242 lines
7.3 KiB
Groff
242 lines
7.3 KiB
Groff
.\" $NetBSD: ld.aout_so.1,v 1.7 2003/06/26 18:09:39 wiz Exp $
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.\"
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.\" Copyright (c) 1998 The NetBSD Foundation, Inc.
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.\" All rights reserved.
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.\"
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.\" This code is derived from software contributed to The NetBSD Foundation
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.\" by Paul Kranenburg.
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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 acknowledgement:
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.\" This product includes software developed by the NetBSD
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.\" Foundation, Inc. and its contributors.
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.\" 4. Neither the name of The NetBSD Foundation nor the names of its
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.\" contributors may be used to endorse or promote products derived
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.\" from this software without specific prior written permission.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS
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.\" ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
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.\" TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
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.\" PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS
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.\" BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
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.\" CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
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.\" SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
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.\" INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
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.\" CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
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.\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
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.\" POSSIBILITY OF SUCH DAMAGE.
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.\"
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.Dd March 24, 2000
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.Dt LD.AOUT_SO 1
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.Os
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.Sh NAME
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.Nm ld.so
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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
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providing run-time support for loading and link-editing
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shared objects into a process' address space.
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It uses the data structures
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.Po
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see
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.Xr link 5
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.Pc
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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.
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A mechanism is provided for initialization routines to be called,
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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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.Nm
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looks for a symbol named
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.Em .init
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in each object's symbol table.
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If present, this symbol is assumed to represent a C-function declared as
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.Ft void
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.Fn .init "void" ,
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which is then called.
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Similarly, a
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.Ft void
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.Fn .fini "void"
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function is called just before an object is unloaded from the process
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address space as a result of calling
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.Xr dlclose 3 .
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Note that while an object's
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.Em .init
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is always called, whether the object is loaded automatically at program
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startup or programmatically by using
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.Xr dlopen 3 ,
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the
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.Em .fini
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function is called only on
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.Sq last Xr dlclose 3 .
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.Pp
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This mechanism is exploited by the system-supplied C++ constructor
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initialization code located in
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.Pa /usr/lib/c++rt.o .
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This file should be included in the list of object-code files passed to
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.Xr ld 1
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when building a shared C++ library.
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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.
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Whenever 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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.Po
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see
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.Xr link 5
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for details
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.Pc .
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.Em crt0
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passes control to
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.Nm rtld 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.
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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 filesystem,
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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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.Ao
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library-name, major-version-number, minor-version-number
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.Ac Ns \&.
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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 behavior as follows:
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.Pp
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.Bl -tag -width LD_TRACE_LOADED_OBJECTS \" intentionally not the longest
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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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.It Ev LD_PRELOAD
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A colon separated list of shared object filenames to be loaded
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.Em after
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the main program but
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.Em before
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its shared object dependencies.
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.It Ev LD_WARN_NON_PURE_CODE
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When set, issue a warning whenever a link-editing operation requires
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modification of the text segment of some loaded object.
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This is usually indicative of an incorrectly built library.
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.It Ev LD_SUPPRESS_WARNINGS
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When set, no warning messages of any kind are issued.
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Normally, a warning is given if satisfactorily versioned
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library could not be found.
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.It Ev LD_TRACE_LOADED_OBJECTS
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When set, 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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.Po also known as
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.Dq __progname
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.Pc .
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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.
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This might be useful when running on a system with a completely
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non-standard filesystem layout.
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.El
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.Sh FILES
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.Bl -tag -width /var/run/ld.so.hintsXXX -compact
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.It Pa /var/run/ld.so.hints
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library location hints built by
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.Xr ldconfig 8
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.El
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.Sh SEE ALSO
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.Xr ld 1 ,
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.Xr ld.elf_so 1 ,
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.Xr ld.so 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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.Sh SECURITY CONSIDERATIONS
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The environment variables
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.Ev LD_LIBRARY_PATH
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and
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.Ev LD_PRELOAD
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are not honored when executing in a set-user-ID or set-group-ID environment.
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This action is taken to prevent malicious substitution of shared object
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dependencies or interposition of symbols.
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