//------------------------------------------------------------------------------
// A simple recursive C programme.  Is it *reentrant*?

#include <stdlib.h>
#include <stdio.h>

#define  TRUE (0 == 0)	// In C, "#define" is a -text- substitution
#define FALSE (0 == 1)	// These statements say what they mean

int factorial(int x);	// Function declaration (for later forward reference)

int error;		// Global variable

// - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

main (int argc, char *argv[])
{
int i, j;

if (argc < 2) printf("Needs a parameter.\n");
else
  {
  error = FALSE;		// This is set if the integer range is exceeded
  i = atoi(argv[1]);		// Convert the argument string to an integer

  if (i < 0) printf("Positive numbers only, please.\n");
  else
    {
    if (i == 0) j = factorial(1);	// 0! is same as 1!
    else        j = factorial(i);	// First call
    if (!error) printf("The factorial of %d is %d.\n", i, j);
    else        printf("Sorry, factorial %d overflowed.\n", i);
    }
  }

exit(0);
}

//------------------------------------------------------------------------------
// Recursive factorial evaluation.

int factorial(int x)
{
int k;

if (x == 1) return 1;
else
  {
  k = x * factorial(x-1);	// Recursive call
  if (k < 0) error = TRUE;	// Exceeding the number range will make
  return k;			// the value appear negative (at first).
  }
}

//------------------------------------------------------------------------------
//
// Factorial evaluation is often used as an -example- of recursion; it is
// not the most efficient way to do this in most cases.
//
//------------------------------------------------------------------------------