219 lines
7.2 KiB
Plaintext
219 lines
7.2 KiB
Plaintext
.\" $FreeBSD$
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.\"
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.\" Copyright (c) 1995 Paul Kranenburg
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.\" All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\" 3. All advertising materials mentioning features or use of this software
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.\" must display the following acknowledgment:
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.\" This product includes software developed by Paul Kranenburg.
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.\" 3. The name of the author may not be used to endorse or promote products
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.\" derived from this software without specific prior written permission
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
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.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
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.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
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.\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
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.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
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.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
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.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
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.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
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.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
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.\"
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.Dd June 27, 1995
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.Dt RTLD 1
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.Os
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.Sh NAME
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.Nm ld.so ,
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.Nm rtld
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.Nd run-time link-editor
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.Sh DESCRIPTION
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.Nm
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is a self-contained, position independent program image providing run-time
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support for loading and link-editing shared objects into a process'
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address space. It uses the data structures
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(see
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.Xr link 5 )
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contained within dynamically linked programs to determine which shared
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libraries are needed and loads them at a convenient virtual address
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using the
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.Xr mmap 2
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system call.
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.Pp
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After all shared libraries have been successfully loaded,
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.Nm
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proceeds to resolve external references from both the main program and
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all objects loaded. A mechanism is provided for initialization routines
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to be called, on a per-object basis, giving a shared object an opportunity
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to perform any extra set-up, before execution of the program proper begins.
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This is useful for C++ libraries that contain static constructors.
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.Pp
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.Nm
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is itself a shared object that is initially loaded by the startup module
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.Em crt0 .
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Since
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.Xr a.out 5
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formats do not provide easy access to the file header from within a running
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process,
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.Em crt0
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uses the special symbol
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.Va _DYNAMIC
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to determine whether a program is in fact dynamically linked or not. Whenever
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the linker
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.Xr ld 1
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has relocated this symbol to a location other than 0,
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.Em crt0
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assumes the services of
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.Nm
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are needed
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(see
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.Xr link 5
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for details).
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.Em crt0
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passes control to
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.Nm Ns 's
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entry point before the program's
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.Fn main
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routine is called. Thus,
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.Nm
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can complete the link-editing process before the dynamic program calls upon
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services of any dynamic library.
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.Pp
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To quickly locate the required shared objects in the file system,
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.Nm
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may use a
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.Dq hints
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file, prepared by the
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.Xr ldconfig 8
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utility, in which the full path specification of the shared objects can be
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looked up by hashing on the 3-tuple
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.Aq library-name , major-version-number , minor-version-number .
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.Pp
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.Nm
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recognizes a number of environment variables that can be used to modify
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its behaviour as follows:
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.Pp
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.Bl -tag -width "LD_IGNORE_MISSING_OBJECTS"
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.It Ev LD_LIBRARY_PATH
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A colon separated list of directories, overriding the default search path
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for shared libraries.
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This is ignored for set-user-ID and set-group-ID programs.
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.It Ev LD_PRELOAD
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A colon separated list of shared libraries, to be linked in before any
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other shared libraries. If the directory is not specified then
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the directories specified by LD_LIBRARY_PATH will be searched first
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followed by the set of built-in standard directories.
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This is ignored for set-user-ID and set-group-ID programs.
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.It Ev LD_BIND_NOW
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When set to a nonempty string, causes
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.Nm
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to relocate all external function calls before starting execution of the
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program. Normally, function calls are bound lazily, at the first call
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of each function.
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.Ev LD_BIND_NOW
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increases the start-up time of a program, but it avoids run-time
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surprises caused by unexpectedly undefined functions.
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.It Ev LD_WARN_NON_PURE_CODE
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When set to a nonempty string, issue a warning whenever a link-editing
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operation requires modification of the text segment of some loaded
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object. This is usually indicative of an incorrectly built library.
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.It Ev LD_SUPPRESS_WARNINGS
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When set to a nonempty string, no warning messages of any kind are
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issued. Normally, a warning is given if satisfactorily versioned
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library could not be found.
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.It Ev LD_IGNORE_MISSING_OBJECTS
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When set to a nonempty string, makes it a nonfatal condition if
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one or more required shared objects cannot be loaded.
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Loading and execution proceeds using the objects that are
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available.
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A warning is produced for each missing object, unless the environment
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variable
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.Ev LD_SUPPRESS_WARNINGS
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is set to a nonempty string.
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.Pp
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This is ignored for set-user-ID and set-group-ID programs.
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.Pp
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Missing shared objects can be ignored without errors if all the
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following conditions hold:
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.Bl -bullet
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.It
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They do not supply definitions for any required data symbols.
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.It
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No functions defined by them are called during program execution.
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.It
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The environment variable
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.Ev LD_BIND_NOW
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is unset or is set to the empty string.
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.El
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.It Ev LD_TRACE_LOADED_OBJECTS
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When set to a nonempty string, causes
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.Nm
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to exit after loading the shared objects and printing a summary which includes
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the absolute pathnames of all objects, to standard output.
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.It Ev LD_TRACE_LOADED_OBJECTS_FMT1
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.It Ev LD_TRACE_LOADED_OBJECTS_FMT2
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When set, these variables are interpreted as format strings a la
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.Xr printf 3
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to customize the trace output and are used by
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.Xr ldd 1 Ns 's
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.Fl f
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option and allows
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.Xr ldd 1
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to be operated as a filter more conveniently.
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The following conversions can be used:
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.Bl -tag -width "xxxx"
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.It \&%a
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The main program's name
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(also known as
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.Dq __progname ) .
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.It \&%A
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The value of the environment variable
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.Ev LD_TRACE_LOADED_OBJECTS_PROGNAME
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.It \&%o
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The library name.
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.It \&%m
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The library's major version number.
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.It \&%n
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The library's minor version number.
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.It \&%p
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The full pathname as determined by
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.Nm rtld Ns 's
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library search rules.
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.It \&%x
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The library's load address.
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.El
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.Pp
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Additionally,
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.Sy \en
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and
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.Sy \et
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are recognized and have their usual meaning.
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.\" .It Ev LD_NO_INTERN_SEARCH
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.\" When set,
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.\" .Nm
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.\" does not process any internal search paths that were recorded in the
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.\" executable.
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.\" .It Ev LD_NOSTD_PATH
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.\" When set, do not include a set of built-in standard directory paths for
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.\" searching. This might be useful when running on a system with a completely
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.\" non-standard file system layout.
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.El
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.Pp
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.Sh FILES
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/var/run/ld.so.hints
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.Pp
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.Sh SEE ALSO
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.Xr ld 1 ,
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.Xr link 5 ,
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.Xr ldconfig 8
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.Sh HISTORY
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The shared library model employed first appeared in SunOS 4.0.
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