1.\" $Id: rtld.1,v 1.2 1995/10/05 05:16:52 nate Exp $
2.\"
3.\" Copyright (c) 1995 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 June 27, 1995
32.Dt RTLD 1
33.Os FreeBSD
34.Sh NAME
35.Nm ld.so
36.Nd run-time link-editor
37.Sh DESCRIPTION
38.Nm
39is a self-contained, position independent program image providing run-time
40support for loading and link-editing shared objects into a process'
41address space. It uses the data structures
42.Po
43see
44.Xr link 5
45.Pc
46contained within dynamically linked programs to determine which shared
47libraries are needed and loads them at a convenient virtual address
48using the
49.Xr mmap 2
50system call.
51.Pp
52After all shared libraries have been succesfully loaded,
53.Nm
54proceeds to resolve external references from both the main program and
55all objects loaded. A mechanism is provided for initialisation routines
56to be called, on a per-object basis, giving a shared object an opportunity
57to perfrom any extra set-up, before execution of the program proper begins.
58This is useful for C++ libraries that contain static constrictors.
59.Pp
60.Nm
61is itself a shared object that is initially loaded by the startup module
62.Em crt0 .
63Since
64.Xr a.out 5
65formats do not provide easy access to the file header from within a running
66process,
67.Em crt0
68uses the special symbol
69.Va _DYNAMIC
70to determine whether a program is in fact dynamically linked or not. Whenever
71the linker
72.Xr ld 1
73has relocated this symbol to a location other then 0,
74.Em crt0
75assumes the services of
76.Nm
77are needed
78.Po
79see
80.Xr link 5
81for details
82.Pc \&.
83.Em crt0
84passes control to
85.Nm
86\&'s entry point before the program's
87.Fn main
88routine is called. Thus,
89.Nm
90can complete the link-editing process before the dynamic program calls upon
91services of any dynamic library.
92.Pp
93To quickly locate the required shared objects in the filesystem,
94.Nm
95may use a
96.Dq hints
97file, prepared by the
98.Xr ldconfig 8
99utility, in which the full path specification of the shared objects can be
100looked up by hashing on the 3-tuple
101.Ao
102library-name, major-version-number, minor-version-number
103.Ac \&.
104.Pp
105.Nm
106recognises a number of environment variables that can be used to modify
107its behaviour as follows:
108.Pp
109.Bl -tag -width "LD_TRACE_LOADED_OBJECTS"
110.It Ev LD_LIBRARY_PATH
111A colon separated list of directories, overriding the default search path
112for shared libraries.
113This is ignored for set-user-ID and set-group-ID programs.
114.It Ev LD_PRELOAD
115A colon separated list of shared libraries, to be linked in before any
116other shared libraries. If the directory is not specified then
117the directories specified by LD_LIBRARY_PATH will be searched first
118followed by the set of built-in standard directories.
119This is ignored for set-user-ID and set-group-ID programs.
120.It Ev LD_WARN_NON_PURE_CODE
121When set, issue a warning whenever a link-editing operation requires
122modification of the text segment of some loaded object. This is usually
123indicative of an incorrectly built library.
124.It Ev LD_SUPPRESS_WARNINGS
125When set, no warning messages of any kind are issued. Normally, a warning
126is given if satisfactorily versioned library could not be found.
127.It Ev LD_TRACE_LOADED_OBJECTS
128When set, causes
129.Nm
130to exit after loading the shared objects and printing a summary which includes
131the absolute pathnames of all objects, to standard output.
132.El
133.Pp
134.Sh FILES
135/var/run/ld.so.hints
136.Pp
137.Sh SEE ALSO
138.Xr ld 1
139.Xr ldconfig 8
140.Xr link 5
141.Sh HISTORY
142The shared library model employed first appeared in SunOS 4.0
2.\"
3.\" Copyright (c) 1995 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 June 27, 1995
32.Dt RTLD 1
33.Os FreeBSD
34.Sh NAME
35.Nm ld.so
36.Nd run-time link-editor
37.Sh DESCRIPTION
38.Nm
39is a self-contained, position independent program image providing run-time
40support for loading and link-editing shared objects into a process'
41address space. It uses the data structures
42.Po
43see
44.Xr link 5
45.Pc
46contained within dynamically linked programs to determine which shared
47libraries are needed and loads them at a convenient virtual address
48using the
49.Xr mmap 2
50system call.
51.Pp
52After all shared libraries have been succesfully loaded,
53.Nm
54proceeds to resolve external references from both the main program and
55all objects loaded. A mechanism is provided for initialisation routines
56to be called, on a per-object basis, giving a shared object an opportunity
57to perfrom any extra set-up, before execution of the program proper begins.
58This is useful for C++ libraries that contain static constrictors.
59.Pp
60.Nm
61is itself a shared object that is initially loaded by the startup module
62.Em crt0 .
63Since
64.Xr a.out 5
65formats do not provide easy access to the file header from within a running
66process,
67.Em crt0
68uses the special symbol
69.Va _DYNAMIC
70to determine whether a program is in fact dynamically linked or not. Whenever
71the linker
72.Xr ld 1
73has relocated this symbol to a location other then 0,
74.Em crt0
75assumes the services of
76.Nm
77are needed
78.Po
79see
80.Xr link 5
81for details
82.Pc \&.
83.Em crt0
84passes control to
85.Nm
86\&'s entry point before the program's
87.Fn main
88routine is called. Thus,
89.Nm
90can complete the link-editing process before the dynamic program calls upon
91services of any dynamic library.
92.Pp
93To quickly locate the required shared objects in the filesystem,
94.Nm
95may use a
96.Dq hints
97file, prepared by the
98.Xr ldconfig 8
99utility, in which the full path specification of the shared objects can be
100looked up by hashing on the 3-tuple
101.Ao
102library-name, major-version-number, minor-version-number
103.Ac \&.
104.Pp
105.Nm
106recognises a number of environment variables that can be used to modify
107its behaviour as follows:
108.Pp
109.Bl -tag -width "LD_TRACE_LOADED_OBJECTS"
110.It Ev LD_LIBRARY_PATH
111A colon separated list of directories, overriding the default search path
112for shared libraries.
113This is ignored for set-user-ID and set-group-ID programs.
114.It Ev LD_PRELOAD
115A colon separated list of shared libraries, to be linked in before any
116other shared libraries. If the directory is not specified then
117the directories specified by LD_LIBRARY_PATH will be searched first
118followed by the set of built-in standard directories.
119This is ignored for set-user-ID and set-group-ID programs.
120.It Ev LD_WARN_NON_PURE_CODE
121When set, issue a warning whenever a link-editing operation requires
122modification of the text segment of some loaded object. This is usually
123indicative of an incorrectly built library.
124.It Ev LD_SUPPRESS_WARNINGS
125When set, no warning messages of any kind are issued. Normally, a warning
126is given if satisfactorily versioned library could not be found.
127.It Ev LD_TRACE_LOADED_OBJECTS
128When set, causes
129.Nm
130to exit after loading the shared objects and printing a summary which includes
131the absolute pathnames of all objects, to standard output.
132.El
133.Pp
134.Sh FILES
135/var/run/ld.so.hints
136.Pp
137.Sh SEE ALSO
138.Xr ld 1
139.Xr ldconfig 8
140.Xr link 5
141.Sh HISTORY
142The shared library model employed first appeared in SunOS 4.0