99ac8154ff
Create libdl.so.1 as a filter for libc.so.7 which exports public dl* functions. The functions are resolved from the rtld instead, the goal of creating library is to avoid errors from the static linker due to missed libdl. For static binaries, an empty .o is compiled into libdl.a so that static binaries still get dl stubs from libc.a. Right now lld cannot create filter objects, disable libdl on arm64 when binutils are not used. Reviewed by: bdrewery, dim (previos version); emaste Exp run: PR 220525, done by antoine Sponsored by: The FreeBSD Foundation MFC after: 1 month Differential revision: https://reviews.freebsd.org/D11504
417 lines
11 KiB
Groff
417 lines
11 KiB
Groff
.\" This source code is a product of Sun Microsystems, Inc. and is provided
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.\" for unrestricted use provided that this legend is included on all tape
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.\" media and as a part of the software program in whole or part. Users
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.\" may copy or modify this source code without charge, but are not authorized
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.\" to license or distribute it to anyone else except as part of a product or
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.\" program developed by the user.
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.\"
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.\" THIS PROGRAM CONTAINS SOURCE CODE COPYRIGHTED BY SUN MICROSYSTEMS, INC.
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.\" SUN MICROSYSTEMS, INC., MAKES NO REPRESENTATIONS ABOUT THE SUITABLITY
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.\" OF SUCH SOURCE CODE FOR ANY PURPOSE. IT IS PROVIDED "AS IS" WITHOUT
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.\" EXPRESS OR IMPLIED WARRANTY OF ANY KIND. SUN MICROSYSTEMS, INC. DISCLAIMS
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.\" ALL WARRANTIES WITH REGARD TO SUCH SOURCE CODE, INCLUDING ALL IMPLIED
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.\" WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. IN
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.\" NO EVENT SHALL SUN MICROSYSTEMS, INC. BE LIABLE FOR ANY SPECIAL, INDIRECT,
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.\" INCIDENTAL, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING
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.\" FROM USE OF SUCH SOURCE CODE, REGARDLESS OF THE THEORY OF LIABILITY.
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.\"
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.\" This source code is provided with no support and without any obligation on
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.\" the part of Sun Microsystems, Inc. to assist in its use, correction,
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.\" modification or enhancement.
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.\"
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.\" SUN MICROSYSTEMS, INC. SHALL HAVE NO LIABILITY WITH RESPECT TO THE
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.\" INFRINGEMENT OF COPYRIGHTS, TRADE SECRETS OR ANY PATENTS BY THIS
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.\" SOURCE CODE OR ANY PART THEREOF.
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.\"
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.\" Sun Microsystems, Inc.
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.\" 2550 Garcia Avenue
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.\" Mountain View, California 94043
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.\"
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.\" Copyright (c) 1991 Sun Microsystems, Inc.
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.\"
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.\" @(#) dlopen.3 1.6 90/01/31 SMI
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.\" $FreeBSD$
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.\"
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.Dd July 7, 2017
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.Dt DLOPEN 3
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.Os
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.Sh NAME
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.Nm dlopen ,
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.Nm fdlopen ,
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.Nm dlsym ,
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.Nm dlfunc ,
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.Nm dlerror ,
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.Nm dlclose
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.Nd programmatic interface to the dynamic linker
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.Sh LIBRARY
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.Lb libc
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.Sh SYNOPSIS
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.In dlfcn.h
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.Ft void *
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.Fn dlopen "const char *path" "int mode"
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.Ft void *
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.Fn fdlopen "int fd" "int mode"
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.Ft void *
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.Fn dlsym "void * restrict handle" "const char * restrict symbol"
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.Ft dlfunc_t
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.Fn dlfunc "void * restrict handle" "const char * restrict symbol"
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.Ft char *
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.Fn dlerror "void"
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.Ft int
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.Fn dlclose "void *handle"
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.Sh DESCRIPTION
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These functions provide a simple programmatic interface to the services of the
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dynamic linker.
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Operations are provided to add new shared objects to a
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program's address space, to obtain the address bindings of symbols
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defined by such
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objects, and to remove such objects when their use is no longer required.
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.Pp
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The
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.Fn dlopen
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function
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provides access to the shared object in
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.Fa path ,
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returning a descriptor that can be used for later
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references to the object in calls to
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.Fn dlsym
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and
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.Fn dlclose .
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If
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.Fa path
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was not in the address space prior to the call to
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.Fn dlopen ,
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it is placed in the address space.
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When an object is first loaded into the address space in this way, its
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function
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.Fn _init ,
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if any, is called by the dynamic linker.
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If
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.Fa path
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has already been placed in the address space in a previous call to
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.Fn dlopen ,
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it is not added a second time, although a reference count of
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.Fn dlopen
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operations on
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.Fa path
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is maintained.
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A null pointer supplied for
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.Fa path
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is interpreted as a reference to the main
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executable of the process.
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The
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.Fa mode
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argument
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controls the way in which external function references from the
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loaded object are bound to their referents.
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It must contain one of the following values, possibly ORed with
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additional flags which will be described subsequently:
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.Bl -tag -width RTLD_LAZYX
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.It Dv RTLD_LAZY
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Each external function reference is resolved when the function is first
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called.
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.It Dv RTLD_NOW
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All external function references are bound immediately by
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.Fn dlopen .
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.El
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.Pp
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.Dv RTLD_LAZY
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is normally preferred, for reasons of efficiency.
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However,
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.Dv RTLD_NOW
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is useful to ensure that any undefined symbols are discovered during the
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call to
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.Fn dlopen .
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.Pp
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One of the following flags may be ORed into the
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.Fa mode
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argument:
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.Bl -tag -width RTLD_NODELETE
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.It Dv RTLD_GLOBAL
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Symbols from this shared object and its directed acyclic graph (DAG)
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of needed objects will be available for resolving undefined references
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from all other shared objects.
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.It Dv RTLD_LOCAL
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Symbols in this shared object and its DAG of needed objects will be
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available for resolving undefined references only from other objects
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in the same DAG.
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This is the default, but it may be specified
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explicitly with this flag.
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.It Dv RTLD_TRACE
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When set, causes dynamic linker to exit after loading all objects
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needed by this shared object and printing a summary which includes
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the absolute pathnames of all objects, to standard output.
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With this flag
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.Fn dlopen
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will return to the caller only in the case of error.
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.It Dv RTLD_NODELETE
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Prevents unload of the loaded object on
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.Fn dlclose .
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The same behaviour may be requested by
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.Fl "z nodelete"
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option of the static linker
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.Xr ld 1 .
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.It Dv RTLD_NOLOAD
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Only return valid handle for the object if it is already loaded in
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the process address space, otherwise
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.Dv NULL
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is returned.
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Other mode flags may be specified, which will be applied for promotion
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for the found object.
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.El
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.Pp
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If
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.Fn dlopen
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fails, it returns a null pointer, and sets an error condition which may
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be interrogated with
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.Fn dlerror .
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.Pp
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The
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.Fn fdlopen
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function is similar to
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.Fn dlopen ,
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but it takes the file descriptor argument
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.Fa fd ,
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which is used for the file operations needed to load an object
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into the address space.
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The file descriptor
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.Fa fd
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is not closed by the function regardless a result of execution,
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but a duplicate of the file descriptor is.
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This may be important if a
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.Xr lockf 3
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lock is held on the passed descriptor.
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The
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.Fa fd
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argument -1 is interpreted as a reference to the main
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executable of the process, similar to
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.Va NULL
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value for the
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.Fa name
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argument to
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.Fn dlopen .
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The
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.Fn fdlopen
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function can be used by the code that needs to perform
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additional checks on the loaded objects, to prevent races with
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symlinking or renames.
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.Pp
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The
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.Fn dlsym
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function
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returns the address binding of the symbol described in the null-terminated
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character string
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.Fa symbol ,
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as it occurs in the shared object identified by
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.Fa handle .
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The symbols exported by objects added to the address space by
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.Fn dlopen
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can be accessed only through calls to
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.Fn dlsym .
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Such symbols do not supersede any definition of those symbols already present
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in the address space when the object is loaded, nor are they available to
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satisfy normal dynamic linking references.
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.Pp
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If
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.Fn dlsym
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is called with the special
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.Fa handle
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.Dv NULL ,
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it is interpreted as a reference to the executable or shared object
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from which the call
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is being made.
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Thus a shared object can reference its own symbols.
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.Pp
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If
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.Fn dlsym
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is called with the special
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.Fa handle
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.Dv RTLD_DEFAULT ,
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the search for the symbol follows the algorithm used for resolving
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undefined symbols when objects are loaded.
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The objects searched are
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as follows, in the given order:
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.Bl -enum
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.It
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The referencing object itself (or the object from which the call to
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.Fn dlsym
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is made), if that object was linked using the
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.Fl Bsymbolic
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option to
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.Xr ld 1 .
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.It
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All objects loaded at program start-up.
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.It
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All objects loaded via
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.Fn dlopen
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with the
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.Dv RTLD_GLOBAL
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flag set in the
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.Fa mode
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argument.
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.It
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All objects loaded via
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.Fn dlopen
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which are in needed-object DAGs that also contain the referencing object.
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.El
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.Pp
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If
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.Fn dlsym
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is called with the special
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.Fa handle
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.Dv RTLD_NEXT ,
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then the search for the symbol is limited to the shared objects
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which were loaded after the one issuing the call to
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.Fn dlsym .
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Thus, if the function is called from the main program, all
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the shared libraries are searched.
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If it is called from a shared library, all subsequent shared
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libraries are searched.
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.Dv RTLD_NEXT
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is useful for implementing wrappers around library functions.
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For example, a wrapper function
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.Fn getpid
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could access the
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.Dq real
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.Fn getpid
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with
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.Li dlsym(RTLD_NEXT, \&"getpid\&") .
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(Actually, the
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.Fn dlfunc
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interface, below, should be used, since
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.Fn getpid
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is a function and not a data object.)
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.Pp
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If
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.Fn dlsym
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is called with the special
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.Fa handle
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.Dv RTLD_SELF ,
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then the search for the symbol is limited to the shared object
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issuing the call to
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.Fn dlsym
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and those shared objects which were loaded after it.
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.Pp
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The
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.Fn dlsym
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function
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returns a null pointer if the symbol cannot be found, and sets an error
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condition which may be queried with
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.Fn dlerror .
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.Pp
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The
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.Fn dlfunc
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function
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implements all of the behavior of
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.Fn dlsym ,
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but has a return type which can be cast to a function pointer without
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triggering compiler diagnostics.
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(The
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.Fn dlsym
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function
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returns a data pointer; in the C standard, conversions between
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data and function pointer types are undefined.
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Some compilers and
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.Xr lint 1
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utilities warn about such casts.)
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The precise return type of
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.Fn dlfunc
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is unspecified; applications must cast it to an appropriate function pointer
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type.
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.Pp
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The
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.Fn dlerror
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function
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returns a null-terminated character string describing the last error that
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occurred during a call to
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.Fn dlopen ,
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.Fn dladdr ,
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.Fn dlinfo ,
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.Fn dlsym ,
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.Fn dlfunc ,
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or
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.Fn dlclose .
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If no such error has occurred,
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.Fn dlerror
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returns a null pointer.
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At each call to
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.Fn dlerror ,
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the error indication is reset.
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Thus in the case of two calls
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to
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.Fn dlerror ,
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where the second call follows the first immediately, the second call
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will always return a null pointer.
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.Pp
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The
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.Fn dlclose
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function
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deletes a reference to the shared object referenced by
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.Fa handle .
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If the reference count drops to 0, the object is removed from the
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address space, and
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.Fa handle
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is rendered invalid.
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Just before removing a shared object in this way, the dynamic linker
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calls the object's
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.Fn _fini
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function, if such a function is defined by the object.
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If
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.Fn dlclose
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is successful, it returns a value of 0.
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Otherwise it returns -1, and sets an error condition that can be
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interrogated with
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.Fn dlerror .
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.Pp
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The object-intrinsic functions
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.Fn _init
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and
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.Fn _fini
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are called with no arguments, and are not expected to return values.
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.Sh NOTES
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ELF executables need to be linked
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using the
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.Fl export-dynamic
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option to
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.Xr ld 1
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for symbols defined in the executable to become visible to
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.Fn dlsym .
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.Pp
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Other ELF platforms require linking with
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.Lb libdl
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to provide
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.Fn dlopen
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and other functions.
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.Fx
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does not require linking with the library, but supports it for compatibility.
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.Pp
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In previous implementations, it was necessary to prepend an underscore
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to all external symbols in order to gain symbol
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compatibility with object code compiled from the C language.
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This is
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still the case when using the (obsolete)
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.Fl aout
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option to the C language compiler.
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.Sh ERRORS
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The
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.Fn dlopen ,
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.Fn fdlopen ,
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.Fn dlsym ,
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and
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.Fn dlfunc
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functions
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return a null pointer in the event of errors.
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The
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.Fn dlclose
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function
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returns 0 on success, or -1 if an error occurred.
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Whenever an error has been detected, a message detailing it can be
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retrieved via a call to
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.Fn dlerror .
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.Sh SEE ALSO
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.Xr ld 1 ,
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.Xr rtld 1 ,
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.Xr dladdr 3 ,
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.Xr dlinfo 3 ,
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.Xr link 5
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