DLOPEN(3)

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

SYNOPSIS
#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.

DESCRIPTION
dlopen loads a dynamic library from the file named by the null terminated 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_ELF_LIBRARY path environment variable. If and only if LD_ELF_LIBRARY_PATH is not defined, LD_LIBRARY_PATH is used in its place.

    The list of libraries specified in /etc/ld.so.cache.

    /usr/lib, followed by /lib.

External references in the library are resolved using other dynamic libraries previously loaded with dlopen, and the dynamic libraries that the executable was linked against. 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 can 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. If the library exports a routine named _init, then that code is executed 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 deallocated 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.

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.

EXAMPLES
Load the math library, and print the cosine of 2.0:
    #include <dlfcn.h>
    int main(int argc, char **argv) {
        void *handle = dlopen ("/lib/libm.so", RTLD_LAZY);
        double (*cosine)(double) = dlsym(handle, "cos");
        printf ("%f\n", (*cosine)(2.0));
        dlclose(handle);
    }

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

        gcc -rdynamic -o foo foo.c -ldl

Do the same thing, but check for errors at every step:
    #include <stdio.h>
    #include <dlfcn.h>
    int main(int argc, char **argv) {
        void *handle;
        double (*cosine)(double);
        char *error;
        handle = dlopen ("/lib/libm.so", RTLD_LAZY);
        if (!handle) {
            fputs (dlerror(), stderr);
            exit(1);
        }
        cosine = dlsym(handle, "cos");
        if ((error = dlerror()) != NULL)  {
            fputs(error, stderr);
            exit(1);
        }
        printf ("%f\n", (*cosine)(2.0));
        dlclose(handle);
    }

ACKNOWLEDGEMENTS
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.

SEE ALSO
ld(1), ld.so(8), ldconfig(8), ldd(1).