freebsd-dev/libexec/rtld-aout/rtld.1aout

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.\" Copyright (c) 1995 Paul Kranenburg
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.Dd June 27, 1995
.Dt RTLD 1
.Os FreeBSD
.Sh NAME
.Nm ld.so ,
.Nm rtld
.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
(see
.Xr link 5 )
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 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
.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 than 0,
.Em crt0
assumes the services of
.Nm
are needed
(see
.Xr link 5
for details).
.Em crt0
passes control to
.Nm Ns '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
.Aq library-name , major-version-number , minor-version-number .
.Pp
.Nm
recognizes a number of environment variables that can be used to modify
its behaviour as follows:
.Pp
.Bl -tag -width "LD_IGNORE_MISSING_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_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_WARN_NON_PURE_CODE
When set to a nonempty string, 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 to a nonempty string, no warning messages of any kind are
issued. Normally, a warning is given if satisfactorily versioned
library could not be found.
.It Ev LD_IGNORE_MISSING_OBJECTS
When set to a nonempty string, makes it a nonfatal condition if
one or more required shared objects cannot be loaded.
Loading and execution proceeds using the objects that are
available.
A warning is produced for each missing object, unless the environment
variable
.Ev LD_SUPPRESS_WARNINGS
is set to a nonempty string.
.Pp
This is ignored for set-user-ID and set-group-ID programs.
.Pp
Missing shared objects can be ignored without errors if all the
following conditions hold:
.Bl -bullet
.It
They do not supply definitions for any required data symbols.
.It
No functions defined by them are called during program execution.
.It
The environment variable
.Ev LD_BIND_NOW
is unset or is set to the empty string.
.El
.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_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 "xxxx"
.It \&%a
The main program's name
(also known as
.Dq __progname ) .
.It \&%A
The value of the environment variable
.Ev LD_TRACE_LOADED_OBJECTS_PROGNAME
.It \&%o
The library name.
.It \&%m
The library's major version number.
.It \&%n
The library's minor version number.
.It \&%p
The full pathname as determined by
.Nm rtld Ns 's
library search rules.
.It \&%x
The library's load address.
.El
.Pp
Additionally,
.Sy \en
and
.Sy \et
are recognized and have their usual meaning.
.\" .It Ev LD_NO_INTERN_SEARCH
.\" When set,
.\" .Nm
.\" does not process any internal search paths that were recorded in the
.\" executable.
.\" .It Ev LD_NOSTD_PATH
.\" When set, do not include a set of built-in standard directory paths for
.\" searching. This might be useful when running on a system with a completely
.\" non-standard filesystem layout.
.El
.Pp
.Sh FILES
/var/run/ld.so.hints
.Pp
.Sh SEE ALSO
.Xr ld 1 ,
.Xr link 5 ,
.Xr ldconfig 8
.Sh HISTORY
The shared library model employed first appeared in SunOS 4.0