Assuming you already have Eclipse CDT, the first thing you need to do is create a new project.  After starting Eclipse, click on File->New->C Project and you’ll get a dialog box to choose the type of project (executable, static library, shared library, etc).  For this example, I’m choosing Executable and Hello World ANSI C Project.  Put in a name and click Finish.

Next you’ll see you have a new project in the left-side panel with a .c file containing a simple hello world program.

Now in my example I’ll add the line #include <avcodec.h> to set up my include.  If you need to add an include directory (where your header files are located) then in the Eclipse menu go to Project->Properties then open the C/C++ Build branch and click on Settings.  On the right side click on the Directories branch of GCC C Compiler.  You can then add directories to your include path.

In this same dialog you can click on Libraries in the GCC C Linker branch to add any libraries to the project for linking.  The library I’m adding is the libavcodec.so shared library.  To add this (notice in the screenshot) I only need to add avcodec, without the prefix lib and .so extension.  This might be new to Windows programmers.  Since you’re adding a library it’s assumed to have the lib and .so in the name.

Now let’s just add a couple of lines to the main() so we can test a compile with the libavcodec library.

#include <stdio.h>
#include <stdlib.h>
#include <avcodec.h>
int main(void) {
   avcodec_init();
   avcodec_register_all();
   return 1;
}

And then to run it, in the menu toolbar click on the run arrow and choose Run As->C/C++ Local Application.

Getting no errors means success.