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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ATTRIBUTES | STANDARDS | HISTORY | NOTES | BUGS | EXAMPLES | SEE ALSO | COLOPHON |
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dlopen(3) Library Functions Manual dlopen(3)
dlclose, dlopen, dlmopen - open and close a shared object
Dynamic linking library (libdl, -ldl)
#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);
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):
• (ELF only) If the calling object (i.e., the shared library or
executable from which dlopen() is called) contains a DT_RPATH
tag, and does not contain a DT_RUNPATH tag, then the
directories listed in the DT_RPATH tag are searched.
• 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.)
• (ELF only) If the calling object contains a DT_RUNPATH tag,
then the directories listed in that tag are searched.
• The cache file /etc/ld.so.cache (maintained by ldconfig(8)) is
checked to see whether it contains an entry for filename.
• 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 and global 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(3), 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.
Symbol references in the shared object are resolved using (in
order): symbols in the link map of objects loaded for the main
program and its dependencies; symbols in shared objects (and
their dependencies) that were previously opened with dlopen()
using the RTLD_GLOBAL flag; and definitions in the shared object
itself (and any dependencies that were loaded for that object).
Any global symbols in the executable that were placed into its
dynamic symbol table by ld(1) can also be used to resolve
references in a dynamically loaded shared object. Symbols may be
placed in the dynamic symbol table either because the executable
was linked with the flag "-rdynamic" (or, synonymously,
"--export-dynamic"), which causes all of the executable's global
symbols to be placed in the dynamic symbol table, or because
ld(1) noted a dependency on a symbol in another object during
static linking.
If the same shared object is opened 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.
Constructors (see below) are called only when the object is
actually loaded into memory (i.e., when the reference count
increases to 1).
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. Similarly, an object that was previously
opened with RTLD_LOCAL can be promoted to RTLD_GLOBAL in a
subsequent dlopen().
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 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 RTLD_GLOBAL flag
and one of its symbols satisfied a relocation in another object.)
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 object. 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).
For an explanation of the terms used in this section, see
attributes(7).
┌─────────────────────────────────────┬───────────────┬─────────┐
│ Interface │ Attribute │ Value │
├─────────────────────────────────────┼───────────────┼─────────┤
│ dlopen(), dlmopen(), dlclose() │ Thread safety │ MT-Safe │
└─────────────────────────────────────┴───────────────┴─────────┘
dlopen()
dlclose()
POSIX.1-2008.
dlmopen()
RTLD_NOLOAD
RTLD_NODELETE
GNU.
RTLD_DEEPBIND
Solaris.
dlopen()
dlclose()
glibc 2.0. POSIX.1-2001.
dlmopen()
glibc 2.3.4.
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 <dlfcn.h>
#include <stdio.h>
#include <stdlib.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.1-2001 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 1 to POSIX.1-2008 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 the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
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⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.9.1.tar.gz
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2024-06-26. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
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[email protected]
Linux man-pages 6.9.1 2024-06-15 dlopen(3)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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