dlopen.3: Note that symbol use might keep a dlclose'd object in memory

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

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.\" Modified by David A. Wheeler <dwheeler@dwheeler.com> 2000-11-28.
.\" Applied patch by Terran Melconian, aeb, 2001-12-14.
.\" Modified by Hacksaw <hacksaw@hacksaw.org> 2003-03-13.
.\" Modified by Matt Domsch, 2003-04-09: _init and _fini obsolete
.\" Modified by Michael Kerrisk <mtk.manpages@gmail.com> 2003-05-16.
.\" Modified by Walter Harms: dladdr, dlvsym
.\" Modified by Petr Baudis <pasky@suse.cz>, 2008-12-04: dladdr caveat
.\"
.TH DLOPEN 3 2017-09-15 "Linux" "Linux Programmer's Manual"
.SH NAME
dlclose, dlopen, dlmopen \-
open and close a shared object
.SH SYNOPSIS
.B #include <dlfcn.h>
.PP
.BI "void *dlopen(const char *" filename ", int " flags );
.PP
.BI "int dlclose(void *" handle );
.PP
.B #define _GNU_SOURCE
.br
.B #include <dlfcn.h>
.PP
.BI "void *dlmopen (Lmid_t " lmid ", const char *" filename ", int " flags );
.PP
Link with \fI\-ldl\fP.
.SH DESCRIPTION
.SS dlopen()
The function
.BR dlopen ()
loads the dynamic shared object (shared library)
file named by the null-terminated
string
.I filename
and returns an opaque "handle" for the loaded object.
This handle is employed with other functions in the dlopen API, such as
.BR dlsym (3),
.BR dladdr (3),
.BR dlinfo (3),
and
.BR dlclose ().
.PP
If
.I filename
.\" FIXME On Solaris, when handle is NULL, we seem to get back
.\" a handle for (something like) the root of the namespace.
.\" The point here is that if we do a dlmopen(LM_ID_NEWLM), then
.\" the filename==NULL case returns a different handle than
.\" in the initial namespace. But, on glibc, the same handle is
.\" returned. This is probably a bug in glibc.
.\"
is NULL, then the returned handle is for the main program.
If
.I filename
contains a slash ("/"), then it is interpreted as a (relative
or absolute) pathname.
Otherwise, the dynamic linker searches for the object as follows
(see
.BR ld.so (8)
for further details):
.IP o 4
(ELF only) If the executable file for the calling program
contains a DT_RPATH tag, and does not contain a DT_RUNPATH tag,
then the directories listed in the DT_RPATH tag are searched.
.IP o
If, at the time that the program was started, the environment variable
.B LD_LIBRARY_PATH
was defined to contain a colon-separated list of directories,
then these are searched.
(As a security measure, this variable is ignored for set-user-ID and
set-group-ID programs.)
.IP o
(ELF only) If the executable file for the calling program
contains a DT_RUNPATH tag, then the directories listed in that tag
are searched.
.IP o
The cache file
.I /etc/ld.so.cache
(maintained by
.BR ldconfig (8))
is checked to see whether it contains an entry for
.IR filename .
.IP o
The directories
.I /lib
and
.I /usr/lib
are searched (in that order).
.PP
If the object specified by
.I filename
has dependencies on other shared objects,
then these are also automatically loaded by the dynamic linker
using the same rules.
(This process may occur recursively,
if those objects in turn have dependencies, and so on.)
.PP
One of the following two values must be included in
.IR flags :
.TP
.B RTLD_LAZY
Perform lazy binding.
Resolve symbols only as the code that references them is executed.
If the symbol is never referenced, then it is never resolved.
(Lazy binding is performed only for function references;
references to variables are always immediately bound when
the shared object is loaded.)
Since glibc 2.1.1,
.\" commit 12b5b6b7f78ea111e89bbf638294a5413c791072
this flag is overridden by the effect of the
.B LD_BIND_NOW
environment variable.
.TP
.B RTLD_NOW
If this value is specified, or the environment variable
.B LD_BIND_NOW
is set to a nonempty string,
all undefined symbols in the shared object are resolved before
.BR dlopen ()
returns.
If this cannot be done, an error is returned.
.PP
Zero or more of the following values may also be ORed in
.IR flags :
.TP
.B RTLD_GLOBAL
The symbols defined by this shared object will be
made available for symbol resolution of subsequently loaded shared objects.
.TP
.B RTLD_LOCAL
This is the converse of
.BR RTLD_GLOBAL ,
and the default if neither flag is specified.
Symbols defined in this shared object are not made available to resolve
references in subsequently loaded shared objects.
.TP
.BR RTLD_NODELETE " (since glibc 2.2)"
Do not unload the shared object during
.BR dlclose ().
Consequently, the object's static variables are not reinitialized
if the object is reloaded with
.BR dlopen ()
at a later time.
.TP
.BR RTLD_NOLOAD " (since glibc 2.2)"
Don't load the shared object.
This can be used to test if the object is already resident
.RB ( dlopen ()
returns NULL if it is not, or the object's handle if it is resident).
This flag can also be used to promote the flags on a shared object
that is already loaded.
For example, a shared object that was previously loaded with
.B RTLD_LOCAL
can be reopened with
.BR RTLD_NOLOAD\ |\ RTLD_GLOBAL .
.\"
.TP
.BR RTLD_DEEPBIND " (since glibc 2.3.4)"
.\" Inimitably described by UD in
.\" http://sources.redhat.com/ml/libc-hacker/2004-09/msg00083.html.
Place the lookup scope of the symbols in this
shared object ahead of the global scope.
This means that a self-contained object will use
its own symbols in preference to global symbols with the same name
contained in objects that have already been loaded.
.PP
If
.I filename
is NULL, then the returned handle is for the main program.
When given to
.BR dlsym (),
this handle causes a search for a symbol in the main program,
followed by all shared objects loaded at program startup,
and then all shared objects loaded by
.BR dlopen ()
with the flag
.BR RTLD_GLOBAL .
.PP
External references in the shared object are resolved using the
shared objects in that object's dependency list and any other
objects previously opened with the
.B RTLD_GLOBAL
flag.
If the executable was linked with the flag "\-rdynamic"
(or, synonymously, "\-\-export\-dynamic"),
then the global symbols in the executable will also be used
to resolve references in a dynamically loaded shared object.
.PP
If the same shared object is opened again with
.BR dlopen (),
the same object handle is returned.
The dynamic linker maintains reference
counts for object handles, so a dynamically loaded shared object is not
deallocated until
.BR dlclose ()
has been called on it as many times as
.BR dlopen ()
has succeeded on it.
Constructors (see below) are called only when the library is actually loaded
into memory (i.e., when the reference count increases to 1).
However, a subsequent
.BR dlopen ()
call that loads the same shared object with
.B RTLD_NOW
may force symbol resolution for a shared object earlier loaded with
.BR RTLD_LAZY .
.PP
If
.BR dlopen ()
fails for any reason, it returns NULL.
.\"
.SS dlmopen()
This function performs the same task as
.BR dlopen ()\(emthe
.I filename
and
.I flags
arguments, as well as the return value, are the same,
except for the differences noted below.
.PP
The
.BR dlmopen ()
function differs from
.BR dlopen ()
primarily in that it accepts an additional argument,
.IR lmid ,
that specifies the link-map list (also referred to as a
.IR namespace )
in which the shared object should be loaded.
(By comparison,
.BR dlopen ()
adds the dynamically loaded shared object to the same namespace as
the shared object from which the
.BR dlopen ()
call is made.)
The
.I Lmid_t
type is an opaque handle that refers to a namespace.
.PP
The
.I lmid
argument is either the ID of an existing namespace
.\" FIXME: Is using dlinfo() RTLD_DI_LMID the right technique?
(which can be obtained using the
.BR dlinfo (3)
.B RTLD_DI_LMID
request) or one of the following special values:
.TP
.B LM_ID_BASE
Load the shared object in the initial namespace
(i.e., the application's namespace).
.TP
.B LM_ID_NEWLM
Create a new namespace and load the shared object in that namespace.
The object must have been correctly linked
to reference all of the other shared objects that it requires,
since the new namespace is initially empty.
.PP
If
.I filename
is NULL, then the only permitted value for
.I lmid
is
.BR LM_ID_BASE .
.SS dlclose()
The function
.BR dlclose ()
decrements the reference count on the
dynamically loaded shared object referred to by
.IR handle .
.PP
If the object's reference count drops to zero
and no symbols in this object are required by other objects,
then the object is unloaded
after first calling any destructors defined for the object.
(Symbols in this object might be required in another object
because this object was opened with the
.BR RTLD_GLOBAL
flag and one of its symbols satisfied a relocation in another object.)
.PP
All shared objects that were automatically loaded when
.BR dlopen ()
was invoked on the object referred to by
.I handle
are recursively closed in the same manner.
.PP
A successful return from
.BR dlclose ()
does not guarantee that the symbols associated with
.I handle
are removed from the caller's address space.
In addition to references resulting from explicit
.BR dlopen ()
calls, a shared object may have been implicitly loaded
(and reference counted) because of dependencies in other shared objects.
Only when all references have been released can the shared object
be removed from the address space.
.SH RETURN VALUE
On success,
.BR dlopen ()
and
.BR dlmopen ()
return a non-NULL handle for the loaded library.
On error
(file could not be found, was not readable, had the wrong format,
or caused errors during loading),
these functions return NULL.
.PP
On success,
.BR dlclose ()
returns 0; on error, it returns a nonzero value.
.PP
Errors from these functions can be diagnosed using
.BR dlerror (3).
.SH VERSIONS
.BR dlopen ()
and
.BR dlclose ()
are present in glibc 2.0 and later.
.BR dlmopen ()
first appeared in glibc 2.3.4.
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.TS
allbox;
lbw30 lb lb
l l l.
Interface       Attribute       Value
T{
.BR dlopen (),
.BR dlmopen (),
.BR dlclose ()
T}      Thread safety   MT-Safe
.TE
.SH CONFORMING TO
POSIX.1-2001 describes
.BR dlclose ()
and
.BR dlopen ().
The
.BR dlmopen ()
function is a GNU extension.
.PP
The
.BR RTLD_NOLOAD ,
.BR RTLD_NODELETE ,
and
.BR RTLD_DEEPBIND
flags are GNU extensions;
the first two of these flags are also present on Solaris.
.SH NOTES
.SS dlmopen() and namespaces
A link-map list defines an isolated namespace for the
resolution of symbols by the dynamic linker.
Within a namespace,
dependent shared objects are implicitly loaded according to the usual rules,
and symbol references are likewise resolved according to the usual rules,
but such resolution is confined to the definitions provided by the
objects that have been (explicitly and implicitly) loaded into the namespace.
.PP
The
.BR dlmopen ()
function permits object-load isolation\(emthe ability
to load a shared object in a new namespace without
exposing the rest of the application to the symbols
made available by the new object.
Note that the use of the
.B RTLD_LOCAL
flag is not sufficient for this purpose,
since it prevents a shared object's symbols from being available to
.I any
other shared object.
In some cases,
we may want to make the symbols provided by a dynamically
loaded shared object available to (a subset of) other shared objects
without exposing those symbols to the entire application.
This can be achieved by using a separate namespace and the
.B RTLD_GLOBAL
flag.
.PP
The
.BR dlmopen ()
function also can be used to provide better isolation than the
.BR RTLD_LOCAL
flag.
In particular, shared objects loaded with
.BR RTLD_LOCAL
may be promoted to
.BR RTLD_GLOBAL
if they are dependencies of another shared object loaded with
.BR RTLD_GLOBAL .
Thus,
.BR RTLD_LOCAL
is insufficient to isolate a loaded shared object except in the (uncommon)
case where one has explicit control over all shared object dependencies.
.PP
Possible uses of
.BR dlmopen ()
are plugins where the author of the plugin-loading framework
can't trust the plugin authors and does not wish
any undefined symbols from the plugin framework to be resolved to plugin
symbols.
Another use is to load the same object more than once.
Without the use of
.BR dlmopen (),
this would require the creation of distinct copies of the shared object file.
Using
.BR dlmopen (),
this can be achieved by loading the same shared object file into
different namespaces.
.PP
The glibc implementation supports a maximum of
.\" DL_NNS
16 namespaces.
.\"
.SS Initialization and finalization functions
Shared objects may export functions using the
.B __attribute__((constructor))
and
.B __attribute__((destructor))
function attributes.
Constructor functions are executed before
.BR dlopen ()
returns, and destructor functions are executed before
.BR dlclose ()
returns.
A shared object may export multiple constructors and destructors,
and priorities can be associated with each function
to determine the order in which they are executed.
See the
.BR gcc
info pages (under "Function attributes")
.\" info gcc "C Extensions" "Function attributes"
for further information.
.PP
An older method of (partially) achieving the same result is via the use of
two special symbols recognized by the linker:
.B _init
and
.BR _fini .
If a dynamically loaded shared object exports a routine named
.BR _init (),
then that code is executed after loading a shared object, before
.BR dlopen ()
returns.
If the shared object exports a routine named
.BR _fini (),
then that routine is called just before the object is unloaded.
In this case, one must avoid linking against the system startup files,
which contain default versions of these files;
this can be done by using the
.BR gcc (1)
.I \-nostartfiles
command-line option.
.PP
Use of
.B _init
and
.BR _fini
is now deprecated in favor of the aforementioned
constructors and destructors,
which among other advantages,
permit multiple initialization and finalization functions to be defined.
.\"
.\" Using these routines, or the gcc
.\" .B \-nostartfiles
.\" or
.\" .B \-nostdlib
.\" options, is not recommended.
.\" Their use may result in undesired behavior,
.\" since the constructor/destructor routines will not be executed
.\" (unless special measures are taken).
.\" .\" void _init(void) __attribute__((constructor));
.\" .\" void _fini(void) __attribute__((destructor));
.\"
.PP
Since glibc 2.2.3,
.BR atexit (3)
can be used to register an exit handler that is automatically
called when a shared object is unloaded.
.SS History
These functions are part of the dlopen API, derived from SunOS.
.SH BUGS
As at glibc 2.24, specifying the
.BR RTLD_GLOBAL
flag when calling
.BR dlmopen ()
.\" dlerror(): "invalid mode"
generates an error.
Furthermore, specifying
.BR RTLD_GLOBAL
when calling
.BR dlopen ()
results in a program crash
.RB ( SIGSEGV )
if the call is made from any object loaded in a
namespace other than the initial namespace.
.SH EXAMPLE
The program below loads the (glibc) math library,
looks up the address of the
.BR cos (3)
function, and prints the cosine of 2.0.
The following is an example of building and running the program:
.PP
.in +4n
.EX
$ \fBcc dlopen_demo.c \-ldl\fP
$ \fB./a.out\fP
\-0.416147
.EE
.in
.SS Program source
\&
.EX
#include <stdio.h>
#include <stdlib.h>
#include <dlfcn.h>
#include <gnu/lib-names.h>  /* Defines LIBM_SO (which will be a
                               string such as "libm.so.6") */
int
main(void)
{
    void *handle;
    double (*cosine)(double);
    char *error;

    handle = dlopen(LIBM_SO, RTLD_LAZY);
    if (!handle) {
        fprintf(stderr, "%s\en", dlerror());
        exit(EXIT_FAILURE);
    }

    dlerror();    /* Clear any existing error */

    cosine = (double (*)(double)) dlsym(handle, "cos");

    /* According to the ISO C standard, casting between function
       pointers and 'void *', as done above, produces undefined results.
       POSIX.1-2003 and POSIX.1-2008 accepted this state of affairs and
       proposed the following workaround:

           *(void **) (&cosine) = dlsym(handle, "cos");

       This (clumsy) cast conforms with the ISO C standard and will
       avoid any compiler warnings.

       The 2013 Technical Corrigendum to POSIX.1-2008 (a.k.a.
       POSIX.1-2013) improved matters by requiring that conforming
       implementations support casting 'void *' to a function pointer.
       Nevertheless, some compilers (e.g., gcc with the '-pedantic'
       option) may complain about the cast used in this program. */
.\" http://pubs.opengroup.org/onlinepubs/009695399/functions/dlsym.html#tag_03_112_08
.\" http://pubs.opengroup.org/onlinepubs/9699919799/functions/dlsym.html#tag_16_96_07
.\" http://austingroupbugs.net/view.php?id=74

    error = dlerror();
    if (error != NULL) {
        fprintf(stderr, "%s\en", error);
        exit(EXIT_FAILURE);
    }

    printf("%f\en", (*cosine)(2.0));
    dlclose(handle);
    exit(EXIT_SUCCESS);
}
.EE
.SH SEE ALSO
.BR ld (1),
.BR ldd (1),
.BR pldd (1),
.BR dl_iterate_phdr (3),
.BR dladdr (3),
.BR dlerror (3),
.BR dlinfo (3),
.BR dlsym (3),
.BR rtld-audit (7),
.BR ld.so (8),
.BR ldconfig (8)
.PP
gcc info pages, ld info pages