|
NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | VERSIONS | ATTRIBUTES | CONFORMING TO | NOTES | BUGS | EXAMPLE | SEE ALSO | COLOPHON |
DLOPEN(3) Linux Programmer's Manual DLOPEN(3)
dlclose, dlopen, dlmopen - open and close a shared object
#include <dlfcn.h>
void *dlopen(const char *filename, int flags);
int dlclose(void *handle);
#define _GNU_SOURCE
#include <dlfcn.h>
void *dlmopen (Lmid_t lmid, const char *filename, int flags);
Link with -ldl.
dlopen()
The function dlopen() loads the dynamic shared object (shared
library) file named by the null-terminated string filename and
returns an opaque "handle" for the loaded object. This handle is
employed with other functions in the dlopen API, such as dlsym(3),
dladdr(3), dlinfo(3), and dlclose().
If filename is NULL, then the returned handle is for the main
program. If 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 ld.so(8) for further
details):
o (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.
o If, at the time that the program was started, the environment
variable 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.)
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.
o The cache file /etc/ld.so.cache (maintained by ldconfig(8)) is
checked to see whether it contains an entry for filename.
o The directories /lib and /usr/lib are searched (in that order).
If the object specified by 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.)
One of the following two values must be included in flags:
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, this flag is overridden by the
effect of the LD_BIND_NOW environment variable.
RTLD_NOW
If this value is specified, or the environment variable
LD_BIND_NOW is set to a nonempty string, all undefined symbols
in the shared object are resolved before dlopen() returns. If
this cannot be done, an error is returned.
Zero or more of the following values may also be ORed in flags:
RTLD_GLOBAL
The symbols defined by this shared object will be made
available for symbol resolution of subsequently loaded shared
objects.
RTLD_LOCAL
This is the converse of 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.
RTLD_NODELETE (since glibc 2.2)
Do not unload the shared object during dlclose().
Consequently, the object's static variables are not
reinitialized if the object is reloaded with dlopen() at a
later time.
RTLD_NOLOAD (since glibc 2.2)
Don't load the shared object. This can be used to test if the
object is already resident (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 RTLD_LOCAL can be reopened with
RTLD_NOLOAD | RTLD_GLOBAL.
RTLD_DEEPBIND (since glibc 2.3.4)
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.
If filename is NULL, then the returned handle is for the main
program. When given to 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 dlopen() with
the flag RTLD_GLOBAL.
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 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.
If the same shared object is loaded again with 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 dlclose() has been called on it as many times
as dlopen() has succeeded on it. Any initialization returns (see
below) are called just once. However, a subsequent dlopen() call
that loads the same shared object with RTLD_NOW may force symbol
resolution for a shared object earlier loaded with RTLD_LAZY.
If dlopen() fails for any reason, it returns NULL.
dlmopen()
This function performs the same task as dlopen()—the filename and
flags arguments, as well as the return value, are the same, except
for the differences noted below.
The dlmopen() function differs from dlopen() primarily in that it
accepts an additional argument, lmid, that specifies the link-map
list (also referred to as a namespace) in which the shared object
should be loaded. (By comparison, dlopen() adds the dynamically
loaded shared object to the same namespace as the shared object from
which the dlopen() call is made.) The Lmid_t type is an opaque
handle that refers to a namespace.
The lmid argument is either the ID of an existing namespace (which
can be obtained using the dlinfo(3) RTLD_DI_LMID request) or one of
the following special values:
LM_ID_BASE
Load the shared object in the initial namespace (i.e., the
application's namespace).
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.
If filename is NULL, then the only permitted value for lmid is
LM_ID_BASE.
dlclose()
The function dlclose() decrements the reference count on the
dynamically loaded shared object referred to by handle. If the
reference count drops to zero, then the object is unloaded. All
shared objects that were automatically loaded when dlopen() was
invoked on the object referred to by handle are recursively closed in
the same manner.
A successful return from dlclose() does not guarantee that the
symbols associated with handle are removed from the caller's address
space. In addition to references resulting from explicit 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.
On success, dlopen() and 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.
On success, dlclose() returns 0; on error, it returns a nonzero
value.
Errors from these functions can be diagnosed using dlerror(3).
dlopen() and dlclose() are present in glibc 2.0 and later. dlmopen()
first appeared in glibc 2.3.4.
For an explanation of the terms used in this section, see
attributes(7).
┌───────────────────────────────┬───────────────┬─────────┐
│Interface │ Attribute │ Value │
├───────────────────────────────┼───────────────┼─────────┤
│dlopen(), dlmopen(), dlclose() │ Thread safety │ MT-Safe │
└───────────────────────────────┴───────────────┴─────────┘
POSIX.1-2001 describes dlclose() and dlopen(). The dlmopen()
function is a GNU extension.
The RTLD_NOLOAD, RTLD_NODELETE, and RTLD_DEEPBIND flags are GNU
extensions; the first two of these flags are also present on Solaris.
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.
The dlmopen() function permits object-load isolation—the 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 RTLD_LOCAL flag is not sufficient for this
purpose, since it prevents a shared object's symbols from being
available to 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 RTLD_GLOBAL flag.
The dlmopen() function also can be used to provide better isolation
than the RTLD_LOCAL flag. In particular, shared objects loaded with
RTLD_LOCAL may be promoted to RTLD_GLOBAL if they are dependencies of
another shared object loaded with RTLD_GLOBAL. Thus, 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.
Possible uses of 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 dlmopen(), this would require the creation
of distinct copies of the shared object file. Using dlmopen(), this
can be achieved by loading the same shared object file into different
namespaces.
The glibc implementation supports a maximum of 16 namespaces.
Initialization and finalization functions
Shared objects may export functions using the
__attribute__((constructor)) and __attribute__((destructor)) function
attributes. Constructor functions are executed before dlopen()
returns, and destructor functions are executed before 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 gcc info
pages (under "Function attributes") for further information.
An older method of (partially) achieving the same result is via the
use of two special symbols recognized by the linker: _init and _fini.
If a dynamically loaded shared object exports a routine named
_init(), then that code is executed after loading a shared object,
before dlopen() returns. If the shared object exports a routine
named _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 gcc(1) -nostartfiles command-line option.
Use of _init and _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.
Since glibc 2.2.3, atexit(3) can be used to register an exit handler
that is automatically called when a shared object is unloaded.
History
These functions are part of the dlopen API, derived from SunOS.
As at glibc 2.24, specifying the RTLD_GLOBAL flag when calling
dlmopen() generates an error. Furthermore, specifying RTLD_GLOBAL
when calling dlopen() results in a program crash (SIGSEGV) if the
call is made from any object loaded in a namespace other than the
initial namespace.
The program below loads the (glibc) math library, looks up the
address of the cos(3) function, and prints the cosine of 2.0. The
following is an example of building and running the program:
$ cc dlopen_demo.c -ldl
$ ./a.out
-0.416147
Program source
#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\n", 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. */
error = dlerror();
if (error != NULL) {
fprintf(stderr, "%s\n", error);
exit(EXIT_FAILURE);
}
printf("%f\n", (*cosine)(2.0));
dlclose(handle);
exit(EXIT_SUCCESS);
}
ld(1), ldd(1), pldd(1), dl_iterate_phdr(3), dladdr(3), dlerror(3),
dlinfo(3), dlsym(3), rtld-audit(7), ld.so(8), ldconfig(8)
gcc info pages, ld info pages
This page is part of release 4.12 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
Linux 2016-10-08 DLOPEN(3)
Pages that refer to this page: pldd(1), pmcd(1), uselib(2), vfork(2), atexit(3), backtrace(3), dladdr(3), dlerror(3), dlinfo(3), dl_iterate_phdr(3), dlsym(3), lttng-ust(3), lttng-ust-dl(3), pmda(3), rtld-audit(7), ld.so(8)