409 lines
11 KiB
Groff
409 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 February 14, 2015
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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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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
|