MacLochlainns Weblog

Somebody wanted a quick example of using a user-defined header file for a constant value. While I think there a lot of examples on the Internet already, here’s quick example.

You can define a header (or global.h) file in the local directory with the value of pi, like this:

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// A global header file for constants.
double pi = 3.1415926535;

Then, you can write a little circle.c program like so:

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#include <stdio.h>
#include "global.h"
 
int main() {
  int r;
  double area;
 
  // Get the radius.
  printf("Enter a radius in inches: ");
  scanf("%i",&r);
 
  // Calculate the area of a circle.
  area = pi * (r * r);
 
  // Print the radius and circle area.
  printf("Area of a %d inch circle is %lf square inches!\n",r,area);
  return(0); }

The local global.h file is enclosed in double quotes rather than less than and great than symbols. You can compile circle.c like this on Linux, provided both files are in the same directory:

gcc -o circle circle.c

According to the gcc-documentation, the priority for include <> is, on a “standard Unix system”, as follows:

/usr/local/include
libdir/gcc/target/version/include
/usr/target/include
/usr/include

You should chmod the circle file as executable and then you can run it like so:

./circle

You’ll see more or less the following:

Enter a radius in inches: 3
Area of a 3 inch circle is 28.274334 square inches!

As always, I hope the example helps those looking for a starting point.