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):

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:

Zero or more of the following values may also be ORed in flags:

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()\(emthe 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:

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.

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.

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);
}