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.\" Copyright (c) 1995 Paul Kranenburg
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.Dd January 28, 2000
.Dt RTLD 1
.Os
.Sh NAME
.Nm ld-elf.so.1 ,
.Nm ld.so ,
.Nm rtld
.Nd run-time link-editor
.Sh DESCRIPTION
The
.Nm
utility is a self-contained shared object providing run-time
support for loading and link-editing shared objects into a process'
address space.
It is also commonly known as the dynamic linker.
It uses the data structures
contained within dynamically linked programs to determine which shared
libraries are needed and loads them using the
.Xr mmap 2
system call.
.Pp
After all shared libraries have been successfully loaded,
.Nm
proceeds to resolve external references from both the main program and
all objects loaded.
A mechanism is provided for initialization routines
to be called on a per-object basis, giving a shared object an opportunity
to perform any extra set-up before execution of the program proper begins.
This is useful for C++ libraries that contain static constructors.
.Pp
The
.Nm
utility itself is loaded by the kernel together with any dynamically-linked
program that is to be executed.
The kernel transfers control to the
dynamic linker.
After the dynamic linker has finished loading,
relocating, and initializing the program and its required shared
objects, it transfers control to the entry point of the program.
.Pp
To locate the required shared objects in the file system,
.Nm
may use a
.Dq hints
file prepared by the
.Xr ldconfig 8
utility.
.Pp
The
.Nm
utility
recognizes a number of environment variables that can be used to modify
its behaviour as follows:
.Pp
.Bl -tag -width ".Ev LD_LIBRARY_PATH"
.It Ev LD_LIBRARY_PATH
A colon separated list of directories, overriding the default search path
for shared libraries.
This is ignored for set-user-ID and set-group-ID programs.
.It Ev LD_PRELOAD
A list of shared libraries, separated by colons and/or white space,
to be linked in before any
other shared libraries.
If the directory is not specified then
the directories specified by
.Ev LD_LIBRARY_PATH
will be searched first
followed by the set of built-in standard directories.
This is ignored for set-user-ID and set-group-ID programs.
.It Ev LD_BIND_NOW
When set to a nonempty string, causes
.Nm
to relocate all external function calls before starting execution of the
program.
Normally, function calls are bound lazily, at the first call
of each function.
.Ev LD_BIND_NOW
increases the start-up time of a program, but it avoids run-time
surprises caused by unexpectedly undefined functions.
.It Ev LD_TRACE_LOADED_OBJECTS
When set to a nonempty string, causes
.Nm
to exit after loading the shared objects and printing a summary which includes
the absolute pathnames of all objects, to standard output.
.It Ev LD_TRACE_LOADED_OBJECTS_ALL
When set to a nonempty string, causes
.Nm
to expand the summary to indicate which objects caused each object to
be loaded.
.It Ev LD_TRACE_LOADED_OBJECTS_FMT1
.It Ev LD_TRACE_LOADED_OBJECTS_FMT2
When set, these variables are interpreted as format strings a la
.Xr printf 3
to customize the trace output and are used by
.Xr ldd 1 Ns 's
.Fl f
option and allows
.Xr ldd 1
to be operated as a filter more conveniently.
The following conversions can be used:
.Bl -tag -width 4n
.It Li %a
The main program's name
(also known as
.Dq __progname ) .
.It Li \&%A
The value of the environment variable
.Ev LD_TRACE_LOADED_OBJECTS_PROGNAME
.It Li %o
The library name.
.It Li %m
The library's major version number.
.It Li %p
The full pathname as determined by
.Nm rtld Ns 's
library search rules.
.It Li %x
The library's load address.
.El
.Pp
Additionally,
.Ql \en
and
.Ql \et
are recognized and have their usual meaning.
.El
.Sh FILES
.Bl -tag -width indent
.It Pa /var/run/ld-elf.so.hints
.El
.Sh SEE ALSO
.Xr ld 1 ,
.Xr ldd 1 ,
.Xr elf 5 ,
.Xr ldconfig 8