1.\" $FreeBSD: src/libexec/rtld-elf/rtld.1,v 1.18.2.7 2002/01/10 17:51:28 ru Exp $ 2.\" $DragonFly: src/libexec/rtld-elf/rtld.1,v 1.6 2008/01/08 00:02:04 corecode Exp $ 3.\" 4.\" Copyright (c) 1995 Paul Kranenburg 5.\" All rights reserved. 6.\" 7.\" Redistribution and use in source and binary forms, with or without 8.\" modification, are permitted provided that the following conditions 9.\" are met: 10.\" 1. Redistributions of source code must retain the above copyright 11.\" notice, this list of conditions and the following disclaimer. 12.\" 2. Redistributions in binary form must reproduce the above copyright 13.\" notice, this list of conditions and the following disclaimer in the 14.\" documentation and/or other materials provided with the distribution. 15.\" 3. All advertising materials mentioning features or use of this software 16.\" must display the following acknowledgment: 17.\" This product includes software developed by Paul Kranenburg. 18.\" 3. The name of the author may not be used to endorse or promote products 19.\" derived from this software without specific prior written permission 20.\" 21.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR 22.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES 23.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. 24.\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, 25.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT 26.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, 27.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY 28.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT 29.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF 30.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. 31.\" 32.Dd January 7, 2008 33.Dt RTLD 1 34.Os 35.Sh NAME 36.Nm ld-elf.so.1 , 37.Nm ld-elf.so.2 , 38.Nm rtld 39.Nd run-time link-editor 40.Sh DESCRIPTION 41.Nm 42is a self-contained shared object providing run-time 43support for loading and link-editing shared objects into a process' 44address space. 45It is also commonly known as the dynamic linker. 46It uses the data structures 47contained within dynamically linked programs to determine which shared 48libraries are needed and loads them using the 49.Xr mmap 2 50system call. 51.Pp 52After all shared libraries have been successfully loaded, 53.Nm 54proceeds to resolve external references from both the main program and 55all objects loaded. 56A mechanism is provided for initialization routines 57to be called on a per-object basis, giving a shared object an opportunity 58to perform any extra set-up before execution of the program proper begins. 59This is useful for C++ libraries that contain static constructors. 60.Pp 61.Nm 62itself is loaded by the kernel together with any dynamically-linked 63program that is to be executed. 64The kernel transfers control to the 65dynamic linker. 66After the dynamic linker has finished loading, 67relocating, and initializing the program and its required shared 68objects, it transfers control to the entry point of the program. 69.Pp 70To locate the required shared objects in the filesystem, 71.Nm 72may use a 73.Dq hints 74file prepared by the 75.Xr ldconfig 8 76utility. 77.Pp 78.Nm 79recognizes a number of environment variables that can be used to modify 80its behaviour as follows: 81.Pp 82.Bl -tag -width ".Ev LD_LIBRARY_PATH" 83.It Ev LD_LIBRARY_PATH 84A colon separated list of directories, overriding the default search path 85for shared libraries. 86This is ignored for set-user-ID and set-group-ID programs. 87.It Ev LD_PRELOAD 88A list of shared libraries, separated by colons and/or white space, 89to be linked in before any 90other shared libraries. 91If the directory is not specified then 92the directories specified by 93.Ev LD_LIBRARY_PATH 94will be searched first 95followed by the set of built-in standard directories. 96This is ignored for set-user-ID and set-group-ID programs. 97.It Ev LD_BIND_NOW 98When set to a nonempty string, causes 99.Nm 100to relocate all external function calls before starting execution of the 101program. 102Normally, function calls are bound lazily, at the first call 103of each function. 104.Ev LD_BIND_NOW 105increases the start-up time of a program, but it avoids run-time 106surprises caused by unexpectedly undefined functions. 107.It Ev LD_TRACE_LOADED_OBJECTS 108When set to a nonempty string, causes 109.Nm 110to exit after loading the shared objects and printing a summary which includes 111the absolute pathnames of all objects, to standard output. 112.It Ev LD_TRACE_LOADED_OBJECTS_FMT1 113.It Ev LD_TRACE_LOADED_OBJECTS_FMT2 114When set, these variables are interpreted as format strings a la 115.Xr printf 3 116to customize the trace output and are used by 117.Xr ldd 1 Ns 's 118.Fl f 119option and allows 120.Xr ldd 1 121to be operated as a filter more conveniently. 122The following conversions can be used: 123.Bl -tag -width 4n 124.It Li %a 125The main program's name 126(also known as 127.Dq __progname ) . 128.It Li \&%A 129The value of the environment variable 130.Ev LD_TRACE_LOADED_OBJECTS_PROGNAME 131.It Li %o 132The library name. 133.It Li %m 134The library's major version number. 135.It Li %p 136The full pathname as determined by 137.Nm rtld Ns 's 138library search rules. 139.It Li %x 140The library's load address. 141.El 142.Pp 143Additionally, 144.Ql \en 145and 146.Ql \et 147are recognized and have their usual meaning. 148.El 149.Pp 150If a shared object preloaded by the 151.Ev LD_PRELOAD 152mechanism contains a public symbol 153.Dq _rtld_functrace , 154.Nm 155will transfer control to this function each time 156it needs to resolve an unbound function symbol. 157By returning a non-zero value, 158.Fn _rtld_functrace 159can advise the linker to keep tracing the specified 160combination of caller shared object and called function; 161returning 0 will prevent 162.Fn _rtld_functrace 163to be called for this combination again. 164.Pp 165When implementing a custom 166.Fn _rtld_functrace 167function, be aware of the possibility that 168.Fn _rtld_functrace 169might be called for functions called on its behalf, 170or that multiple threads could enter 171.Fn _rtld_functrace 172at the same time. 173.Pp 174The signature for 175.Fn _rtld_functrace 176is specified as follows. 177.Pp 178.Ft int 179.Fn _rtld_functrace "const char *callerso" "const char *calleeso" "const char *calleefun" "void *stack" 180.Sh DIFFERENCES BETWEEN .1 and .2 181ABI changes have been made to support TLS allocation and initialization 182and to give threading libraries a chance to complete initialization of the 183TCB prior to the calling of the _init() functions for the dynamically loaded 184libraries. 185.Sh FILES 186.Bl -tag -width indent 187.It Pa /var/run/ld-elf.so.hints 188.El 189.Sh SEE ALSO 190.Xr ld 1 , 191.Xr ldd 1 , 192.Xr elf 5 , 193.Xr ldconfig 8 194