DLOPEN(3)                  Linux Programmer's Manual                 DLOPEN(3)

       dlclose,  dlerror,  dlopen,  dlsym  -  Programming interface to dynamic
       linking loader.

       #include <dlfcn.h>

       void *dlopen(const char *filename, int flag);
       const char *dlerror(void);
       void *dlsym(void *handle, char *symbol);
       int dlclose(void *handle);

       Special symbols: _init, _fini.

       dlopen loads a dynamic library from the file named by the  null  termi-
       nated  string  filename  and returns an opaque "handle" for the dynamic
       library.  If filename is not an absolute path (i.e., it does not  begin
       with a "/"), then the file is searched for in the following locations:

              A   colon-separated   list   of   directories   in   the  user's
              LD_LIBRARY_PATH environment variable.

              The list of libraries cached in /etc/

              /lib, followed by /usr/lib.

       If filename is a NULL pointer, then the returned handle is for the main

       External  references in the library are resolved using the libraries in
       that library's dependency  list  and  any  other  libraries  previously
       opened  with  the  RTLD_GLOBAL flag.  If the executable was linked with
       the flag "-rdynamic", then the global symbols in  the  executable  will
       also be used to resolve references in a dynamically loaded library.

       flag  must  be  either  RTLD_LAZY, meaning resolve undefined symbols as
       code from the dynamic library is executed, or RTLD_NOW, meaning resolve
       all undefined symbols before dlopen returns, and fail if this cannot be
       done.  Optionally, RTLD_GLOBAL may be or'ed with flag,  in  which  case
       the  external  symbols defined in the library will be made available to
       subsequently loaded libraries.

       If the library exports a routine named _init, then that  code  is  exe-
       cuted  before dlopen returns.  If the same library is loaded twice with
       dlopen(), the same file handle is returned.  The dl  library  maintains
       link counts for dynamic file handles, so a dynamic library is not deal-
       located until dlclose has been called on it as many times as dlopen has
       succeeded on it.

       If  dlopen  fails  for  any  reason, it returns NULL.  A human readable
       string describing the most recent error that occurred from any  of  the
       dl routines (dlopen, dlsym or dlclose) can be extracted with dlerror().
       dlerror returns NULL if no errors have occurred since initialization or
       since it was last called.  (Calling dlerror() twice consecutively, will
       always result in the second call returning NULL.)

       dlsym takes a "handle" of a dynamic library returned by dlopen and  the
       null terminated symbol name, returning the address where that symbol is
       loaded.  If the symbol is not found, dlsym returns NULL;  however,  the
       correct  way  to  test for an error from dlsym is to save the result of
       dlerror into a variable, and then check if saved  value  is  not  NULL.
       This  is because the value of the symbol could actually be NULL.  It is
       also necessary to save the results of dlerror into a  variable  because
       if dlerror is called again, it will return NULL.

       There  are two special pseudo-handles, RTLD_DEFAULT and RTLD_NEXT.  The
       former will find the first occurrence of the desired symbol  using  the
       default  library  search  order.  The latter, which is usable only from
       within a dynamic library, will find the next occurrence of  a  function
       in the search order after the current library.  This allows one to pro-
       vide a wrapper around a function in another shared library.

       dlclose decrements the reference count on the  dynamic  library  handle
       handle.   If  the  reference  count  drops  to zero and no other loaded
       libraries use symbols in it, then the dynamic library is unloaded.   If
       the dynamic library exports a routine named _fini, then that routine is
       called just before the library is unloaded.

       dlclose returns 0 on success, and non-zero on error.

       Load the math library, and print the cosine of 2.0:
              #include <stdio.h>
              #include <dlfcn.h>

              int main(int argc, char **argv) {
                  void *handle;
                  double (*cosine)(double);
                  char *error;

                  handle = dlopen ("", RTLD_LAZY);
                  if (!handle) {
                      fprintf (stderr, "%s\n", dlerror());

                  cosine = dlsym(handle, "cos");
                  if ((error = dlerror()) != NULL)  {
                      fprintf (stderr, "%s\n", error);

                  printf ("%f\n", (*cosine)(2.0));
                  return 0;

       If this program were in a file named "foo.c", you would build the  pro-
       gram with the following command:

              gcc -rdynamic -o foo foo.c -ldl

       The  symbols  RTLD_DEFAULT  and RTLD_NEXT are defined by <dlfcn.h> only
       when _GNU_SOURCE was defined before including it.

       The dlopen interface standard comes from  Solaris.   The  Linux  dlopen
       implementation  was  primarily written by Eric Youngdale with help from
       Mitch D'Souza, David Engel, Hongjiu Lu, Andreas Schwab and others.  The
       manual page was written by Adam Richter.

       ld(1),, ldconfig(8), ldd(1),

Linux                             2001-12-14                         DLOPEN(3)