 112The data structures supporting dynamic linking and run-time relocation 113reside both in the text and data segments of the image they apply to. 114The text segments contain read-only data such as symbols descriptions and 115names, while the data segments contain the tables that need to be modified by 116during the relocation process. 117.Pp 118The _DYNAMIC symbol references a 119.Fa _dynamic 120structure: 121.Bd -literal -offset indent 122struct _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 133This field provides for different versions of the dynamic linking 134implementation. 135The current version numbers understood by 136.Xr ld 1 137and 138.Xr ld.so 1 139are 140.Em LD_VERSION_SUN (3) , 141which is used by the 142.Tn SunOS 1434.x releases, and 144.Em LD_VERSION_BSD (8) , 145which has been in use since 146.Fx 1.1 . 147.It Fa d_un 148Refers to a 149.Em d_version 150dependent data structure. 151.It Fa so_debug 152this field provides debuggers with a hook to access symbol tables of shared 153objects loaded as a result of the actions of the run-time link-editor. 154.El 155.Pp 156The 157.Fa section_dispatch_table 158structure is the main 159.Dq dispatcher 160table, containing offsets into the image's segments where various symbol 161and relocation information is located. 162.Bd -literal -offset indent 163struct 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.Pp 181.Bl -tag -width sdt_filler1 182.It Fa sdt_loaded 183A pointer to the first link map loaded (see below). 184This field is set by 185.Nm ld.so 186.It Fa sdt_sods 187The start of a (linked) list of shared object descriptors needed by 188.Em this 189object. 190.It Fa sdt_filler1 191Deprecated (used by SunOS to specify library search rules). 192.It Fa sdt_got 193The location of the Global Offset Table within this image. 194.It Fa sdt_plt 195The location of the Procedure Linkage Table within this image. 196.It Fa sdt_rel 197The location of an array of 198.Fa relocation_info 199structures 200(see 201.Xr a.out 5 ) 202specifying run-time relocations. 203.It Fa sdt_hash 204The location of the hash table for fast symbol lookup in this object's 205symbol table. 206.It Fa sdt_nzlist 207The location of the symbol table. 208.It Fa sdt_filler2 209Currently unused. 210.It Fa sdt_buckets 211The number of buckets in 212.Fa sdt_hash 213.It Fa sdt_strings 214The location of the symbol string table that goes with 215.Fa sdt_nzlist . 216.It Fa sdt_str_sz 217The size of the string table. 218.It Fa sdt_text_sz 219The size of the object's text segment. 220.It Fa sdt_plt_sz 221The size of the Procedure Linkage Table. 222.El 223.Pp 224A 225.Fa sod 226structure describes a shared object that is needed 227to complete the link edit process of the object containing it. 228A list of such objects 229(chained through 230.Fa sod_next ) 231is pointed at 232by the 233.Fa sdt_sods 234in the section_dispatch_table structure. 235.Bd -literal -offset indent 236struct sod { 237 long sod_name; 238 u_int sod_library : 1, 239 sod_reserved : 31; 240 short sod_major; 241 short sod_minor; 242 long sod_next; 243}; 244.Ed 245.Pp 246.Bl -tag -width sod_library 247.It Fa sod_name 248The offset in the text segment of a string describing this link object. 249.It Fa sod_library 250If set, 251.Fa sod_name 252specifies a library that is to be searched for by 253.Nm ld.so . 254The path name 255is obtained by searching a set of directories 256(see also 257.Xr ldconfig 8 ) 258for a shared object matching 259.Em lib\&<sod_name>\&.so.n.m . 260If not set, 261.Fa sod_name 262should point at a full path name for the desired shared object. 263.It Fa sod_major 264Specifies the major version number of the shared object to load. 265.It Fa sod_minor 266Specifies the preferred minor version number of the shared object to load. 267.El 268.Pp 269The run-time link-editor maintains a list of structures called 270.Em link maps 271to keep track of all shared objects loaded into a process' address space. 272These structures are only used at run-time and do not occur within 273the text or data segment of an executable or shared library. 274.Bd -literal -offset indent 275struct so_map { 276 caddr_t som_addr; 277 char *som_path; 278 struct so_map *som_next; 279 struct sod *som_sod; 280 caddr_t som_sodbase; 281 u_int som_write : 1; 282 struct _dynamic *som_dynamic; 283 caddr_t som_spd; 284}; 285.Ed 286.Bl -tag -width som_dynamic 287.It Fa som_addr 288The address at which the shared object associated with this link map has 289been loaded. 290.It Fa som_path 291The full path name of the loaded object. 292.It Fa som_next 293Pointer to the next link map. 294.It Fa som_sod 295The 296.Fa sod 297structure that was responsible for loading this shared object. 298.It Fa som_sodbase 299Tossed out in later versions of the run-time linker. 300.It Fa som_write 301Set if (some portion of) this object's text segment is currently writable. 302.It Fa som_dynamic 303Pointer to this object's 304.Fa _dynamic 305structure. 306.It Fa som_spd 307Hook for attaching private data maintained by the run-time link-editor. 308.El 309.Pp 310Symbol description with size. 311This is simply an 312.Fa nlist 313structure with one field 314.Pq Fa nz_size 315added. 316Used to convey size information on items in the data segment 317of shared objects. 318An array of these lives in the shared object's 319text segment and is addressed by the 320.Fa sdt_nzlist 321field of 322.Fa section_dispatch_table . 323.Bd -literal -offset indent 324struct nzlist { 325 struct nlist nlist; 326 u_long nz_size; 327#define nz_un nlist.n_un 328#define nz_strx nlist.n_un.n_strx 329#define nz_name nlist.n_un.n_name 330#define nz_type nlist.n_type 331#define nz_value nlist.n_value 332#define nz_desc nlist.n_desc 333#define nz_other nlist.n_other 334}; 335.Ed 336.Bl -tag -width nz_size 337.It Fa nlist 338(see 339.Xr nlist 3 ) . 340.It Fa nz_size 341The size of the data represented by this symbol. 342.El 343.Pp 344A hash table is included within the text segment of shared object 345to facilitate quick lookup of symbols during run-time link-editing. 346The 347.Fa sdt_hash 348field of the 349.Fa section_dispatch_table 350structure points at an array of 351.Fa rrs_hash 352structures: 353.Bd -literal -offset indent 354struct rrs_hash { 355 int rh_symbolnum; /* symbol number */ 356 int rh_next; /* next hash entry */ 357}; 358.Ed 359.Pp 360.Bl -tag -width rh_symbolnum 361.It Fa rh_symbolnum 362The index of the symbol in the shared object's symbol table (as given by the 363.Fa ld_symbols 364field). 365.It Fa rh_next 366In case of collisions, this field is the offset of the next entry in this 367hash table bucket. 368It is zero for the last bucket element. 369.El 370The 371.Fa rt_symbol 372structure is used to keep track of run-time allocated commons 373and data items copied from shared objects. 374These items are kept on linked list 375and is exported through the 376.Fa dd_cc 377field in the 378.Fa so_debug 379structure (see below) for use by debuggers. 380.Bd -literal -offset indent 381struct rt_symbol { 382 struct nzlist *rt_sp; 383 struct rt_symbol *rt_next; 384 struct rt_symbol *rt_link; 385 caddr_t rt_srcaddr; 386 struct so_map *rt_smp; 387}; 388.Ed 389.Pp 390.Bl -tag -width rt_scraddr 391.It Fa rt_sp 392The symbol description. 393.It Fa rt_next 394Virtual address of next rt_symbol. 395.It Fa rt_link 396Next in hash bucket. 397Used internally by 398.Nm ld.so . 399.It Fa rt_srcaddr 400Location of the source of initialized data within a shared object. 401.It Fa rt_smp 402The shared object which is the original source of the data that this 403run-time symbol describes. 404.El 405.Pp 406The 407.Fa so_debug 408structure is used by debuggers to gain knowledge of any shared objects 409that have been loaded in the process's address space as a result of run-time 410link-editing. 411Since the run-time link-editor runs as a part of process 412initialization, a debugger that wishes to access symbols from shared objects 413can only do so after the link-editor has been called from crt0. 414A dynamically linked binary contains a 415.Fa so_debug 416structure which can be located by means of the 417.Fa d_debug 418field in 419.Fa _dynamic . 420.Bd -literal -offset indent 421struct so_debug { 422 int dd_version; 423 int dd_in_debugger; 424 int dd_sym_loaded; 425 char *dd_bpt_addr; 426 int dd_bpt_shadow; 427 struct rt_symbol *dd_cc; 428}; 429.Ed 430.Pp 431.Bl -tag -width dd_in_debugger 432.It Fa dd_version 433Version number of this interface. 434.It Fa dd_in_debugger 435Set by the debugger to indicate to the run-time linker that the program is 436run under control of a debugger. 437.It Fa dd_sym_loaded 438Set by the run-time linker whenever it adds symbols by loading shared objects. 439.It Fa dd_bpt_addr 440The address where a breakpoint will be set by the run-time linker to 441divert control to the debugger. 442This address is determined by the start-up 443module, 444.Pa crt0.o , 445to be some convenient place before the call to _main. 446.It Fa dd_bpt_shadow 447Contains the original instruction that was at 448.Fa dd_bpt_addr . 449The debugger is expected to put this instruction back before continuing the 450program. 451.It Fa dd_cc 452A pointer to the linked list of run-time allocated symbols that the debugger 453may be interested in. 454.El 455.Pp 456The 457.Em ld_entry 458structure defines a set of service routines within 459.Nm ld.so . 460.\" See 461.\" .Xr libdl.a 462.\" for more information. 463.Bd -literal -offset indent 464struct ld_entry { 465 void *(*dlopen)(char *, int); 466 int (*dlclose)(void *); 467 void *(*dlsym)(void *, char *); 468 char *(*dlerror)(void); 469}; 470.Ed 471.Pp 472The 473.Fa crt_ldso 474structure defines the interface between the start-up code in crt0 and 475.Nm ld.so . 476.Bd -literal -offset indent 477struct crt_ldso { 478 int crt_ba; 479 int crt_dzfd; 480 int crt_ldfd; 481 struct _dynamic *crt_dp; 482 char **crt_ep; 483 caddr_t crt_bp; 484 char *crt_prog; 485 char *crt_ldso; 486 struct ld_entry *crt_ldentry; 487}; 488#define CRT_VERSION_SUN 1 489#define CRT_VERSION_BSD_2 2 490#define CRT_VERSION_BSD_3 3 491#define CRT_VERSION_BSD_4 4 492.Ed 493.Bl -tag -width crt_dzfd 494.It Fa crt_ba 495The virtual address at which 496.Nm ld.so 497was loaded by crt0. 498.It Fa crt_dzfd 499On SunOS systems, this field contains an open file descriptor to 500.Dq Pa /dev/zero 501used to get demand paged zeroed pages. 502On 503.Fx 504systems it contains -1. 505.It Fa crt_ldfd 506Contains an open file descriptor that was used by crt0 to load 507.Nm ld.so . 508.It Fa crt_dp 509A pointer to main's 510.Fa _dynamic 511structure. 512.It Fa crt_ep 513A pointer to the environment strings. 514.It Fa crt_bp 515The address at which a breakpoint will be placed by the run-time linker 516if the main program is run by a debugger. 517See 518.Fa so_debug 519.It Fa crt_prog 520The name of the main program as determined by crt0 (CRT_VERSION_BSD3 only). 521.It Fa crt_ldso 522The path of the run-time linker as mapped by crt0 (CRT_VERSION_BSD4 only). 523.El 524.Pp 525The 526.Fa hints_header 527and 528.Fa hints_bucket 529structures define the layout of the library hints, normally found in 530.Dq Pa /var/run/ld.so.hints , 531which is used by 532.Nm ld.so 533to quickly locate the shared object images in the 534file system. 535The organization of the hints file is not unlike that of an 536.Dq a.out 537object file, in that it contains a header determining the offset and size 538of a table of fixed sized hash buckets and a common string pool. 539.Bd -literal -offset indent 540struct hints_header { 541 long hh_magic; 542#define HH_MAGIC 011421044151 543 long hh_version; 544#define LD_HINTS_VERSION_1 1 545 long hh_hashtab; 546 long hh_nbucket; 547 long hh_strtab; 548 long hh_strtab_sz; 549 long hh_ehints; 550}; 551.Ed 552.Bl -tag -width hh_strtab_sz 553.It Fa hh_magic 554Hints file magic number. 555.It Fa hh_version 556Interface version number. 557.It Fa hh_hashtab 558Offset of hash table. 559.It Fa hh_strtab 560Offset of string table. 561.It Fa hh_strtab_sz 562Size of strings. 563.It Fa hh_ehints 564Maximum usable offset in hints file. 565.El 566.Pp 567.Bd -literal -offset indent 568/* 569 * Hash table element in hints file. 570 */ 571struct hints_bucket { 572 int hi_namex; 573 int hi_pathx; 574 int hi_dewey[MAXDEWEY]; 575 int hi_ndewey; 576#define hi_major hi_dewey[0] 577#define hi_minor hi_dewey[1] 578 int hi_next; 579}; 580.Ed 581.Bl -tag -width hi_ndewey 582.It Fa hi_namex 583Index of the string identifying the library. 584.It Fa hi_pathx 585Index of the string representing the full path name of the library. 586.It Fa hi_dewey 587The version numbers of the shared library. 588.It Fa hi_ndewey 589The number of valid entries in 590.Fa hi_dewey . 591.It Fa hi_next 592Next bucket in case of hashing collisions. 593.El 594.Sh CAVEATS 595Only the (GNU) C compiler currently supports the creation of shared libraries. 596Other programming languages cannot be used. 597