Chapter: Other topics
...Section: Multiple Files, and make

Here's file1.c:

// A global variable in this file:
#include < stdio.h >
int i;
// A global variable from another file:
extern int j;
int main () {
  i = 5;                     // Access the file variable, as usual.
  printf ("i = %d\n", i);
  j = 6;                     // Access the other file's variable.
  printf ("j = %d\n", j);
}

j is declared in file2.c:

#include < stdio.h >
int j;

To create an executable executablename from the two files file1.c and file2.c you'll need to:

  gcc -o executablename file1.c file2.c
  (or) gcc -o executablename file2.c file1.c

Notes:

• The extern keyword is used to indicate a variable in another file.
• There is no special declaration needed for j in the other file.
• Local variables (declared inside a function) cannot be accessed outside the function, let alone another file.

Function access works similarly: You can define a function to be extern by placing the word extern in its prototype in this file.

It is good practice to create function prototypes for all your functions in each file as follows:

• Declare static prototypes for functions you don't want to export to other files.
• Declare non-static prototypes for those that you wish to export.

Large C projects use a makefile which facilitates specification of compilation options, compilation order, and other compilation-related features.