2.4.2 Creating executables from object files

The final step in creating an executable file is to use gcc to link the object files together and fill in the missing addresses of external functions. To link object files together, they are simply listed on the command line:

$ gcc main.o hello_fn.o -o hello

This is one of the few occasions where there is no need to use the -Wall warning option, since the individual source files have already been successfully compiled to object code. Once the source files have been compiled, linking is an unambiguous process which either succeeds or fails (it fails only if there are references which cannot be resolved).

To perform the linking step gcc uses the linker ld, which is a separate program. On GNU systems the GNU linker, GNU ld, is used. Other systems may use the GNU linker with GCC, or may have their own linkers. The linker itself will be discussed later (see section 11 How the compiler works). By running the linker, gcc creates an executable file from the object files.

The resulting executable file can now be run:

$ ./hello
Hello, world!

It produces the same output as the version of the program using a single source file in the previous section.