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.\"
| 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: head/share/man/man5/link.5 68716 2000-11-14 11:20:58Z ru $
| 29.\" $FreeBSD: head/share/man/man5/link.5 70466 2000-12-29 09:18:45Z ru $
|
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.
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.Em _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.Pp
112.Sh DATA STRUCTURES
113The data structures supporting dynamic linking and run-time relocation
114reside both in the text and data segments of the image they apply to.
115The text segments contain read-only data such as symbols descriptions and
116names, while the data segments contain the tables that need to be modified by
117during the relocation process.
118.Pp
119The _DYNAMIC symbol references a
120.Fa _dynamic
121structure:
122.Bd -literal -offset indent
123struct _dynamic {
124 int d_version;
125 struct so_debug *d_debug;
126 union {
127 struct section_dispatch_table *d_sdt;
128 } d_un;
129 struct ld_entry *d_entry;
130};
131.Ed
132.Bl -tag -width d_version
133.It Fa d_version
134This field provides for different versions of the dynamic linking
135implementation.
136The current version numbers understood by
137.Xr ld 1
138and
139.Xr ld.so 1
140are
141.Em LD_VERSION_SUN (3),
142which is used by the
143.Tn SunOS
1444.x releases, and
145.Em LD_VERSION_BSD (8),
146which has been in use since
147.Fx 1.1 .
148.It Fa d_un
149Refers to a
150.Em d_version
151dependent data structure.
152.It Fa so_debug
153this field provides debuggers with a hook to access symbol tables of shared
154objects loaded as a result of the actions of the run-time link-editor.
155.El
156.Pp
157The
158.Fa section_dispatch_table
159structure is the main
160.Dq dispatcher
161table, containing offsets into the image's segments where various symbol
162and relocation information is located.
163.Bd -literal -offset indent
164struct section_dispatch_table {
165 struct so_map *sdt_loaded;
166 long sdt_sods;
167 long sdt_filler1;
168 long sdt_got;
169 long sdt_plt;
170 long sdt_rel;
171 long sdt_hash;
172 long sdt_nzlist;
173 long sdt_filler2;
174 long sdt_buckets;
175 long sdt_strings;
176 long sdt_str_sz;
177 long sdt_text_sz;
178 long sdt_plt_sz;
179};
180.Ed
181.Pp
182.Bl -tag -width sdt_filler1
183.It Fa sdt_loaded
184A pointer to the first link map loaded (see below). This 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.Po
201see
202.Xr a.out 5
203.Pc
204specifying run-time relocations.
205.It Fa sdt_hash
206The location of the hash table for fast symbol lookup in this object's
207symbol table.
208.It Fa sdt_nzlist
209The location of the symbol table.
210.It Fa sdt_filler2
211Currently unused.
212.It Fa sdt_buckets
213The number of buckets in
214.Fa sdt_hash
215.It Fa sdt_strings
216The location of the symbol string table that goes with
217.Fa sdt_nzlist.
218.It Fa sdt_str_sz
219The size of the string table.
220.It Fa sdt_text_sz
221The size of the object's text segment.
222.It Fa sdt_plt_sz
223The size of the Procedure Linkage Table.
224.El
225.Pp
226A
227.Fa sod
228structure describes a shared object that is needed
229to complete the link edit process of the object containing it.
230A list of such objects
231.Po
232chained through
233.Fa sod_next
234.Pc
235is pointed at
236by the
237.Fa sdt_sods
238in the section_dispatch_table structure.
239.Bd -literal -offset indent
240struct sod {
241 long sod_name;
242 u_int sod_library : 1,
243 sod_reserved : 31;
244 short sod_major;
245 short sod_minor;
246 long sod_next;
247};
248.Ed
249.Pp
250.Bl -tag -width sod_library
251.It Fa sod_name
252The offset in the text segment of a string describing this link object.
253.It Fa sod_library
254If set,
255.Fa sod_name
256specifies a library that is to be searched for by
257.Nm ld.so .
258The path name
259is obtained by searching a set of directories
260.Po
261see also
262.Xr ldconfig 8
263.Pc
264for a shared object matching
265.Em lib\&<sod_name>\&.so.n.m.
266If not set,
267.Fa sod_name
268should point at a full path name for the desired shared object.
269.It Fa sod_major
270Specifies the major version number of the shared object to load.
271.It Fa sod_minor
272Specifies the prefered minor version number of the shared object to load.
273.El
274.Pp
275The run-time link-editor maintains a list of structures called
276.Em link maps
277to keep track of all shared objects loaded into a process' address space.
278These structures are only used at run-time and do not occur within
279the text or data segment of an executable or shared library.
280.Bd -literal -offset indent
281struct so_map {
282 caddr_t som_addr;
283 char *som_path;
284 struct so_map *som_next;
285 struct sod *som_sod;
286 caddr_t som_sodbase;
287 u_int som_write : 1;
288 struct _dynamic *som_dynamic;
289 caddr_t som_spd;
290};
291.Ed
292.Bl -tag -width som_dynamic
293.It Fa som_addr
294The address at which the shared object associated with this link map has
295been loaded.
296.It Fa som_path
297The full path name of the loaded object.
298.It Fa som_next
299Pointer to the next link map.
300.It Fa som_sod
301The
302.Fa sod
303structure that was responsible for loading this shared object.
304.It Fa som_sodbase
305Tossed in later versions the run-time linker.
306.It Fa som_write
307Set if (some portion of) this object's text segment is currently writable.
308.It Fa som_dynamic
309Pointer to this object's
310.Fa _dynamic
311structure.
312.It Fa som_spd
313Hook for attaching private data maintained by the run-time link-editor.
314.El
315.Pp
316Symbol description with size.
317This is simply an
318.Fa nlist
319structure with one field
320.Pq Fa nz_size
321added.
322Used to convey size information on items in the data segment
323of shared objects.
324An array of these lives in the shared object's
325text segment and is addressed by the
326.Fa sdt_nzlist
327field of
328.Fa section_dispatch_table.
329.Bd -literal -offset indent
330struct nzlist {
331 struct nlist nlist;
332 u_long nz_size;
333#define nz_un nlist.n_un
334#define nz_strx nlist.n_un.n_strx
335#define nz_name nlist.n_un.n_name
336#define nz_type nlist.n_type
337#define nz_value nlist.n_value
338#define nz_desc nlist.n_desc
339#define nz_other nlist.n_other
340};
341.Ed
342.Bl -tag -width nz_size
343.It Fa nlist
344.Po
345see
346.Xr nlist 3
347.Pc .
348.It Fa nz_size
349The size of the data represented by this symbol.
350.El
351.Pp
352A hash table is included within the text segment of shared object to
353to facilitate quick lookup of symbols during run-time link-editing.
354The
355.Fa sdt_hash
356field of the
357.Fa section_dispatch_table
358structure points at an array of
359.Fa rrs_hash
360structures:
361.Bd -literal -offset indent
362struct rrs_hash {
363 int rh_symbolnum; /* symbol number */
364 int rh_next; /* next hash entry */
365};
366.Ed
367.Pp
368.Bl -tag -width rh_symbolnum
369.It Fa rh_symbolnum
370The index of the symbol in the shared object's symbol table (as given by the
371.Fa ld_symbols
372field).
373.It Fa rh_next
374In case of collisions, this field is the offset of the next entry in this
375hash table bucket.
376It is zero for the last bucket element.
377.El
378The
379.Fa rt_symbol
380structure is used to keep track of run-time allocated commons
381and data items copied from shared objects.
382These items are kept on linked list
383and is exported through the
384.Fa dd_cc
385field in the
386.Fa so_debug
387structure (see below) for use by debuggers.
388.Bd -literal -offset indent
389struct rt_symbol {
390 struct nzlist *rt_sp;
391 struct rt_symbol *rt_next;
392 struct rt_symbol *rt_link;
393 caddr_t rt_srcaddr;
394 struct so_map *rt_smp;
395};
396.Ed
397.Pp
398.Bl -tag -width rt_scraddr
399.It Fa rt_sp
400The symbol description.
401.It Fa rt_next
402Virtual address of next rt_symbol.
403.It Fa rt_link
404Next in hash bucket.
405Used by internally by
406.Nm ld.so .
407.It Fa rt_srcaddr
408Location of the source of initialized data within a shared object.
409.It Fa rt_smp
410The shared object which is the original source of the data that this
411run-time symbol describes.
412.El
413.Pp
414The
415.Fa so_debug
416structure is used by debuggers to gain knowledge of any shared objects
417that have been loaded in the process's address space as a result of run-time
418link-editing.
419Since the run-time link-editor runs as a part of process
420initialization, a debugger that wishes to access symbols from shared objects
421can only do so after the link-editor has been called from crt0.
422A dynamically linked binary contains a
423.Fa so_debug
424structure which can be located by means of the
425.Fa d_debug
426field in
427.Fa _dynamic .
428.Bd -literal -offset indent
429struct so_debug {
430 int dd_version;
431 int dd_in_debugger;
432 int dd_sym_loaded;
433 char *dd_bpt_addr;
434 int dd_bpt_shadow;
435 struct rt_symbol *dd_cc;
436};
437.Ed
438.Pp
439.Bl -tag -width dd_in_debugger
440.It Fa dd_version
441Version number of this interface.
442.It Fa dd_in_debugger
443Set by the debugger to indicate to the run-time linker that the program is
444run under control of a debugger.
445.It Fa dd_sym_loaded
446Set by the run-time linker whenever it adds symbols by loading shared objects.
447.It Fa dd_bpt_addr
448The address were a breakpoint will be set by the run-time linker to
449divert control to the debugger.
450This address is determined by the start-up
451module,
452.Em crt0.o,
453to be some convenient place before the call to _main.
454.It Fa dd_bpt_shadow
455Contains the original instruction that was at
456.Fa dd_bpt_addr .
457The debugger is expected to put this instruction back before continuing the
458program.
459.It Fa dd_cc
460A pointer to the linked list of run-time allocated symbols that the debugger
461may be interested in.
462.El
463.Pp
464The
465.Em ld_entry
466structure defines a set of service routines within
467.Nm ld.so .
468.\" See
469.\" .Xr libdl.a
470.\" for more information.
471.Bd -literal -offset indent
472struct ld_entry {
473 void *(*dlopen)(char *, int);
474 int (*dlclose)(void *);
475 void *(*dlsym)(void *, char *);
476 char *(*dlerror)(void);
477};
478.Ed
| 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.
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.Em _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.Pp
112.Sh DATA STRUCTURES
113The data structures supporting dynamic linking and run-time relocation
114reside both in the text and data segments of the image they apply to.
115The text segments contain read-only data such as symbols descriptions and
116names, while the data segments contain the tables that need to be modified by
117during the relocation process.
118.Pp
119The _DYNAMIC symbol references a
120.Fa _dynamic
121structure:
122.Bd -literal -offset indent
123struct _dynamic {
124 int d_version;
125 struct so_debug *d_debug;
126 union {
127 struct section_dispatch_table *d_sdt;
128 } d_un;
129 struct ld_entry *d_entry;
130};
131.Ed
132.Bl -tag -width d_version
133.It Fa d_version
134This field provides for different versions of the dynamic linking
135implementation.
136The current version numbers understood by
137.Xr ld 1
138and
139.Xr ld.so 1
140are
141.Em LD_VERSION_SUN (3),
142which is used by the
143.Tn SunOS
1444.x releases, and
145.Em LD_VERSION_BSD (8),
146which has been in use since
147.Fx 1.1 .
148.It Fa d_un
149Refers to a
150.Em d_version
151dependent data structure.
152.It Fa so_debug
153this field provides debuggers with a hook to access symbol tables of shared
154objects loaded as a result of the actions of the run-time link-editor.
155.El
156.Pp
157The
158.Fa section_dispatch_table
159structure is the main
160.Dq dispatcher
161table, containing offsets into the image's segments where various symbol
162and relocation information is located.
163.Bd -literal -offset indent
164struct section_dispatch_table {
165 struct so_map *sdt_loaded;
166 long sdt_sods;
167 long sdt_filler1;
168 long sdt_got;
169 long sdt_plt;
170 long sdt_rel;
171 long sdt_hash;
172 long sdt_nzlist;
173 long sdt_filler2;
174 long sdt_buckets;
175 long sdt_strings;
176 long sdt_str_sz;
177 long sdt_text_sz;
178 long sdt_plt_sz;
179};
180.Ed
181.Pp
182.Bl -tag -width sdt_filler1
183.It Fa sdt_loaded
184A pointer to the first link map loaded (see below). This 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.Po
201see
202.Xr a.out 5
203.Pc
204specifying run-time relocations.
205.It Fa sdt_hash
206The location of the hash table for fast symbol lookup in this object's
207symbol table.
208.It Fa sdt_nzlist
209The location of the symbol table.
210.It Fa sdt_filler2
211Currently unused.
212.It Fa sdt_buckets
213The number of buckets in
214.Fa sdt_hash
215.It Fa sdt_strings
216The location of the symbol string table that goes with
217.Fa sdt_nzlist.
218.It Fa sdt_str_sz
219The size of the string table.
220.It Fa sdt_text_sz
221The size of the object's text segment.
222.It Fa sdt_plt_sz
223The size of the Procedure Linkage Table.
224.El
225.Pp
226A
227.Fa sod
228structure describes a shared object that is needed
229to complete the link edit process of the object containing it.
230A list of such objects
231.Po
232chained through
233.Fa sod_next
234.Pc
235is pointed at
236by the
237.Fa sdt_sods
238in the section_dispatch_table structure.
239.Bd -literal -offset indent
240struct sod {
241 long sod_name;
242 u_int sod_library : 1,
243 sod_reserved : 31;
244 short sod_major;
245 short sod_minor;
246 long sod_next;
247};
248.Ed
249.Pp
250.Bl -tag -width sod_library
251.It Fa sod_name
252The offset in the text segment of a string describing this link object.
253.It Fa sod_library
254If set,
255.Fa sod_name
256specifies a library that is to be searched for by
257.Nm ld.so .
258The path name
259is obtained by searching a set of directories
260.Po
261see also
262.Xr ldconfig 8
263.Pc
264for a shared object matching
265.Em lib\&<sod_name>\&.so.n.m.
266If not set,
267.Fa sod_name
268should point at a full path name for the desired shared object.
269.It Fa sod_major
270Specifies the major version number of the shared object to load.
271.It Fa sod_minor
272Specifies the prefered minor version number of the shared object to load.
273.El
274.Pp
275The run-time link-editor maintains a list of structures called
276.Em link maps
277to keep track of all shared objects loaded into a process' address space.
278These structures are only used at run-time and do not occur within
279the text or data segment of an executable or shared library.
280.Bd -literal -offset indent
281struct so_map {
282 caddr_t som_addr;
283 char *som_path;
284 struct so_map *som_next;
285 struct sod *som_sod;
286 caddr_t som_sodbase;
287 u_int som_write : 1;
288 struct _dynamic *som_dynamic;
289 caddr_t som_spd;
290};
291.Ed
292.Bl -tag -width som_dynamic
293.It Fa som_addr
294The address at which the shared object associated with this link map has
295been loaded.
296.It Fa som_path
297The full path name of the loaded object.
298.It Fa som_next
299Pointer to the next link map.
300.It Fa som_sod
301The
302.Fa sod
303structure that was responsible for loading this shared object.
304.It Fa som_sodbase
305Tossed in later versions the run-time linker.
306.It Fa som_write
307Set if (some portion of) this object's text segment is currently writable.
308.It Fa som_dynamic
309Pointer to this object's
310.Fa _dynamic
311structure.
312.It Fa som_spd
313Hook for attaching private data maintained by the run-time link-editor.
314.El
315.Pp
316Symbol description with size.
317This is simply an
318.Fa nlist
319structure with one field
320.Pq Fa nz_size
321added.
322Used to convey size information on items in the data segment
323of shared objects.
324An array of these lives in the shared object's
325text segment and is addressed by the
326.Fa sdt_nzlist
327field of
328.Fa section_dispatch_table.
329.Bd -literal -offset indent
330struct nzlist {
331 struct nlist nlist;
332 u_long nz_size;
333#define nz_un nlist.n_un
334#define nz_strx nlist.n_un.n_strx
335#define nz_name nlist.n_un.n_name
336#define nz_type nlist.n_type
337#define nz_value nlist.n_value
338#define nz_desc nlist.n_desc
339#define nz_other nlist.n_other
340};
341.Ed
342.Bl -tag -width nz_size
343.It Fa nlist
344.Po
345see
346.Xr nlist 3
347.Pc .
348.It Fa nz_size
349The size of the data represented by this symbol.
350.El
351.Pp
352A hash table is included within the text segment of shared object to
353to facilitate quick lookup of symbols during run-time link-editing.
354The
355.Fa sdt_hash
356field of the
357.Fa section_dispatch_table
358structure points at an array of
359.Fa rrs_hash
360structures:
361.Bd -literal -offset indent
362struct rrs_hash {
363 int rh_symbolnum; /* symbol number */
364 int rh_next; /* next hash entry */
365};
366.Ed
367.Pp
368.Bl -tag -width rh_symbolnum
369.It Fa rh_symbolnum
370The index of the symbol in the shared object's symbol table (as given by the
371.Fa ld_symbols
372field).
373.It Fa rh_next
374In case of collisions, this field is the offset of the next entry in this
375hash table bucket.
376It is zero for the last bucket element.
377.El
378The
379.Fa rt_symbol
380structure is used to keep track of run-time allocated commons
381and data items copied from shared objects.
382These items are kept on linked list
383and is exported through the
384.Fa dd_cc
385field in the
386.Fa so_debug
387structure (see below) for use by debuggers.
388.Bd -literal -offset indent
389struct rt_symbol {
390 struct nzlist *rt_sp;
391 struct rt_symbol *rt_next;
392 struct rt_symbol *rt_link;
393 caddr_t rt_srcaddr;
394 struct so_map *rt_smp;
395};
396.Ed
397.Pp
398.Bl -tag -width rt_scraddr
399.It Fa rt_sp
400The symbol description.
401.It Fa rt_next
402Virtual address of next rt_symbol.
403.It Fa rt_link
404Next in hash bucket.
405Used by internally by
406.Nm ld.so .
407.It Fa rt_srcaddr
408Location of the source of initialized data within a shared object.
409.It Fa rt_smp
410The shared object which is the original source of the data that this
411run-time symbol describes.
412.El
413.Pp
414The
415.Fa so_debug
416structure is used by debuggers to gain knowledge of any shared objects
417that have been loaded in the process's address space as a result of run-time
418link-editing.
419Since the run-time link-editor runs as a part of process
420initialization, a debugger that wishes to access symbols from shared objects
421can only do so after the link-editor has been called from crt0.
422A dynamically linked binary contains a
423.Fa so_debug
424structure which can be located by means of the
425.Fa d_debug
426field in
427.Fa _dynamic .
428.Bd -literal -offset indent
429struct so_debug {
430 int dd_version;
431 int dd_in_debugger;
432 int dd_sym_loaded;
433 char *dd_bpt_addr;
434 int dd_bpt_shadow;
435 struct rt_symbol *dd_cc;
436};
437.Ed
438.Pp
439.Bl -tag -width dd_in_debugger
440.It Fa dd_version
441Version number of this interface.
442.It Fa dd_in_debugger
443Set by the debugger to indicate to the run-time linker that the program is
444run under control of a debugger.
445.It Fa dd_sym_loaded
446Set by the run-time linker whenever it adds symbols by loading shared objects.
447.It Fa dd_bpt_addr
448The address were a breakpoint will be set by the run-time linker to
449divert control to the debugger.
450This address is determined by the start-up
451module,
452.Em crt0.o,
453to be some convenient place before the call to _main.
454.It Fa dd_bpt_shadow
455Contains the original instruction that was at
456.Fa dd_bpt_addr .
457The debugger is expected to put this instruction back before continuing the
458program.
459.It Fa dd_cc
460A pointer to the linked list of run-time allocated symbols that the debugger
461may be interested in.
462.El
463.Pp
464The
465.Em ld_entry
466structure defines a set of service routines within
467.Nm ld.so .
468.\" See
469.\" .Xr libdl.a
470.\" for more information.
471.Bd -literal -offset indent
472struct ld_entry {
473 void *(*dlopen)(char *, int);
474 int (*dlclose)(void *);
475 void *(*dlsym)(void *, char *);
476 char *(*dlerror)(void);
477};
478.Ed
|
479
| 479.Pp
|
480The
481.Fa crt_ldso
482structure defines the interface between the start-up code in crt0 and
483.Nm ld.so .
484.Bd -literal -offset indent
485struct crt_ldso {
486 int crt_ba;
487 int crt_dzfd;
488 int crt_ldfd;
489 struct _dynamic *crt_dp;
490 char **crt_ep;
491 caddr_t crt_bp;
492 char *crt_prog;
493 char *crt_ldso;
494 struct ld_entry *crt_ldentry;
495};
496#define CRT_VERSION_SUN 1
497#define CRT_VERSION_BSD_2 2
498#define CRT_VERSION_BSD_3 3
499#define CRT_VERSION_BSD_4 4
500.Ed
501.Bl -tag -width crt_dzfd
502.It Fa crt_ba
503The virtual address at which
504.Nm ld.so
505was loaded by crt0.
506.It Fa crt_dzfd
507On SunOS systems, this field contains an open file descriptor to
508.Dq Pa /dev/zero
509used to get demand paged zeroed pages.
510On
511.Fx
512systems it contains -1.
513.It Fa crt_ldfd
514Contains an open file descriptor that was used by crt0 to load
515.Nm ld.so .
516.It Fa crt_dp
517A pointer to main's
518.Fa _dynamic
519structure.
520.It Fa crt_ep
521A pointer to the environment strings.
522.It Fa crt_bp
523The address at which a breakpoint will be placed by the run-time linker
524if the main program is run by a debugger.
525See
526.Fa so_debug
527.It Fa crt_prog
528The name of the main program as determined by crt0 (CRT_VERSION_BSD3 only).
529.It Fa crt_ldso
530The path of the run-time linker as mapped by crt0 (CRT_VERSION_BSD4 only).
531.El
532.Pp
533The
534.Fa hints_header
535and
536.Fa hints_bucket
537structures define the layout of the library hints, normally found in
538.Dq Pa /var/run/ld.so.hints ,
539which is used by
540.Nm ld.so
541to quickly locate the shared object images in the
542filesystem.
543The organization of the hints file is not unlike that of an
544.Dq a.out
545object file, in that it contains a header determining the offset and size
546of a table of fixed sized hash buckets and a common string pool.
547.Bd -literal -offset indent
548struct hints_header {
549 long hh_magic;
550#define HH_MAGIC 011421044151
551 long hh_version;
552#define LD_HINTS_VERSION_1 1
553 long hh_hashtab;
554 long hh_nbucket;
555 long hh_strtab;
556 long hh_strtab_sz;
557 long hh_ehints;
558};
559.Ed
560.Bl -tag -width hh_strtab_sz
561.It Fa hh_magic
562Hints file magic number.
563.It Fa hh_version
564Interface version number.
565.It Fa hh_hashtab
566Offset of hash table.
567.It Fa hh_strtab
568Offset of string table.
569.It Fa hh_strtab_sz
570Size of strings.
571.It Fa hh_ehints
572Maximum usable offset in hints file.
573.El
574.Pp
575.Bd -literal -offset indent
576/*
577 * Hash table element in hints file.
578 */
579struct hints_bucket {
580 int hi_namex;
581 int hi_pathx;
582 int hi_dewey[MAXDEWEY];
583 int hi_ndewey;
584#define hi_major hi_dewey[0]
585#define hi_minor hi_dewey[1]
586 int hi_next;
587};
588.Ed
589.Bl -tag -width hi_ndewey
590.It Fa hi_namex
591Index of the string identifying the library.
592.It Fa hi_pathx
593Index of the string representing the full path name of the library.
594.It Fa hi_dewey
595The version numbers of the shared library.
596.It Fa hi_ndewey
597The number of valid entries in
598.Fa hi_dewey .
599.It Fa hi_next
600Next bucket in case of hashing collisions.
601.El
602.Sh CAVEATS
603Only the (GNU) C compiler currently supports the creation of shared libraries.
604Other programming languages can not be used.
| 480The
481.Fa crt_ldso
482structure defines the interface between the start-up code in crt0 and
483.Nm ld.so .
484.Bd -literal -offset indent
485struct crt_ldso {
486 int crt_ba;
487 int crt_dzfd;
488 int crt_ldfd;
489 struct _dynamic *crt_dp;
490 char **crt_ep;
491 caddr_t crt_bp;
492 char *crt_prog;
493 char *crt_ldso;
494 struct ld_entry *crt_ldentry;
495};
496#define CRT_VERSION_SUN 1
497#define CRT_VERSION_BSD_2 2
498#define CRT_VERSION_BSD_3 3
499#define CRT_VERSION_BSD_4 4
500.Ed
501.Bl -tag -width crt_dzfd
502.It Fa crt_ba
503The virtual address at which
504.Nm ld.so
505was loaded by crt0.
506.It Fa crt_dzfd
507On SunOS systems, this field contains an open file descriptor to
508.Dq Pa /dev/zero
509used to get demand paged zeroed pages.
510On
511.Fx
512systems it contains -1.
513.It Fa crt_ldfd
514Contains an open file descriptor that was used by crt0 to load
515.Nm ld.so .
516.It Fa crt_dp
517A pointer to main's
518.Fa _dynamic
519structure.
520.It Fa crt_ep
521A pointer to the environment strings.
522.It Fa crt_bp
523The address at which a breakpoint will be placed by the run-time linker
524if the main program is run by a debugger.
525See
526.Fa so_debug
527.It Fa crt_prog
528The name of the main program as determined by crt0 (CRT_VERSION_BSD3 only).
529.It Fa crt_ldso
530The path of the run-time linker as mapped by crt0 (CRT_VERSION_BSD4 only).
531.El
532.Pp
533The
534.Fa hints_header
535and
536.Fa hints_bucket
537structures define the layout of the library hints, normally found in
538.Dq Pa /var/run/ld.so.hints ,
539which is used by
540.Nm ld.so
541to quickly locate the shared object images in the
542filesystem.
543The organization of the hints file is not unlike that of an
544.Dq a.out
545object file, in that it contains a header determining the offset and size
546of a table of fixed sized hash buckets and a common string pool.
547.Bd -literal -offset indent
548struct hints_header {
549 long hh_magic;
550#define HH_MAGIC 011421044151
551 long hh_version;
552#define LD_HINTS_VERSION_1 1
553 long hh_hashtab;
554 long hh_nbucket;
555 long hh_strtab;
556 long hh_strtab_sz;
557 long hh_ehints;
558};
559.Ed
560.Bl -tag -width hh_strtab_sz
561.It Fa hh_magic
562Hints file magic number.
563.It Fa hh_version
564Interface version number.
565.It Fa hh_hashtab
566Offset of hash table.
567.It Fa hh_strtab
568Offset of string table.
569.It Fa hh_strtab_sz
570Size of strings.
571.It Fa hh_ehints
572Maximum usable offset in hints file.
573.El
574.Pp
575.Bd -literal -offset indent
576/*
577 * Hash table element in hints file.
578 */
579struct hints_bucket {
580 int hi_namex;
581 int hi_pathx;
582 int hi_dewey[MAXDEWEY];
583 int hi_ndewey;
584#define hi_major hi_dewey[0]
585#define hi_minor hi_dewey[1]
586 int hi_next;
587};
588.Ed
589.Bl -tag -width hi_ndewey
590.It Fa hi_namex
591Index of the string identifying the library.
592.It Fa hi_pathx
593Index of the string representing the full path name of the library.
594.It Fa hi_dewey
595The version numbers of the shared library.
596.It Fa hi_ndewey
597The number of valid entries in
598.Fa hi_dewey .
599.It Fa hi_next
600Next bucket in case of hashing collisions.
601.El
602.Sh CAVEATS
603Only the (GNU) C compiler currently supports the creation of shared libraries.
604Other programming languages can not be used.
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