2.5 Recompiling and relinking

To show how source files can be compiled independently we will edit the main program 'main.c' and modify it to print a greeting to everyone instead of world:

#include "hello.h"

int
main (void)
{
  hello ("everyone");  /* changed from "world" */
  return 0;
}

The updated file 'main.c' can now be recompiled with the following command:

$ gcc -Wall -c main.c

This produces a new object file 'main.o'. There is no need to create a new object file for 'hello_fn.c', since that file and the related files that it depends on, such as header files, have not changed.

The new object file can be relinked with the hello function to create a new executable file:

$ gcc main.o hello_fn.o -o hello

The resulting executable 'hello' now uses the new main function to produce the following output:

$ ./hello
Hello, everyone!

Note that only the file 'main.c' has been recompiled, and then relinked with the existing object file for the hello function. If the file 'hello_fn.c' had been modified instead, we could have recompiled 'hello_fn.c' to create a new object file 'hello_fn.o' and relinked this with the existing file 'main.o'.⁽³⁾

In a large project with many source files, recompiling only those that have been modified can make a significant saving. The process of recompiling only the modified files in a project can be automated with the standard Unix program make.