.\"	$OpenBSD: dlfcn.3,v 1.14 2002/12/30 13:51:59 millert Exp $
.\"	$NetBSD: dlfcn.3,v 1.3 1996/01/09 19:43:34 pk Exp $
.\"
.\" Copyright (c) 1995 Paul Kranenburg
.\" All rights reserved.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
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.\" 1. Redistributions of source code must retain the above copyright
.\"    notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\"    notice, this list of conditions and the following disclaimer in the
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.\"    must display the following acknowledgement:
.\"      This product includes software developed by Paul Kranenburg.
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.\"    derived from this software without specific prior written permission
.\"
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.Dd September 30, 1995
.Dt DLFCN 3
.Os
.Sh NAME
.Nm dlopen ,
.Nm dlclose ,
.Nm dlsym ,
.Nm dlctl ,
.Nm dlerror
.Nd dynamic link interface
.Sh SYNOPSIS
.Fd #include <dlfcn.h>
.Ft "void *"
.Fn dlopen "const char *path" "int mode"
.Ft "int"
.Fn dlclose "void *handle"
.Ft "void *"
.Fn dlsym "void *handle" "const char *symbol"
.Ft "int"
.Fn dlctl "void *handle" "int cmd" "void *data"
.Ft "const char *"
.Fn dlerror "void"
.Sh DESCRIPTION
These functions provide an interface to the run-time linker
.Xr ld.so 1 .
They allow new shared objects to be loaded into a process's address space
under program control.
.Pp
The
.Fn dlopen
function takes a name of a shared object as its first argument.
The shared object is mapped into the address space, relocated, and its external
references are resolved in the same way as is done with the implicitly loaded
shared libraries at program startup.
.Pp
The
.Fa path
argument can either be an absolute pathname or it can be of the form
.Sm off
.Do Xo lib Ao name Ac .so
.Op .xx Op .yy Xc
.Dc
.Sm on
in which case the same library search rules apply that are used for
.Dq intrinsic
shared library searches.
The second argument currently has no effect, but should be set to
.Dv DL_LAZY
for future compatibility.
.Pp
.Fn dlopen
returns a handle to be used in calls to
.Fn dlclose ,
.Fn dlsym ,
and
.Fn dlctl .
If the named shared object has already been loaded by a previous call to
.Fn dlopen
.Pq and not yet unloaded by Fn dlclose ,
a handle referring to the resident copy is returned.
.Pp
.Fn dlclose
unlinks and removes the object referred to by
.Fa handle
from the process address space.
If multiple calls to
.Fn dlopen
have been done on this object
.Po or the object was once loaded at startup time
.Pc
the object is removed when its reference count drops to zero.
.Pp
.Fn dlsym
looks for a definition of
.Fa symbol
in the shared object designated by
.Fa handle .
The symbol's address is returned.
If the symbol cannot be resolved,
.Dv NULL
is returned.
.Pp
.Fn dlctl
provides an interface similar to
.Xr ioctl 2
to control several aspects of the run-time linker's operation.
This interface is
.Ud .
.Pp
.Fn dlerror
returns a character string representing the most recent error that has
occurred while processing one of the other functions described here.
.Sh SEE ALSO
.Xr ld 1 ,
.Xr ld.so 1 ,
.Xr link 5
.Sh HISTORY
Some of the
.Nm dl*
functions first appeared in SunOS 4.
.Sh BUGS
An error that occurs while processing a
.Fn dlopen
request results in the termination of the program.