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.Fd #include <sys/types.h> 39.Fd #include <nlist.h> 40.Fd #include <link.h> 41.Sh DESCRIPTION 42The include file 43.Aq Pa 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. The layout of a number of these 48structures within the binaries resembles the a.out format in many places 49as it serves such similar functions as symbol definitions (including the 50accompanying string table) and relocation records needed to resolve 51references to external entities. It also records a number of data structures 52unique to the dynamic loading and linking process. These include references 53to other objects that are required to complete the link-editing process and 54indirection tables to facilitate 55.Em Position Independent Code 56(PIC for short) to improve sharing of code pages among different processes. 57The collection of data structures described here will be referred to as the 58.Em Run-time Relocation Section (RRS) 59and is embedded in the standard text and data segments of the dynamically 60linked program or shared object image as the existing 61.Xr a.out 5 62format offers no room for it elsewhere. 63.Pp 64Several utilities cooperate to ensure that the task of getting a program 65ready to run can complete successfully in a way that optimizes the use 66of system resources. The compiler emits PIC code from which shared libraries 67can be built by 68.Xr ld 1 . 69The compiler also includes size information of any initialized data items 70through the .size assembler directive. PIC code differs from conventional code 71in that it accesses data variables through an indirection table, the 72Global Offset Table, by convention accessible by the reserved name 73.Em _GLOBAL_OFFSET_TABLE_. 74The exact mechanism used for this is machine dependent, usually a machine 75register is reserved for the purpose. The rational behind this construct 76is to generate code that is independent of the actual load address. Only 77the values contained in the Global Offset Table may need updating at run-time 78depending on the load addresses of the various shared objects in the address 79space. 80.Pp 81Likewise, procedure calls to globally defined functions are redirected through 82the Procedure Linkage Table (PLT) residing in the data segment of the core 83image. Again, this is done to avoid run-time modifications to the text segment. 84.Pp 85The linker-editor allocates the Global Offset Table and Procedure Linkage Table 86when combining PIC object files into an image suitable for mapping into the 87process address space. It also collects all symbols that may be needed by the 88run-time link-editor and stores these along with the image's text and data bits. 89Another reserved symbol, 90.Em _DYNAMIC 91is used to indicate the presence of the run-time linker structures. Whenever 92_DYNAMIC is relocated to 0, there is no need to invoke the run-time 93link-editor. If this symbol is non-zero, it points at a data structure from 94which the location of the necessary relocation- and symbol information can 95be derived. This is most notably used by the start-up module, 96.Em crt0. 97The _DYNAMIC structure is conventionally located at the start of the data 98segment of the image to which it pertains. 99.Pp 100.Sh DATA STRUCTURES 101The data structures supporting dynamic linking and run-time relocation 102reside both in the text and data segments of the image they apply to. 103The text segments contain read-only data such as symbols descriptions and 104names, while the data segments contain the tables that need to be modified by 105during the relocation process. 106.Pp 107The _DYNAMIC symbol references a 108.Fa _dynamic 109structure: 110.Bd -literal -offset indent 111struct _dynamic { 112 int d_version; 113 struct so_debug *d_debug; 114 union { 115 struct section_dispatch_table *d_sdt; 116 } d_un; 117 struct ld_entry *d_entry; 118}; 119.Ed 120.Bl -tag -width d_version 121.It Fa d_version 122This field provides for different versions of the dynamic linking 123implementation. The current version numbers understood by 124.Xr ld 1 125and 126.Xr ld.so 1 127are 128.Em LD_VERSION_SUN (3), 129which is used by the 130.Tn SunOS 1314.x releases, and 132.Em LD_VERSION_BSD (8), 133which has been in use since 134.Fx 1.1 . 135.It Fa d_un 136Refers to a 137.Em d_version 138dependent data structure. 139.It Fa so_debug 140this field provides debuggers with a hook to access symbol tables of shared 141objects loaded as a result of the actions of the run-time link-editor. 142.El 143.Pp 144The 145.Fa section_dispatch_table 146structure is the main 147.Dq dispatcher 148table, containing offsets into the image's segments where various symbol 149and relocation information is located. 150.Bd -literal -offset indent 151struct section_dispatch_table { 152 struct so_map *sdt_loaded; 153 long sdt_sods; 154 long sdt_filler1; 155 long sdt_got; 156 long sdt_plt; 157 long sdt_rel; 158 long sdt_hash; 159 long sdt_nzlist; 160 long sdt_filler2; 161 long sdt_buckets; 162 long sdt_strings; 163 long sdt_str_sz; 164 long sdt_text_sz; 165 long sdt_plt_sz; 166}; 167.Ed 168.Pp 169.Bl -tag -width sdt_filler1 170.It Fa sdt_loaded 171A pointer to the first link map loaded (see below). This field is set by 172.Nm ld.so 173.It Fa sdt_sods 174The start of a (linked) list of shared object descriptors needed by 175.Em this 176object. 177.It Fa sdt_filler1 178Deprecated (used by SunOS to specify library search rules). 179.It Fa sdt_got 180The location of the Global Offset Table within this image. 181.It Fa sdt_plt 182The location of the Procedure Linkage Table within this image. 183.It Fa sdt_rel 184The location of an array of 185.Fa relocation_info 186structures 187.Po 188see 189.Xr a.out 5 190.Pc 191specifying run-time relocations. 192.It Fa sdt_hash 193The location of the hash table for fast symbol lookup in this object's 194symbol table. 195.It Fa sdt_nzlist 196The location of the symbol table. 197.It Fa sdt_filler2 198Currently unused. 199.It Fa sdt_buckets 200The number of buckets in 201.Fa sdt_hash 202.It Fa sdt_strings 203The location of the symbol string table that goes with 204.Fa sdt_nzlist. 205.It Fa sdt_str_sz 206The size of the string table. 207.It Fa sdt_text_sz 208The size of the object's text segment. 209.It Fa sdt_plt_sz 210The size of the Procedure Linkage Table. 211.El 212.Pp 213A 214.Fa sod 215structure describes a shared object that is needed 216to complete the link edit process of the object containing it. 217A list of such objects 218.Po 219chained through 220.Fa sod_next 221.Pc 222is pointed at 223by the 224.Fa sdt_sods 225in the section_dispatch_table structure. 226.Bd -literal -offset indent 227struct sod { 228 long sod_name; 229 u_int sod_library : 1, 230 sod_reserved : 31; 231 short sod_major; 232 short sod_minor; 233 long sod_next; 234}; 235.Ed 236.Pp 237.Bl -tag -width sod_library 238.It Fa sod_name 239The offset in the text segment of a string describing this link object. 240.It Fa sod_library 241If set, 242.Fa sod_name 243specifies a library that is to be searched for by 244.Nm ld.so . 245The path name 246is obtained by searching a set of directories 247.Po 248see also 249.Xr ldconfig 8 250.Pc 251for a shared object matching 252.Em lib\&<sod_name>\&.so.n.m. 253If not set, 254.Fa sod_name 255should point at a full path name for the desired shared object. 256.It Fa sod_major 257Specifies the major version number of the shared object to load. 258.It Fa sod_minor 259Specifies the prefered minor version number of the shared object to load. 260.El 261.Pp 262The run-time link-editor maintains a list of structures called 263.Em link maps 264to keep track of all shared objects loaded into a process' address space. 265These structures are only used at run-time and do not occur within 266the text or data segment of an executable or shared library. 267.Bd -literal -offset indent 268struct so_map { 269 caddr_t som_addr; 270 char *som_path; 271 struct so_map *som_next; 272 struct sod *som_sod; 273 caddr_t som_sodbase; 274 u_int som_write : 1; 275 struct _dynamic *som_dynamic; 276 caddr_t som_spd; 277}; 278.Ed 279.Bl -tag -width som_dynamic 280.It Fa som_addr 281The address at which the shared object associated with this link map has 282been loaded. 283.It Fa som_path 284The full path name of the loaded object. 285.It Fa som_next 286Pointer to the next link map. 287.It Fa som_sod 288The 289.Fa sod 290structure that was responsible for loading this shared object. 291.It Fa som_sodbase 292Tossed in later versions the run-time linker. 293.It Fa som_write 294Set if (some portion of) this object's text segment is currently writable. 295.It Fa som_dynamic 296Pointer to this object's 297.Fa _dynamic 298structure. 299.It Fa som_spd 300Hook for attaching private data maintained by the run-time link-editor. 301.El 302.Pp 303Symbol description with size. This is simply an 304.Fa nlist 305structure with one field 306.Pq Fa nz_size 307added. Used to convey size information on items in the data segment 308of shared objects. An array of these lives in the shared object's 309text segment and is addressed by the 310.Fa sdt_nzlist 311field of 312.Fa section_dispatch_table. 313.Bd -literal -offset indent 314struct nzlist { 315 struct nlist nlist; 316 u_long nz_size; 317#define nz_un nlist.n_un 318#define nz_strx nlist.n_un.n_strx 319#define nz_name nlist.n_un.n_name 320#define nz_type nlist.n_type 321#define nz_value nlist.n_value 322#define nz_desc nlist.n_desc 323#define nz_other nlist.n_other 324}; 325.Ed 326.Bl -tag -width nz_size 327.It Fa nlist 328.Po 329see 330.Xr nlist 3 331.Pc . 332.It Fa nz_size 333The size of the data represented by this symbol. 334.El 335.Pp 336A hash table is included within the text segment of shared object to 337to facilitate quick lookup of symbols during run-time link-editing. 338The 339.Fa sdt_hash 340field of the 341.Fa section_dispatch_table 342structure points at an array of 343.Fa rrs_hash 344structures: 345.Bd -literal -offset indent 346struct rrs_hash { 347 int rh_symbolnum; /* symbol number */ 348 int rh_next; /* next hash entry */ 349}; 350.Ed 351.Pp 352.Bl -tag -width rh_symbolnum 353.It Fa rh_symbolnum 354The index of the symbol in the shared object's symbol table (as given by the 355.Fa ld_symbols 356field). 357.It Fa rh_next 358In case of collisions, this field is the offset of the next entry in this 359hash table bucket. It is zero for the last bucket element. 360.El 361The 362.Fa rt_symbol 363structure is used to keep track of run-time allocated commons 364and data items copied from shared objects. These items are kept on linked list 365and is exported through the 366.Fa dd_cc 367field in the 368.Fa so_debug 369structure (see below) for use by debuggers. 370.Bd -literal -offset indent 371struct rt_symbol { 372 struct nzlist *rt_sp; 373 struct rt_symbol *rt_next; 374 struct rt_symbol *rt_link; 375 caddr_t rt_srcaddr; 376 struct so_map *rt_smp; 377}; 378.Ed 379.Pp 380.Bl -tag -width rt_scraddr 381.It Fa rt_sp 382The symbol description. 383.It Fa rt_next 384Virtual address of next rt_symbol. 385.It Fa rt_link 386Next in hash bucket. Used by internally by 387.Nm ld.so . 388.It Fa rt_srcaddr 389Location of the source of initialized data within a shared object. 390.It Fa rt_smp 391The shared object which is the original source of the data that this 392run-time symbol describes. 393.El 394.Pp 395The 396.Fa so_debug 397structure is used by debuggers to gain knowledge of any shared objects 398that have been loaded in the process's address space as a result of run-time 399link-editing. Since the run-time link-editor runs as a part of process 400initialization, a debugger that wishes to access symbols from shared objects 401can only do so after the link-editor has been called from crt0. 402A dynamically linked binary contains a 403.Fa so_debug 404structure which can be located by means of the 405.Fa d_debug 406field in 407.Fa _dynamic . 408.Bd -literal -offset indent 409struct so_debug { 410 int dd_version; 411 int dd_in_debugger; 412 int dd_sym_loaded; 413 char *dd_bpt_addr; 414 int dd_bpt_shadow; 415 struct rt_symbol *dd_cc; 416}; 417.Ed 418.Pp 419.Bl -tag -width dd_in_debugger 420.It Fa dd_version 421Version number of this interface. 422.It Fa dd_in_debugger 423Set by the debugger to indicate to the run-time linker that the program is 424run under control of a debugger. 425.It Fa dd_sym_loaded 426Set by the run-time linker whenever it adds symbols by loading shared objects. 427.It Fa dd_bpt_addr 428The address were a breakpoint will be set by the run-time linker to 429divert control to the debugger. This address is determined by the start-up 430module, 431.Em crt0.o, 432to be some convenient place before the call to _main. 433.It Fa dd_bpt_shadow 434Contains the original instruction that was at 435.Fa dd_bpt_addr . 436The debugger is expected to put this instruction back before continuing the 437program. 438.It Fa dd_cc 439A pointer to the linked list of run-time allocated symbols that the debugger 440may be interested in. 441.El 442.Pp 443The 444.Em ld_entry 445structure defines a set of service routines within 446.Nm ld.so . 447.\" See 448.\" .Xr libdl.a 449.\" for more information. 450.Bd -literal -offset indent 451struct ld_entry { 452 void *(*dlopen)(char *, int); 453 int (*dlclose)(void *); 454 void *(*dlsym)(void *, char *); 455 char *(*dlerror)(void); 456}; 457.Ed 458 459The 460.Fa crt_ldso 461structure defines the interface between the start-up code in crt0 and 462.Nm ld.so . 463.Bd -literal -offset indent 464struct crt_ldso { 465 int crt_ba; 466 int crt_dzfd; 467 int crt_ldfd; 468 struct _dynamic *crt_dp; 469 char **crt_ep; 470 caddr_t crt_bp; 471 char *crt_prog; 472 char *crt_ldso; 473 struct ld_entry *crt_ldentry; 474}; 475#define CRT_VERSION_SUN 1 476#define CRT_VERSION_BSD_2 2 477#define CRT_VERSION_BSD_3 3 478#define CRT_VERSION_BSD_4 4 479.Ed 480.Bl -tag -width crt_dzfd 481.It Fa crt_ba 482The virtual address at which 483.Nm ld.so 484was loaded by crt0. 485.It Fa crt_dzfd 486On SunOS systems, this field contains an open file descriptor to 487.Dq Pa /dev/zero 488used to get demand paged zeroed pages. On 489.Tn FreeBSD 490systems it contains -1. 491.It Fa crt_ldfd 492Contains an open file descriptor that was used by crt0 to load 493.Nm ld.so . 494.It Fa crt_dp 495A pointer to main's 496.Fa _dynamic 497structure. 498.It Fa crt_ep 499A pointer to the environment strings. 500.It Fa crt_bp 501The address at which a breakpoint will be placed by the run-time linker 502if the main program is run by a debugger. 503See 504.Fa so_debug 505.It Fa crt_prog 506The name of the main program as determined by crt0 (CRT_VERSION_BSD3 only). 507.It Fa crt_ldso 508The path of the run-time linker as mapped by crt0 (CRT_VERSION_BSD4 only). 509.El 510.Pp 511The 512.Fa hints_header 513and 514.Fa hints_bucket 515structures define the layout of the library hints, normally found in 516.Dq Pa /var/run/ld.so.hints , 517which is used by 518.Nm ld.so 519to quickly locate the shared object images in the 520filesystem. 521The organization of the hints file is not unlike that of an 522.Dq a.out 523object file, in that it contains a header determining the offset and size 524of a table of fixed sized hash buckets and a common string pool. 525.Bd -literal -offset indent 526struct hints_header { 527 long hh_magic; 528#define HH_MAGIC 011421044151 529 long hh_version; 530#define LD_HINTS_VERSION_1 1 531 long hh_hashtab; 532 long hh_nbucket; 533 long hh_strtab; 534 long hh_strtab_sz; 535 long hh_ehints; 536}; 537.Ed 538.Bl -tag -width hh_strtab_sz 539.It Fa hh_magic 540Hints file magic number. 541.It Fa hh_version 542Interface version number. 543.It Fa hh_hashtab 544Offset of hash table. 545.It Fa hh_strtab 546Offset of string table. 547.It Fa hh_strtab_sz 548Size of strings. 549.It Fa hh_ehints 550Maximum usable offset in hints file. 551.El 552.Pp 553.Bd -literal -offset indent 554/* 555 * Hash table element in hints file. 556 */ 557struct hints_bucket { 558 int hi_namex; 559 int hi_pathx; 560 int hi_dewey[MAXDEWEY]; 561 int hi_ndewey; 562#define hi_major hi_dewey[0] 563#define hi_minor hi_dewey[1] 564 int hi_next; 565}; 566.Ed 567.Bl -tag -width hi_ndewey 568.It Fa hi_namex 569Index of the string identifying the library. 570.It Fa hi_pathx 571Index of the string representing the full path name of the library. 572.It Fa hi_dewey 573The version numbers of the shared library. 574.It Fa hi_ndewey 575The number of valid entries in 576.Fa hi_dewey . 577.It Fa hi_next 578Next bucket in case of hashing collisions. 579.El 580.Sh CAVEATS 581Only the (GNU) C compiler currently supports the creation of shared libraries. 582Other programming languages can not be used. 583