dlopen

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 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(), 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.

RETURN VALUE
       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).

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 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.

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\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);
       }

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 5.03 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                             2019-08-02                         DLOPEN(3)