freebsd/libexec/rtld-elf/rtld.1

.\"	$Id: rtld.1,v 1.3 1996/04/20 18:29:48 jdp Exp $
.\"
.\" Copyright (c) 1995 Paul Kranenburg
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.Dd June 27, 1995
.Dt RTLD 1
.Os FreeBSD
.Sh NAME
.Nm ld.so
.Nd run-time link-editor
.Sh DESCRIPTION
.Nm
is a self-contained, position independent program image providing run-time
support for loading and link-editing shared objects into a process'
address space. It uses the data structures
.Po
see
.Xr link 5
.Pc
contained within dynamically linked programs to determine which shared
libraries are needed and loads them at a convenient virtual address
using the
.Xr mmap 2
system call.
.Pp
After all shared libraries have been succesfully loaded,
.Nm
proceeds to resolve external references from both the main program and
all objects loaded. A mechanism is provided for initialisation routines
to be called, on a per-object basis, giving a shared object an opportunity
to perfrom any extra set-up, before execution of the program proper begins.
This is useful for C++ libraries that contain static constrictors.
.Pp
.Nm
is itself a shared object that is initially loaded by the startup module
.Em crt0 .
Since
.Xr a.out 5
formats do not provide easy access to the file header from within a running
process,
.Em crt0
uses the special symbol
.Va _DYNAMIC
to determine whether a program is in fact dynamically linked or not. Whenever
the linker
.Xr ld 1
has relocated this symbol to a location other then 0,
.Em crt0
assumes the services of
.Nm
are needed
.Po
see
.Xr link 5
for details
.Pc \&.
.Em crt0
passes control to
.Nm
\&'s entry point before the program's
.Fn main
routine is called. Thus,
.Nm
can complete the link-editing process before the dynamic program calls upon
services of any dynamic library.
.Pp
To quickly locate the required shared objects in the filesystem,
.Nm
may use a
.Dq hints
file, prepared by the
.Xr ldconfig 8
utility, in which the full path specification of the shared objects can be
looked up by hashing on the 3-tuple
.Ao
library-name, major-version-number, minor-version-number
.Ac \&.
.Pp
.Nm
recognises a number of environment variables that can be used to modify
its behaviour as follows:
.Pp
.Bl -tag -width "LD_TRACE_LOADED_OBJECTS"
.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 colon separated list of shared libraries, to be linked in before any
other shared libraries.  If the directory is not specified then
the directories specified by 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_WARN_NON_PURE_CODE
When set, issue a warning whenever a link-editing operation requires
modification of the text segment of some loaded object. This is usually
indicative of an incorrectly built library.
.It Ev LD_SUPPRESS_WARNINGS
When set, no warning messages of any kind are issued. Normally, a warning
is given if satisfactorily versioned library could not be found.
.It Ev LD_TRACE_LOADED_OBJECTS
When set, causes
.Nm
to exit after loading the shared objects and printing a summary which includes
the absolute pathnames of all objects, to standard output.
.El
.Pp
.Sh FILES
/var/run/ld.so.hints
.Pp
.Sh SEE ALSO
.Xr ld 1 ,
.Xr ldconfig 8 ,
.Xr link 5
.Sh HISTORY
The shared library model employed first appeared in SunOS 4.0