1.\" $NetBSD: link.5,v 1.6 1996/11/11 00:05:05 lukem Exp $ 2.\" 3.\" Copyright (c) 1993 Paul Kranenburg 4.\" All rights reserved. 5.\" 6.\" Redistribution and use in source and binary forms, with or without 7.\" modification, are permitted provided that the following conditions 8.\" are met: 9.\" 1. Redistributions of source code must retain the above copyright 10.\" notice, this list of conditions and the following disclaimer. 11.\" 2. Redistributions in binary form must reproduce the above copyright 12.\" notice, this list of conditions and the following disclaimer in the 13.\" documentation and/or other materials provided with the distribution. 14.\" 3. All advertising materials mentioning features or use of this software 15.\" must display the following acknowledgement: 16.\" This product includes software developed by Paul Kranenburg. 17.\" 3. The name of the author may not be used to endorse or promote products 18.\" derived from this software without specific prior written permission 19.\" 20.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR 21.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES 22.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. 23.\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, 24.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT 25.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, 26.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY 27.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT 28.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF 29.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. 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.Fd #include <link.h> 39.Sh DESCRIPTION 40The include file 41.Aq Pa link.h 42declares several structures that are present in dynamically linked 43programs and libraries. 44The structures define the interface between several components of the 45link-editor and loader mechanism. The layout of a number of these 46structures within the binaries resembles the a.out format in many places 47as it serves such similar functions as symbol definitions (including the 48accompanying string table) and relocation records needed to resolve 49references to external entities. It also records a number of data structures 50unique to the dynamic loading and linking process. These include references 51to other objects that are required to complete the link-editing process and 52indirection tables to facilitate 53.Em Position Independent Code 54(PIC for short) to improve sharing of code pages among different processes. 55The collection of data structures described here will be refered to as the 56.Em Run-time Relocation Section (RRS) 57and is embedded in the standard text and data segments of the dynamically 58linked program or shared object image as the existing 59.Xr a.out 60format offers no room for it elsewhere. 61.Pp 62Several utilities co-operate to ensure that the task of getting a program 63ready to run can complete successfully in a way that optimizes the use 64of system resources. The compiler emits PIC code from which shared libraries 65can be built by 66.Xr ld 1 . 67The compiler also includes size information of any initialized data items 68through the .size assembler directive. PIC code differs from conventional code 69in that it accesses data variables through an indirection table, the 70Global Offset Table, by convention accessable by the reserved name 71.Em _GLOBAL_OFFSET_TABLE_ . 72The exact mechanism used for this is machine dependent, usually a machine 73register is reserved for the purpose. The rational behind this construct 74is to generate code that is independent of the actual load address. Only 75the values contained in the Global Offset Table may need updating at run-time 76depending on the load addresses of the various shared objects in the address 77space. 78.Pp 79Likewise, procedure calls to globally defined functions are redirected through 80the Procedure Linkage Table (PLT) residing in the data segment of the core 81image. Again, this is done to avoid run-time modifications to the text segment. 82.Pp 83The linker-editor allocates the Global Offset Table and Procedure Linkage Table 84when combining PIC object files into an image suitable for mapping into the 85process address space. It also collects all symbols that may be needed by the 86run-time link-editor and stores these along with the image's text and data bits. 87Another reserved symbol, 88.Em _DYNAMIC 89is used to indicate the presence of the run-time linker structures. Whenever 90_DYNAMIC is relocated to 0, there is no need to invoke the run-time 91link-editor. If this symbol is non-zero, it points at a data structure from 92which the location of the necessary relocation- and symbol information can 93be derived. This is most notably used by the start-up module, 94.Em crt0 . 95The _DYNAMIC structure is conventionally located at the start of the data 96segment of the image to which it pertains. 97.Pp 98.Sh DATA STRUCTURES 99The data structures supporting dynamic linking and run-time relocation 100reside both in the text and data segments of the image they apply to. 101The text segments contain read-only data such as symbols descriptions and 102names, while the data segments contain the tables that need to be modified by 103during the relocation process. 104.Pp 105The _DYNAMIC symbol references a 106.Fa _dynamic 107structure: 108.Bd -literal -offset indent 109struct _dynamic { 110 int d_version; 111 struct so_debug *d_debug; 112 union { 113 struct section_dispatch_table *d_sdt; 114 } d_un; 115 struct ld_entry *d_entry; 116}; 117.Ed 118.Bl -tag -width d_version 119.It Fa d_version 120This field provides for different versions of the dynamic linking 121implementation. The current version numbers understood by ld and ld.so are 122.Em LD_VERSION_SUN (3), 123which is used by the SunOS 4.x releases, and 124.Em LD_VERSION_BSD (8), 125which is currently in use by NetBSD. 126.It Fa d_un 127Refers to a 128.Em d_version 129dependent data structure. 130.It Fa d_debug 131this field provides debuggers with a hook to access symbol tables of shared 132objects loaded as a result of the actions of the run-time link-editor. 133.It Fa d_entry 134this field is obsoleted by CRT interface version CRT_VERSION_BSD4, and is 135replaced by the crt_ldentry in 136.Fa crt_ldso . 137.El 138.Pp 139The 140.Fa section_dispatch_table 141structure is the main 142.Dq dispatcher 143table, containing offsets into the image's segments where various symbol 144and relocation information is located. 145.Bd -literal -offset indent 146struct section_dispatch_table { 147 struct so_map *sdt_loaded; 148 long sdt_sods; 149 long sdt_paths; 150 long sdt_got; 151 long sdt_plt; 152 long sdt_rel; 153 long sdt_hash; 154 long sdt_nzlist; 155 long sdt_filler2; 156 long sdt_buckets; 157 long sdt_strings; 158 long sdt_str_sz; 159 long sdt_text_sz; 160 long sdt_plt_sz; 161}; 162.Ed 163.Pp 164.Bl -tag -width sdt_loaded 165.It Fa sdt_loaded 166A pointer to the first link map loaded (see below). This field is set by 167.Xr ld.so 168for the benefit of debuggers that may use it to load a shared object's 169symbol table. 170.It Fa sdt_sods 171The start of a (linked) list of shared object descriptors needed by 172.Em this 173object. 174.It Fa sdt_paths 175Library search rules. A colon separated list of directories corresponding 176to the 177.Fl R 178option of 179.Xr ld 1 . 180.It Fa sdt_got 181The location of the Global Offset Table within this image. 182.It Fa sdt_plt 183The location of the Procedure Linkage Table within this image. 184.It Fa sdt_rel 185The location of an array of 186.Fa relocation_info 187structures 188.Po 189see 190.Xr a.out 5 191.Pc 192specifying run-time relocations. 193.It Fa sdt_hash 194The location of the hash table for fast symbol lookup in this object's 195symbol table. 196.It Fa sdt_nzlist 197The location of the symbol table. 198.It Fa sdt_filler2 199Currently unused. 200.It Fa sdt_buckets 201The number of buckets in 202.Fa sdt_hash 203.It Fa sdt_strings 204The location of the symbol string table that goes with 205.Fa sdt_nzlist . 206.It Fa sdt_str_sz 207The size of the string table. 208.It Fa sdt_text_sz 209The size of the object's text segment. 210.It Fa sdt_plt_sz 211The size of the Procedure Linkage Table. 212.El 213.Pp 214A 215.Fa sod 216structure descibes a shared object that is needed 217to complete the link edit process of the object containing it. 218A list of such objects 219.Po 220chained through 221.Fa sod_next 222.Pc 223is pointed at 224by the 225.Fa sdt_sods 226in the section_dispatch_table structure. 227.Bd -literal -offset indent 228struct sod { 229 long sod_name; 230 u_int sod_library : 1, 231 sod_unused : 31; 232 short sod_major; 233 short sod_minor; 234 long sod_next; 235}; 236.Ed 237.Pp 238.Bl -tag -width sod_library 239.It Fa sod_name 240The offset in the text segment of a string describing this link object. 241.It Fa sod_library 242If set, 243.Fa sod_name 244specifies a library that is to be searched for by ld.so. The path name 245is obtained by searching a set of directories 246.Po 247see also 248.Xr ldconfig 8 249.Pc 250for a shared object matching 251.Em lib\&<sod_name>\&.so.n.m . 252If not set, 253.Fa sod_name 254should point at a full path name for the desired shared object. 255.It Fa sod_major 256Specifies the major version number of the shared object to load. 257.It Fa sod_minor 258Specifies the prefered minor version number of the shared object to load. 259.El 260.Pp 261The run-time link-editor maintains a list of structures called 262.Em link maps 263to keep track of all shared objects loaded into a process' address space. 264These structures are only used at run-time and do not occur within 265the text or data segment of an executable or shared library. 266.Bd -literal -offset indent 267struct so_map { 268 caddr_t som_addr; 269 char *som_path; 270 struct so_map *som_next; 271 struct sod *som_sod; 272 caddr_t som_sodbase; 273 u_int som_write : 1; 274 struct _dynamic *som_dynamic; 275 caddr_t som_spd; 276}; 277.Ed 278.Bl -tag -width som_dynamic 279.It Fa som_addr 280The address at which the shared object associated with this link map has 281been loaded. 282.It Fa som_path 283The full path name of the loaded object. 284.It Fa som_next 285Pointer to the next link map. 286.It Fa som_sod 287The 288.Fa sod 289structure that was responsible for loading this shared object. 290.It Fa som_sodbase 291Tossed in later versions the run-time linker. 292.It Fa som_write 293Set if (some portion of) this object's text segment is currently writable. 294.It Fa som_dynamic 295Pointer to this object's 296.Fa _dynamic 297structure. 298.It Fa som_spd 299Hook for attaching private data maintained by the run-time link-editor. 300.El 301.Pp 302Symbol description with size. This is simply an 303.Fa nlist 304structure with one field 305.Pq Fa nz_size 306added. Used to convey size information on items in the data segment 307of shared objects. An array of these lives in the shared object's 308text segment and is addressed by the 309.Fa sdt_nzlist 310field of 311.Fa section_dispatch_table . 312.Bd -literal -offset indent 313struct nzlist { 314 struct nlist nlist; 315 u_long nz_size; 316#define nz_un nlist.n_un 317#define nz_strx nlist.n_un.n_strx 318#define nz_name nlist.n_un.n_name 319#define nz_type nlist.n_type 320#define nz_value nlist.n_value 321#define nz_desc nlist.n_desc 322#define nz_other nlist.n_other 323}; 324.Ed 325.Bl -tag -width nz_size 326.It Fa nlist 327.Po 328see 329.Xr nlist 5 330.Pc . 331.It Fa nz_size 332The size of the data represented by this symbol. 333.El 334.Pp 335A hash table is included within the text segment of shared object to 336to facilitate quick lookup of symbols during run-time link-editing. 337The 338.Fa sdt_hash 339field of the 340.Fa section_dispatch_table 341structure points at an array of 342.Fa rrs_hash 343structures: 344.Bd -literal -offset indent 345struct rrs_hash { 346 int rh_symbolnum; /* symbol number */ 347 int rh_next; /* next hash entry */ 348}; 349.Ed 350.Pp 351.Bl -tag -width rh_symbolnum 352.It Fa rh_symbolnum 353The index of the symbol in the shared object's symbol table (as given by the 354.Fa ld_symbols 355field). 356.It Fa rh_next 357In case of collisions, this field is the offset of the next entry in this 358hash table bucket. It is zero for the last bucket element. 359.El 360The 361.Fa rt_symbol 362structure is used to keep track of run-time allocated commons 363and data items copied from shared objects. These items are kept on linked list 364and is exported through the 365.Fa dd_cc 366field in the 367.Fa so_debug 368structure (see below) for use by debuggers. 369.Bd -literal -offset indent 370struct rt_symbol { 371 struct nzlist *rt_sp; 372 struct rt_symbol *rt_next; 373 struct rt_symbol *rt_link; 374 caddr_t rt_srcaddr; 375 struct so_map *rt_smp; 376}; 377.Ed 378.Pp 379.Bl -tag -width rt_scraddr 380.It Fa rt_sp 381The symbol description. 382.It Fa rt_next 383Virtual address of next rt_symbol. 384.It Fa rt_link 385Next in hash bucket. Used by internally by ld.so. 386.It Fa rt_srcaddr 387Location of the source of initialized data within a shared object. 388.It Fa rt_smp 389The shared object which is the original source of the data that this 390run-time symbol describes. 391.El 392.Pp 393The 394.Fa so_debug 395structure is used by debuggers to gain knowledge of any shared objects 396that have been loaded in the process's address space as a result of run-time 397link-editing. Since the run-time link-editor runs as a part of process 398initialization, a debugger that wishes to access symbols from shared objects 399can only do so after the link-editor has been called from crt0. 400A dynamically linked binary contains a 401.Fa so_debug 402structure which can be located by means of the 403.Fa d_debug 404field in 405.Fa _dynamic . 406.Bd -literal -offset indent 407struct so_debug { 408 int dd_version; 409 int dd_in_debugger; 410 int dd_sym_loaded; 411 char *dd_bpt_addr; 412 int dd_bpt_shadow; 413 struct rt_symbol *dd_cc; 414}; 415.Ed 416.Pp 417.Bl -tag -width dd_in_debugger 418.It Fa dd_version 419Version number of this interface. 420.It Fa dd_in_debugger 421Set by the debugger to indicate to the run-time linker that the program is 422run under control of a debugger. 423.It Fa dd_sym_loaded 424Set by the run-time linker whenever it adds symbols by loading shared objects. 425.It Fa dd_bpt_addr 426The address were a breakpoint will be set by the run-time linker to 427divert control to the debugger. This address is determined by the start-up 428module, 429.Em crt0.o, 430to be some convenient place before the call to _main. 431.It Fa dd_bpt_shadow 432Contains the original instruction that was at 433.Fa dd_bpt_addr . 434The debugger is expected to put this instruction back before continuing the 435program. 436.It Fa dd_cc 437A pointer to the linked list of run-time allocated symbols that the debugger 438may be interested in. 439.El 440.Pp 441The 442.Em ld_entry 443structure defines a set of service routines within ld.so. See 444.Xr dlfcn 3 445for more information. 446.Bd -literal -offset indent 447struct ld_entry { 448 void *(*dlopen)(char *, int); 449 int (*dlclose)(void *); 450 void *(*dlsym)(void *, char *); 451 int (*dlctl)(void *, int, void *); 452 void (*dlexit) __P((void)); 453}; 454.Ed 455 456The 457.Fa crt_ldso 458structure defines the interface between ld.so and the start-up code in crt0. 459.Bd -literal -offset indent 460struct crt_ldso { 461 int crt_ba; 462 int crt_dzfd; 463 int crt_ldfd; 464 struct _dynamic *crt_dp; 465 char **crt_ep; 466 caddr_t crt_bp; 467 char *crt_prog; 468 char *crt_ldso; 469 char *crt_ldentry; 470}; 471#define CRT_VERSION_SUN 1 472#define CRT_VERSION_BSD2 2 473#define CRT_VERSION_BSD3 3 474#define CRT_VERSION_BSD4 4 475.Ed 476.Bl -tag -width crt_dzfd 477.It Fa crt_ba 478The virtual address at which ld.so was loaded by crt0. 479.It Fa crt_dzfd 480On SunOS systems, this field contains an open file descriptor to 481.Dq /dev/zero 482used to get demand paged zeroed pages. On NetBSD systems it contains -1. 483.It Fa crt_ldfd 484Contains an open file descriptor that was used by crt0 to load ld.so. 485.It Fa crt_dp 486A pointer to main's 487.Fa _dynamic 488structure. 489.It Fa crt_ep 490A pointer to the environment strings. 491.It Fa crt_bp 492The address at which a breakpoint will be placed by the run-time linker 493if the main program is run by a debugger. 494See 495.Fa so_debug 496.It Fa crt_prog 497The name of the main program as determined by crt0 (CRT_VERSION_BSD3 only). 498.It Fa crt_ldso 499The path of the run-time linker as mapped by crt0 (CRT_VERSION_BSD4 only). 500.It Fa crt_ldentry 501The 502.Xr dlfcn 3 503entry points provided by the run-time linker (CRT_VERSION_BSD4 only). 504.El 505.Pp 506The 507.Fa hints_header 508and 509.Fa hints_bucket 510structures define the layout of the library hints, normally found in 511.Dq /var/run/ld.so.hints, 512which is used by ld.so to quickly locate the shared object images in the 513filesystem. 514The organization of the hints file is not unlike that of an 515.Dq a.out 516object file, in that it contains a header determining the offset and size 517of a table of fixed sized hash buckets and a common string pool. 518.Bd -literal -offset indent 519struct hints_header { 520 long hh_magic; 521#define HH_MAGIC 011421044151 522 long hh_version; 523#define LD_HINTS_VERSION_1 1 524#define LD_HINTS_VERSION_2 2 525 long hh_hashtab; 526 long hh_nbucket; 527 long hh_strtab; 528 long hh_strtab_sz; 529 long hh_ehints; 530 long hh_dirlist; 531}; 532.Ed 533.Bl -tag -width hh_strtab_sz 534.It Fa hh_magic 535Hints file magic number. 536.It Fa hh_version 537Interface version number. 538.It Fa hh_hashtab 539Offset of hash table. 540.It Fa hh_strtab 541Offset of string table. 542.It Fa hh_strtab_sz 543Size of strings. 544.It Fa hh_ehints 545Maximum usable offset in hints file. 546.It Fa hh_dirlist 547Offset in string table of a colon-separated list of directories that was 548used in constructing the hints file. See also 549.Xr ldconfig 8 . 550This field is only available with interface version number 551.Dv LD_HINTS_VERSION_2 552and higher. 553.El 554.Pp 555.Bd -literal -offset indent 556/* 557 * Hash table element in hints file. 558 */ 559struct hints_bucket { 560 int hi_namex; 561 int hi_pathx; 562 int hi_dewey[MAXDEWEY]; 563 int hi_ndewey; 564#define hi_major hi_dewey[0] 565#define hi_minor hi_dewey[1] 566 int hi_next; 567}; 568.Ed 569.Bl -tag -width hi_ndewey 570.It Fa hi_namex 571Index of the string identifying the library. 572.It Fa hi_pathx 573Index of the string representing the full path name of the library. 574.It Fa hi_dewey 575The version numbers of the shared library. 576.It Fa hi_ndewey 577The number of valid entries in 578.Fa hi_dewey . 579.It Fa hi_next 580Next bucket in case of hashing collisions. 581.El 582 583.Sh CAVEATS 584Only the (GNU) C compiler currently supports the creation of shared libraries. 585Other programming languages can not be used. 586 587