### 2.3 Compiling multiple source files ¶

A program can be split up into multiple files. This makes it easier to edit and understand, especially in the case of large programs—it also allows the individual parts to be compiled independently.

In the following example we will split up the program Hello World into three files: `main.c`, `hello_fn.c` and the header file `hello.h`. Here is the main program `main.c`:

    #include "hello.h"

    int
    main (void)
    {
      hello ("world");
      return 0;
    }

The original call to the `printf` system function in the previous program `hello.c` has been replaced by a call to a new external function `hello`, which we will define in a separate file `hello_fn.c`.

The main program also includes the header file `hello.h` which will contain the declaration of the function `hello`. The declaration is used to ensure that the types of the arguments and return value match up correctly between the function call and the function definition. We no longer need to include the system header file `stdio.h` in `main.c` to declare the function `printf`, since the file `main.c` does not call `printf` directly.

The declaration in `hello.h` is a single line specifying the prototype of the function `hello`:

    void hello (const char * name);

The definition of the function `hello` itself is contained in the file `hello_fn.c`:

    #include <stdio.h>
    #include "hello.h"

    void
    hello (const char * name)
    {
      printf ("Hello, %s!\n", name);
    }

This function prints the message “`Hello,` name`!`” using its argument as the value of name.

Incidentally, the difference between the two forms of the include statement `#include "FILE.h"` and `#include <FILE.h>` is that the former searches for `FILE.h` in the current directory before looking in the system header file directories. The include statement `#include <FILE.h>` searches the system header files, but does not look in the current directory by default.

To compile these source files with `gcc`, use the following command:

    $ gcc -Wall main.c hello_fn.c -o newhello

In this case, we use the `-o` option to specify a different output file for the executable, `newhello`. Note that the header file `hello.h` is not specified in the list of files on the command line. The directive `#include "hello.h"` in the source files instructs the compiler to include it automatically at the appropriate points.

To run the program, type the path name of the executable:

    $ ./newhello
    Hello, world!

All the parts of the program have been combined into a single executable file, which produces the same result as the executable created from the single source file used earlier.