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1.\" Copyright (c) 1995 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 acknowledgment:
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.\"
f4f4bfd5 29.\" $FreeBSD$
fcf53d9b 30.\"
f560f0d6 31.Dd May 6, 2012
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32.Dt RTLD 1
33.Os
34.Sh NAME
a1eee96a 35.Nm ld-elf.so.2 ,
fcf53d9b 36.Nm ld-elf.so.1 ,
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37.Nm rtld ,
38.Nm _rtld_functrace
984263bc 39.Nd run-time link-editor
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40.Sh SYNOPSIS
41.Ft int
42.Fn _rtld_functrace "const char *callerso" "const char *calleeso" "const char *calleefun" "void *stack"
984263bc 43.Sh DESCRIPTION
fcf53d9b 44The
984263bc 45.Nm
fcf53d9b 46utility is a self-contained shared object providing run-time
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47support for loading and link-editing shared objects into a process'
48address space.
49It is also commonly known as the dynamic linker.
50It uses the data structures
51contained within dynamically linked programs to determine which shared
52libraries are needed and loads them using the
53.Xr mmap 2
54system call.
55.Pp
56After all shared libraries have been successfully loaded,
57.Nm
58proceeds to resolve external references from both the main program and
59all objects loaded.
60A mechanism is provided for initialization routines
61to be called on a per-object basis, giving a shared object an opportunity
62to perform any extra set-up before execution of the program proper begins.
63This is useful for C++ libraries that contain static constructors.
64.Pp
fcf53d9b 65When resolving dependencies for the loaded objects,
984263bc 66.Nm
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67may be allowed to translate dynamic token strings in rpath and soname
68by setting
69.Fl "z origin"
70option of the static linker
71.Xr ld 1 .
72The following strings are recognized now:
73.Bl -tag -width ".Pa $PLATFORM"
74.It Pa $ORIGIN
75Translated to the full path of the loaded object.
76.It Pa $OSNAME
77Translated to the name of the operating system implementation.
78.It Pa $OSREL
79Translated to the release level of the operating system.
80.It Pa $PLATFORM
81Translated to the machine hardware platform.
82.El
83.Pp
84The
85.Nm
86utility itself is loaded by the kernel together with any dynamically-linked
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87program that is to be executed.
88The kernel transfers control to the
89dynamic linker.
90After the dynamic linker has finished loading,
91relocating, and initializing the program and its required shared
92objects, it transfers control to the entry point of the program.
f560f0d6 93The following search order is used to locate required shared objects:
984263bc 94.Pp
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95.Bl -enum -offset indent -compact
96.It
97.Dv DT_RPATH
98of the referencing object unless that object also contains a
99.Dv DT_RUNPATH
100tag
101.It
102.Dv DT_RPATH
103of the program unless the referencing object contains a
104.Dv DT_RUNPATH
105tag
106.It
107Path indicated by
108.Ev LD_LIBRARY_PATH
109environment variable
110.It
111.Dv DT_RUNPATH
112of the referencing object
113.It
114Hints file produced by the
115.Xr ldconfig 8
116utility
117.It
118The
119.Pa /usr/lib
120directory unless the referencing object was linked using the
121.Dq Fl z Ar nodefaultlib
122option
123.El
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fcf53d9b 125The
984263bc 126.Nm
fcf53d9b 127utility
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128recognizes a number of environment variables that can be used to modify
129its behaviour as follows:
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130.Bl -tag -width ".Ev LD_LIBMAP_DISABLE"
131.It Ev LD_DUMP_REL_POST
132If set,
133.Nm
134will print a table containing all relocations after symbol
135binding and relocation.
136.It Ev LD_DUMP_REL_PRE
137If set,
138.Nm
139will print a table containing all relocations before symbol
140binding and relocation.
141.It Ev LD_LIBMAP
142A library replacement list in the same format as
143.Xr libmap.conf 5 .
144For convenience, the characters
145.Ql =
146and
147.Ql \&,
148can be used instead of a space and a newline.
149This variable is parsed after
150.Xr libmap.conf 5 ,
151and will override its entries.
152This variable is unset for set-user-ID and set-group-ID programs.
153.It Ev LD_LIBMAP_DISABLE
154If set, disables the use of
155.Xr libmap.conf 5
156and
157.Ev LD_LIBMAP .
158This variable is unset for set-user-ID and set-group-ID programs.
159.It Ev LD_ELF_HINTS_PATH
160This variable will override the default location of
161.Dq hints
162file.
163This variable is unset for set-user-ID and set-group-ID programs.
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164.It Ev LD_LIBRARY_PATH
165A colon separated list of directories, overriding the default search path
166for shared libraries.
fcf53d9b 167This variable is unset for set-user-ID and set-group-ID programs.
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168.It Ev LD_PRELOAD
169A list of shared libraries, separated by colons and/or white space,
170to be linked in before any
171other shared libraries.
172If the directory is not specified then
173the directories specified by
174.Ev LD_LIBRARY_PATH
175will be searched first
176followed by the set of built-in standard directories.
fcf53d9b 177This variable is unset for set-user-ID and set-group-ID programs.
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178.It Ev LD_BIND_NOW
179When set to a nonempty string, causes
180.Nm
181to relocate all external function calls before starting execution of the
182program.
183Normally, function calls are bound lazily, at the first call
184of each function.
185.Ev LD_BIND_NOW
186increases the start-up time of a program, but it avoids run-time
187surprises caused by unexpectedly undefined functions.
188.It Ev LD_TRACE_LOADED_OBJECTS
189When set to a nonempty string, causes
190.Nm
191to exit after loading the shared objects and printing a summary which includes
192the absolute pathnames of all objects, to standard output.
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193.It Ev LD_TRACE_LOADED_OBJECTS_ALL
194When set to a nonempty string, causes
195.Nm
196to expand the summary to indicate which objects caused each object to
197be loaded.
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198.It Ev LD_TRACE_LOADED_OBJECTS_FMT1
199.It Ev LD_TRACE_LOADED_OBJECTS_FMT2
200When set, these variables are interpreted as format strings a la
201.Xr printf 3
202to customize the trace output and are used by
203.Xr ldd 1 Ns 's
204.Fl f
205option and allows
206.Xr ldd 1
207to be operated as a filter more conveniently.
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208If the dependency name starts with string
209.Pa lib ,
210.Ev LD_TRACE_LOADED_OBJECTS_FMT1
211is used, otherwise
212.Ev LD_TRACE_LOADED_OBJECTS_FMT2
213is used.
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214The following conversions can be used:
215.Bl -tag -width 4n
216.It Li %a
217The main program's name
218(also known as
219.Dq __progname ) .
220.It Li \&%A
221The value of the environment variable
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222.Ev LD_TRACE_LOADED_OBJECTS_PROGNAME .
223Typically used to print both the names of programs and shared libraries
224being inspected using
225.Xr ldd 1 .
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226.It Li %o
227The library name.
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228.It Li %p
229The full pathname as determined by
230.Nm rtld Ns 's
231library search rules.
232.It Li %x
233The library's load address.
234.El
235.Pp
236Additionally,
237.Ql \en
238and
239.Ql \et
240are recognized and have their usual meaning.
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241.It Ev LD_UTRACE
242If set,
243.Nm
244will log events such as the loading and unloading of shared objects via
245.Xr utrace 2 .
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246.It Ev LD_LOADFLTR
247If set,
248.Nm
249will process the filtee dependencies of the loaded objects immediately,
250instead of postponing it until required.
251Normally, the filtees are opened at the time of the first symbol resolution
252from the filter object.
984263bc 253.El
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254.Pp
255If a shared object preloaded by the
256.Ev LD_PRELOAD
257mechanism contains a public symbol
258.Dq _rtld_functrace ,
259.Nm
260will transfer control to this function each time
261it needs to resolve an unbound function symbol.
262By returning a non-zero value,
263.Fn _rtld_functrace
264can advise the linker to keep tracing the specified
265combination of caller shared object and called function;
266returning 0 will prevent
267.Fn _rtld_functrace
268to be called for this combination again.
269.Pp
270When implementing a custom
271.Fn _rtld_functrace
272function, be aware of the possibility that
273.Fn _rtld_functrace
274might be called for functions called on its behalf,
275or that multiple threads could enter
276.Fn _rtld_functrace
277at the same time.
a1eee96a 278.Sh DIFFERENCES BETWEEN .1 and .2
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279ABI changes have been made to support TLS allocation and initialization
280and to give threading libraries a chance to complete initialization of the
281TCB prior to the calling of the _init() functions for the dynamically loaded
282libraries.
984263bc 283.Sh FILES
fcf53d9b 284.Bl -tag -width ".Pa /var/run/ld-elf.so.hints" -compact
984263bc 285.It Pa /var/run/ld-elf.so.hints
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286Hints file.
287.It Pa /etc/libmap.conf
288The libmap configuration file.
984263bc 289.El
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290.Sh EXAMPLES
291To set up an
292.Fn _rtld_functrace
293for printing out the functions as they are called, this code can be used:
294.Bd -literal -offset indent
295#include <string.h>
296
4ee0d876 297static int nl = 10;
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298
299int
300_rtld_functrace(const char *callerso, const char *calleeso,
301 const char *calleefun, void *stack)
302{
303 write(2, "calling ", 8);
304 write(2, calleefun, strlen(calleefun));
305 write(2, &nl, 1);
306 return 1;
307}
308.Ed
309.Pp
310If put in a file named
311.Pa ft.c
312and compiled with
313.Bd -literal -offset indent
0e759e7d 314$ cc -shared -fPIC ft.c -o ft.so
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315.Ed
316.Pp
317setting
318.Ev LD_PRELOAD
319to the path of
320.Pa ft.so
321will activate it.
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322.Sh SEE ALSO
323.Xr ld 1 ,
324.Xr ldd 1 ,
325.Xr elf 5 ,
fcf53d9b 326.Xr libmap.conf 5 ,
984263bc 327.Xr ldconfig 8