358 lines
10 KiB
Groff
358 lines
10 KiB
Groff
.\" $NetBSD: dlfcn.3,v 1.33 2011/06/25 12:44:37 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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.\"
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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 June 25, 2011
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.Dt DLFCN 3
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.Os
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.Sh NAME
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.Nm dlopen ,
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.Nm dlclose ,
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.Nm dlsym ,
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.Nm dlvsym ,
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.Nm dladdr ,
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.Nm dlctl ,
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.Nm dlerror
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.Nd dynamic link interface
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.Sh LIBRARY
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(These functions are not in a library.
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They are included in every
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dynamically linked program automatically.)
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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 "int"
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.Fn dlclose "void *handle"
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.Ft "void *"
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.Fn dlsym "void * restrict handle" "const char * restrict symbol"
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.Ft "void *"
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.Fn dlvsym "void * restrict handle" "const char * restrict symbol" "const char *version"
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.Ft "int"
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.Fn dladdr "void * restrict addr" "Dl_info * restrict dli"
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.Ft "int"
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.Fn dlctl "void *handle" "int cmd" "void *data"
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.Ft "char *"
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.Fn dlerror "void"
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.Sh DESCRIPTION
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These functions provide an interface to the run-time linker
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.Xr ld.so 1 .
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They allow new shared objects to be loaded into the process' address space
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under program control.
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.Pp
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The
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.Fn dlopen
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function takes the name of a shared object as the first argument.
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The
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.Fa path
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argument can be specified as either an absolute pathname to a shared object
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or just the name of the shared object itself.
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When an absolute pathname is specified,
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only the path provided will be searched.
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When just a shared object name is specified, the same search rules apply that
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are used for
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.Dq intrinsic
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shared object searches.
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.Po
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see
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.Xr ld.elf_so 1
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.Pc
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.Pp
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Shared libraries take the following form:
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.Do lib Ns Ao name Ac Ns .so Ns Oo .xx Ns Oo .yy Oc Oc Dc .
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.Pp
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The shared object is mapped into the address space, relocated, and
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its external references are resolved in the same way as is done
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with the implicitly loaded shared libraries at program startup.
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.Pp
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If the first argument is
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.Dv NULL ,
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.Fn dlopen
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returns a
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.Fa handle
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on the global symbol object.
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This object
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provides access to all symbols from an ordered set of objects consisting
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of the original program image and any dependencies loaded during startup.
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.Pp
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The
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.Fa mode
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parameter specifies symbol resolution time and symbol visibility.
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One of the following values may be used to specify symbol resolution time:
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.Bl -tag -width "RTLD_GLOBALXX" -offset indent
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.It Dv RTLD_NOW
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Symbols are resolved immediately.
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.It Dv RTLD_LAZY
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Symbols are resolved when they are first referred to.
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This is the default value if resolution time is unspecified.
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.El
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.Pp
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One of the following values may be used to specify symbol visibility:
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.Pp
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.Bl -tag -width "RTLD_GLOBALXX" -offset indent
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.It Dv RTLD_GLOBAL
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The object's symbols and the symbols of its dependencies will be visible to
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other objects.
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.It Dv RTLD_LOCAL
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The object's symbols and the symbols of its dependencies will not be visible to
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other objects.
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This is the default value if visibility is unspecified.
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.El
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.Pp
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To specify both resolution time and visibility, bitwise inclusive OR one of
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each of the above values together.
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If an object was opened with
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.Dv RTLD_LOCAL
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and later opened with
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.Dv RTLD_GLOBAL ,
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then it is promoted to
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.Dv RTLD_GLOBAL .
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.Pp
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Additionally, 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_NODELETEXX" -offset indent
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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 do not load the object and return
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.Dv NULL .
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.El
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.Pp
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.Fn dlopen
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returns a
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.Fa handle
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to be used in calls to
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.Fn dlclose ,
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.Fn dlsym ,
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.Fn dlvsym ,
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and
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.Fn dlctl .
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If the named shared object has already
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been loaded by a previous call to
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.Fn dlopen
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.Pq and not yet unloaded by Fn dlclose ,
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a
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.Fa handle
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referring to the resident copy is returned.
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.Pp
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.Fn dlclose
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unlinks and removes the object referred to by
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.Fa handle
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from the process address space.
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If multiple calls to
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.Fn dlopen
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have been done on this object, or the object was one loaded at startup time,
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or the object is a dependency of another object
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then the object is removed when its reference count drops to zero.
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.Fn dlclose
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returns 0 on success and non-zero on failure.
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.Pp
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.Fn dlsym
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looks for a definition of
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.Fa symbol
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in the shared object designated by
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.Fa handle ,
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and all shared objects that are listed as dependencies.
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The symbol's address is returned.
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If the symbol cannot be resolved,
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.Dv NULL
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is returned.
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.Pp
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.Fn dlsym
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may also be called with special
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.Fa handle
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values.
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.Fn dlsym
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respects symbol visibility as specified by the
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.Fn dlopen
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.Fa mode
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parameter.
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However, the symbols of an object's dependencies are always visible to it.
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All shared objects loaded at program startup are globally visible.
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Only the symbols in the main executable that are referenced by a
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shared object at link time will be visible unless it has been linked
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with the --export-dynamic option where all of its symbols will be
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visible.
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The following special
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.Fa handle
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values may be used with
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.Fn dlsym :
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.Bl -tag -width "RTLD_DEFAULTXX" -offset indent
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.It Dv NULL
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Interpreted as a reference to the executable or shared object
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from which the call is being made.
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Thus an object can reference its own symbols and the symbols of its
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dependencies without calling
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.Fn dlopen .
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.It Dv RTLD_DEFAULT
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All the visible shared objects and the executable will be searched in the order
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they were loaded.
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.It Dv RTLD_NEXT
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The search for
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.Fa symbol
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is limited to the visible shared objects which were loaded after the one
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issuing the call to
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.Fn dlsym .
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Thus, if
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.Fn dlsym
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is called from the main program, all the visible shared libraries are searched.
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If it is called from a shared library, all subsequently visible shared
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libraries are searched.
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.It Dv RTLD_SELF
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The search for
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.Fa symbol
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is limited to the shared object issuing the call to
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.Fn dlsym
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and those shared objects which were loaded after it that are visible.
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.El
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.Pp
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.Fn dlvsym
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does the same as
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.Fn dlsym
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but takes a
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.Fa version
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string as an additional argument.
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Both the
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.Fa symbol
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and the
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.Fa version
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must match in order for the symbol to be resolved.
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.Pp
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.Fn dladdr
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examines all currently mapped shared objects for a symbol whose address --
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as mapped in the process address space -- is closest to but not exceeding
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the value passed in the first argument
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.Fa addr .
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The symbols of a shared object are only eligible if
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.Va addr
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is between the base address of the shared object and the value of the
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symbol
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.Dq _end
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in the same shared object.
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If no object for which this condition holds
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true can be found,
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.Fn dladdr
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will return 0.
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Otherwise, a non-zero value is returned and the
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.Fa dli
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argument will be used to provide information on the selected symbol
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and the shared object it is contained in.
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The
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.Fa dli
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argument points at a caller-provided
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.Va Dl_info
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structure defined as follows:
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.Bd -literal -offset indent
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typedef struct {
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const char *dli_fname; /* File defining the symbol */
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void *dli_fbase; /* Base address */
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const char *dli_sname; /* Symbol name */
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const void *dli_saddr; /* Symbol address */
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} Dl_info;
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.Ed
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.Pp
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The structure members are further described as follows:
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.Bl -tag -width "dli_fnameXX"
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.It Li "dli_fname"
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The pathname of the shared object containing the address
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.Fa addr .
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.It Li "dli_fbase"
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The base address at which this shared object is loaded in the process
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address space.
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This may be zero if the symbol was found in the internally generated
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.Dq copy
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section
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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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which is not associated with a file.
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.It Li "dli_sname"
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points at the nul-terminated name of the selected symbol
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.It Li "dli_saddr"
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is the actual address
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.Pq as it appears in the process address space
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of the symbol.
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.El
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.Pp
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Note: both strings pointed at by
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.Va dli_fname
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and
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.Va dli_sname
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reside in memory private to the run-time linker module and should not
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be modified by the caller.
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.Pp
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In dynamically linked programs, the address of a global function will
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point to its program linkage table entry, rather than to the entry
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point of the function itself.
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This causes most global functions to appear to be defined within the
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main executable, rather than in the shared libraries where the actual
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code resides.
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.Pp
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.Fn dlctl
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provides an interface similar to
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.Xr ioctl 2
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to control several aspects of the run-time linker's operation.
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This interface is
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.Ud
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.Pp
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.Fn dlerror
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returns a character string representing the most recent error that has
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occurred while processing one of the other functions described here.
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If no dynamic linking errors have occurred since the last invocation of
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.Fn dlerror ,
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.Fn dlerror
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returns
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.Dv NULL .
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Thus, invoking
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.Fn dlerror
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a second time, immediately following a prior invocation, will result in
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.Dv NULL
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being returned.
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.Sh ERRORS
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The error
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.Dq Cannot dlopen non-loadable /usr/lib/libpthread.so.1
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is generated when a program
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.Fn dlopen Ns No s
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a module that needs libpthread but isn't linked against it itself.
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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 link 5
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.Sh HISTORY
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Some of the
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.Nm dl*
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functions first appeared in SunOS 4.
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