[thirdparty/man-pages.git] / man3 / dlsym.3

.\" Copyright 1995 Yggdrasil Computing, Incorporated.
.\" and Copyright 2003, 2015 Michael Kerrisk <mtk.manpages@gmail.com>
.\"
.\" %%%LICENSE_START(GPLv2+_DOC_FULL)
.\" This is free documentation; you can redistribute it and/or
.\" modify it under the terms of the GNU General Public License as
.\" published by the Free Software Foundation; either version 2 of
.\" the License, or (at your option) any later version.
.\"
.\" The GNU General Public License's references to "object code"
.\" and "executables" are to be interpreted as the output of any
.\" document formatting or typesetting system, including
.\" intermediate and printed output.
.\"
.\" This manual is distributed in the hope that it will be useful,
.\" but WITHOUT ANY WARRANTY; without even the implied warranty of
.\" MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
.\" GNU General Public License for more details.
.\"
.\" You should have received a copy of the GNU General Public
.\" License along with this manual; if not, see
.\" <http://www.gnu.org/licenses/>.
.\" %%%LICENSE_END
.\"
.TH DLSYM 3 2017-09-15 "Linux" "Linux Programmer's Manual"
.SH NAME
dlsym, dlvsym \- obtain address of a symbol in a shared object or executable
.SH SYNOPSIS
.B #include <dlfcn.h>
.PP
.BI "void *dlsym(void *" handle ", const char *" symbol );
.PP
.B #define _GNU_SOURCE
.br
.B #include <dlfcn.h>
.PP
.BI "void *dlvsym(void *" handle ", char *" symbol ", char *" version );
.PP
Link with \fI\-ldl\fP.
.SH DESCRIPTION
The function
.BR dlsym ()
takes a "handle" of a dynamic loaded shared object returned by
.BR dlopen (3)
along with a null-terminated symbol name,
and returns the address where that symbol is
loaded into memory.
If the symbol is not found, in the specified
object or any of the shared objects that were automatically loaded by
.BR dlopen (3)
when that object was loaded,
.BR dlsym ()
returns NULL.
(The search performed by
.BR dlsym ()
is breadth first through the dependency tree of these shared objects.)
.PP
Since the value of the symbol could actually be NULL (so that a
NULL return from
.BR dlsym ()
need not indicate an error), the correct way to test for an error
is to call
.BR dlerror (3)
to clear any old error conditions, then call
.BR dlsym (),
and then call
.BR dlerror (3)
again, saving its return value into a variable, and check whether
this saved value is not NULL.
.PP
There are two special pseudo-handles that may be specified in
.IR handle :
.TP
.B RTLD_DEFAULT
Find the first occurrence of the desired symbol
using the default shared object search order.
The search will include global symbols in the executable
and its dependencies,
as well as symbols in shared objects that were dynamically loaded with the
.BR RTLD_GLOBAL
flag.
.TP
.BR RTLD_NEXT
Find the next occurrence of the desired symbol in the search order
after the current object.
This allows one to provide a wrapper
around a function in another shared object, so that, for example,
the definition of a function in a preloaded shared object
(see
.B LD_PRELOAD
in
.BR ld.so (8))
can find and invoke the "real" function provided in another shared object
(or for that matter, the "next" definition of the function in cases
where there are multiple layers of preloading).
.PP
The
.B _GNU_SOURCE
feature test macro must be defined in order to obtain the
definitions of
.B RTLD_DEFAULT
and
.B RTLD_NEXT
from
.IR <dlfcn.h> .
.PP
.PP
The function
.BR dlvsym ()
does the same as
.BR dlsym ()
but takes a version string as an additional argument.
.SH RETURN VALUE
On success,
these functions return the address associated with
.IR symbol .
On failure, they return NULL;
the cause of the error can be diagnosed using
.BR dlerror (3).
.SH VERSIONS
.BR dlsym ()
is present in glibc 2.0 and later.
.BR dlvsym ()
first appeared in glibc 2.1.
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.TS
allbox;
lb lb lb
l l l.
Interface	Attribute	Value
T{
.BR dlsym (),
.BR dlvsym ()
T}	Thread safety	MT-Safe
.TE
.SH CONFORMING TO
POSIX.1-2001 describes
.BR dlsym ().
The
.BR dlvsym ()
function is a GNU extension.
.SH NOTES
.SS History
The
.BR dlsym ()
function is part of the dlopen API, derived from SunOS.
That system does not have
.BR dlvsym ().
.SH EXAMPLE
See
.BR dlopen (3).
.SH SEE ALSO
.BR dl_iterate_phdr (3),
.BR dladdr (3),
.BR dlerror (3),
.BR dlinfo (3),
.BR dlopen (3),
.BR ld.so (8)
Commit	Line	Data
15e1b31b	1	.\" Copyright 1995 Yggdrasil Computing, Incorporated.
616c2730	2	.\" and Copyright 2003, 2015 Michael Kerrisk <mtk.manpages@gmail.com>
15e1b31b MK	3	.\"
	4	.\" %%%LICENSE_START(GPLv2+_DOC_FULL)
	5	.\" This is free documentation; you can redistribute it and/or
	6	.\" modify it under the terms of the GNU General Public License as
	7	.\" published by the Free Software Foundation; either version 2 of
	8	.\" the License, or (at your option) any later version.
	9	.\"
	10	.\" The GNU General Public License's references to "object code"
	11	.\" and "executables" are to be interpreted as the output of any
	12	.\" document formatting or typesetting system, including
	13	.\" intermediate and printed output.
	14	.\"
	15	.\" This manual is distributed in the hope that it will be useful,
	16	.\" but WITHOUT ANY WARRANTY; without even the implied warranty of
	17	.\" MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
	18	.\" GNU General Public License for more details.
	19	.\"
	20	.\" You should have received a copy of the GNU General Public
	21	.\" License along with this manual; if not, see
	22	.\" <http://www.gnu.org/licenses/>.
	23	.\" %%%LICENSE_END
	24	.\"
4b8c67d9	25	.TH DLSYM 3 2017-09-15 "Linux" "Linux Programmer's Manual"
15e1b31b MK	26	.SH NAME
	27	dlsym, dlvsym \- obtain address of a symbol in a shared object or executable
	28	.SH SYNOPSIS
	29	.B #include <dlfcn.h>
68e4db0a	30	.PP
15e1b31b	31	.BI "void dlsym(void " handle ", const char *" symbol );
dbfe9c70	32	.PP
15e1b31b MK	33	.B #define _GNU_SOURCE
	34	.br
	35	.B #include <dlfcn.h>
68e4db0a	36	.PP
15e1b31b	37	.BI "void dlvsym(void " handle ", char " symbol ", char " version );
68e4db0a	38	.PP
15e1b31b MK	39	Link with \fI\-ldl\fP.
	40	.SH DESCRIPTION
	41	The function
	42	.BR dlsym ()
	43	takes a "handle" of a dynamic loaded shared object returned by
	44	.BR dlopen (3)
	45	along with a null-terminated symbol name,
	46	and returns the address where that symbol is
	47	loaded into memory.
	48	If the symbol is not found, in the specified
	49	object or any of the shared objects that were automatically loaded by
	50	.BR dlopen (3)
	51	when that object was loaded,
	52	.BR dlsym ()
	53	returns NULL.
	54	(The search performed by
	55	.BR dlsym ()
	56	is breadth first through the dependency tree of these shared objects.)
847e0d88	57	.PP
15e1b31b MK	58	Since the value of the symbol could actually be NULL (so that a
	59	NULL return from
	60	.BR dlsym ()
	61	need not indicate an error), the correct way to test for an error
	62	is to call
	63	.BR dlerror (3)
	64	to clear any old error conditions, then call
	65	.BR dlsym (),
	66	and then call
	67	.BR dlerror (3)
	68	again, saving its return value into a variable, and check whether
	69	this saved value is not NULL.
	70	.PP
	71	There are two special pseudo-handles that may be specified in
	72	.IR handle :
	73	.TP
	74	.B RTLD_DEFAULT
	75	Find the first occurrence of the desired symbol
	76	using the default shared object search order.
	77	The search will include global symbols in the executable
	78	and its dependencies,
	79	as well as symbols in shared objects that were dynamically loaded with the
	80	.BR RTLD_GLOBAL
	81	flag.
	82	.TP
	83	.BR RTLD_NEXT
	84	Find the next occurrence of the desired symbol in the search order
	85	after the current object.
	86	This allows one to provide a wrapper
	87	around a function in another shared object, so that, for example,
	88	the definition of a function in a preloaded shared object
	89	(see
	90	.B LD_PRELOAD
	91	in
	92	.BR ld.so (8))
	93	can find and invoke the "real" function provided in another shared object
	94	(or for that matter, the "next" definition of the function in cases
	95	where there are multiple layers of preloading).
f9ce70d2 MK	96	.PP
	97	The
	98	.B _GNU_SOURCE
	99	feature test macro must be defined in order to obtain the
	100	definitions of
	101	.B RTLD_DEFAULT
	102	and
	103	.B RTLD_NEXT
	104	from
	105	.IR <dlfcn.h> .
847e0d88	106	.PP
4590829a	107	.PP
15e1b31b MK	108	The function
	109	.BR dlvsym ()
	110	does the same as
	111	.BR dlsym ()
	112	but takes a version string as an additional argument.
	113	.SH RETURN VALUE
	114	On success,
	115	these functions return the address associated with
	116	.IR symbol .
	117	On failure, they return NULL;
	118	the cause of the error can be diagnosed using
	119	.BR dlerror (3).
	120	.SH VERSIONS
	121	.BR dlsym ()
	122	is present in glibc 2.0 and later.
	123	.BR dlvsym ()
	124	first appeared in glibc 2.1.
39720f03 MK	125	.SH ATTRIBUTES
	126	For an explanation of the terms used in this section, see
	127	.BR attributes (7).
	128	.TS
	129	allbox;
	130	lb lb lb
	131	l l l.
	132	Interface Attribute Value
	133	T{
	134	.BR dlsym (),
	135	.BR dlvsym ()
	136	T} Thread safety MT-Safe
	137	.TE
15e1b31b MK	138	.SH CONFORMING TO
	139	POSIX.1-2001 describes
	140	.BR dlsym ().
	141	The
	142	.BR dlvsym ()
	143	function is a GNU extension.
	144	.SH NOTES
	145	.SS History
	146	The
	147	.BR dlsym ()
	148	function is part of the dlopen API, derived from SunOS.
	149	That system does not have
	150	.BR dlvsym ().
	151	.SH EXAMPLE
	152	See
	153	.BR dlopen (3).
	154	.SH SEE ALSO
	155	.BR dl_iterate_phdr (3),
	156	.BR dladdr (3),
936b64f6	157	.BR dlerror (3),
15e1b31b MK	158	.BR dlinfo (3),
	159	.BR dlopen (3),
	160	.BR ld.so (8)