Entry

how to call function in .so files

Aug 12th, 2004 06:16
Hans Holmberg, Jay Nayegandhi, jaya prakash,

Suppose you want to use something in library /somelibdir/libFoo.so.

When you build your program, you will need to add a -L option to tell 
the linker which directory to look for the library, and a -l option for 
which library:

cc -L/somelibdir myprogram.c -lFoo

At run time, libFoo must be visible to the linker.  Different Unices 
have different ways of making this happen.  The dynamic linker will 
have a default search path that includes directories such as /lib 
and /usr/lib.  Other directories may be included through use of the 
environment variable LD_LIBRARY_PATH.  Look at the man page for your 
dynamic linker(usually 'ld.so') for details.

---
Note by Hans Holmberg:

In most modern unixes you can use dlopen() and dlsym() to load a library 
dynamicly on the fly without the need to use ld on your binary.

Here's the example that you will find in the linux manpages:

#include <stdio.h>
#include <dlfcn.h>

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

  handle = dlopen ("libm.so", RTLD_LAZY);
  if (!handle) {
    fprintf (stderr, "%s\n", dlerror());
    exit(1);
  }

  cosine = dlsym(handle, "cos");
  if ((error = dlerror()) != NULL)  {
    fprintf (stderr, "%s\n", error);
    exit(1);
  }

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

Remember, in some instances dlsym() will return 0 or NULL if the symbol 
has that value, allthough it will also return 0 or NULL when the symbol 
doesn't exist. To make sure the returned value is a correct one ALWAYS 
call dlerror() afterwards.

It's also a good idea to make sure libraries are loaded in the correct 
dependency order if you open more than one library.

There's a good writeup on using libraries by David Wheeler at
http://www.dwheeler.com/program-library/Program-Library-HOWTO/index.html