1.\" $FreeBSD$ 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 acknowledgment: 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 January 28, 2000 32.Dt RTLD 1 33.Os 34.Sh NAME 35.Nm ld-elf.so.1 , 36.Nm rtld 37.Nd run-time link-editor 38.Sh DESCRIPTION 39.Nm 40is a self-contained shared object providing run-time 41support for loading and link-editing shared objects into a process' 42address space. 43It is also commonly known as the dynamic linker. 44It uses the data structures 45contained within dynamically linked programs to determine which shared 46libraries are needed and loads them using the 47.Xr mmap 2 48system call. 49.Pp 50After all shared libraries have been successfully loaded, 51.Nm 52proceeds to resolve external references from both the main program and 53all objects loaded. 54A mechanism is provided for initialization routines 55to be called on a per-object basis, giving a shared object an opportunity 56to perform any extra set-up before execution of the program proper begins. 57This is useful for C++ libraries that contain static constructors. 58.Pp 59.Nm 60itself is loaded by the kernel together with any dynamically-linked 61program that is to be executed. 62The kernel transfers control to the 63dynamic linker. 64After the dynamic linker has finished loading, 65relocating, and initializing the program and its required shared 66objects, it transfers control to the entry point of the program. 67.Pp 68To locate the required shared objects in the filesystem, 69.Nm 70may use a 71.Dq hints 72file prepared by the 73.Xr ldconfig 8 74utility. 75.Pp 76.Nm 77recognizes a number of environment variables that can be used to modify 78its behaviour as follows: 79.Pp 80.Bl -tag -width ".Ev LD_LIBRARY_PATH" 81.It Ev LD_LIBRARY_PATH 82A colon separated list of directories, overriding the default search path 83for shared libraries. 84This is ignored for set-user-ID and set-group-ID programs. 85.It Ev LD_PRELOAD 86A list of shared libraries, separated by colons and/or white space, 87to be linked in before any 88other shared libraries. 89If the directory is not specified then 90the directories specified by 91.Ev LD_LIBRARY_PATH 92will be searched first 93followed by the set of built-in standard directories. 94This is ignored for set-user-ID and set-group-ID programs. 95.It Ev LD_BIND_NOW 96When set to a nonempty string, causes 97.Nm 98to relocate all external function calls before starting execution of the 99program. 100Normally, function calls are bound lazily, at the first call 101of each function. 102.Ev LD_BIND_NOW 103increases the start-up time of a program, but it avoids run-time 104surprises caused by unexpectedly undefined functions. 105.It Ev LD_TRACE_LOADED_OBJECTS 106When set to a nonempty string, causes 107.Nm 108to exit after loading the shared objects and printing a summary which includes 109the absolute pathnames of all objects, to standard output. 110.It Ev LD_TRACE_LOADED_OBJECTS_FMT1 111.It Ev LD_TRACE_LOADED_OBJECTS_FMT2 112When set, these variables are interpreted as format strings a la 113.Xr printf 3 114to customize the trace output and are used by 115.Xr ldd 1 Ns 's 116.Fl f 117option and allows 118.Xr ldd 1 119to be operated as a filter more conveniently. 120The following conversions can be used: 121.Bl -tag -width 4n 122.It Li %a 123The main program's name 124(also known as 125.Dq __progname ) . 126.It Li \&%A 127The value of the environment variable 128.Ev LD_TRACE_LOADED_OBJECTS_PROGNAME 129.It Li %o 130The library name. 131.It Li %m 132The library's major version number. 133.It Li %p 134The full pathname as determined by 135.Nm rtld Ns 's 136library search rules. 137.It Li %x 138The library's load address. 139.El 140.Pp 141Additionally, 142.Ql \en 143and 144.Ql \et 145are recognized and have their usual meaning. 146.El 147.Sh FILES 148.Bl -tag -width indent 149.It Pa /var/run/ld-elf.so.hints 150.El 151.Sh SEE ALSO 152.Xr ld 1 , 153.Xr ldd 1 , 154.Xr elf 5 , 155.Xr ldconfig 8 156