DLOPEN(3) Linux Programmer's Manual DLOPEN(3)
NAME
dlclose, dlopen, dlmopen - open and close a shared object
SYNOPSIS
#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.
DESCRIPTION
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 vari-
able.
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 pre-
viously 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.
RETURN VALUE
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).
VERSIONS
dlopen() and dlclose() are present in glibc 2.0 and later. dlmopen() first appeared in glibc 2.3.4.
ATTRIBUTES
For an explanation of the terms used in this section, see attributes(7).
+-------------------------------+---------------+---------+
|Interface | Attribute | Value |
+-------------------------------+---------------+---------+
|dlopen(), dlmopen(), dlclose() | Thread safety | MT-Safe |
+-------------------------------+---------------+---------+
CONFORMING TO
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.
NOTES
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 names-
pace.
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 pur-
pose, 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 depen-
dencies.
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. Construc-
tor 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.
BUGS
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.
EXAMPLE
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
", 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
", error);
exit(EXIT_FAILURE);
}
printf("%f
", (*cosine)(2.0));
dlclose(handle);
exit(EXIT_SUCCESS);
}
SEE ALSO
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
COLOPHON
This page is part of release 4.15 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 2017-09-15 DLOPEN(3)