Sometimes when our program is in multiple files, we still want to define variables that are visible to each file. Here’s how to do this:
- Declare the global variable in a .h file. This can either be done in a .h file with some of your function prototypes, or in a special file, globals.h, that contains the declarations of all global variables. If you do use a special globals header file, it does not need a corresponding .c file.
- Include the .h file wherever you want to use the variable
- When you want to use the variable, declare:
extern type name;
where type
is the type of the global variable, and
name
is its name. The extern
keyword tells the
compiler not to create a new variable, but instead to
find the variable name declared in another file.
As an example, suppose we want to make the array size a global variable in our for our earlier statistics program. Here’s what we’d do:
//start of stats.h
#ifndef STATS_H
#define STATS_H
//define array size
int size;
//functions no longer need array lengths
int min(int*);
int max(int*);
double avg(int*);
#endif
//end of stats.h
////////////////////////////
//start of stats.c
//We must include the corresponding header file
#include "stats.h"
//say we're going to use the size variable
extern int size;
int min(int *vals) {
int min = vals[0];
int i;
for (i = 1; i < size; i++) {
if (vals[i] < min) {
min = vals[i];
}
}
return min;
}
int max(int *vals) {
int max = vals[0];
int i;
for (i = 1; i < size; i++) {
if (vals[i] > max) {
max = vals[i];
}
}
return max;
}
double avg(int *vals) {
int sum = 0;
int i;
for (i = 0; i < size; i++) {
sum += vals[i];
}
return sum/(double)size;
}
//end of stats.c
/////////////////////////
//start of prog.c
#include <stdio.h>
#include <stdlib.h>
//include our header file
//use "" since it is in the current directory
#include "stats.h"
//declare that we're using the size variable
extern int size;
int main() {
int* nums;
int i, length;
printf("Enter size of array: ");
scanf("%d", &length);
nums = malloc(length*sizeof(int));
for (i = 0; i < length; i++) {
printf("Enter a number: ");
scanf("%d", nums+i);
}
printf("The min is: %d\n", min(nums));
printf("The max is: %d\n", max(nums));
printf("The average is: %.2lf\n", avg(nums));
return 0;
}
//end of prog.c
Notice that we declare the variable in a header file. When we want
to use the size
variable, we first include that header file. Then,
we declare size
as an extern variable.