Local Goto with setjmp() and longjmp()

There is no reason why you cannot use setjmp() and longjmp() to perform local goto activities. Let us start with a very simple program. Click here to download a copy.

#include  <stdio.h>
#include  <setjmp.h>

void  main(int argc, char *argv[])
{
     jmp_buf   Buffer;
     long      fact, i, number;

     number = atoi(argv[1]);
     fact   = i = 1;  
     setjmp(Buffer); 
     fact *= i;      
     printf("%d! = %d\n", i, fact);
     i++;
     if (i <= number)         
          longjmp(Buffer, 1); 
}

With the power of setjmp() and longjmp(), we can certainly do more interesting (or weird, depending on your point of view) programming practice. The following is another, but longer, program. Click here to download a copy.

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

void  main(int argc,  char *argv[])
{
     jmp_buf   Return;           
     jmp_buf   Counting;          
     int       MaxCount;          
     int       even, odd, count;
     int       number;

     MaxCount = atoi(argv[1]);

     printf("Even-Odd Random Number Counting:\n");
     switch (setjmp(Return)) {       
          case 0:  goto LOOP;        
          case 1:  longjmp(Counting, 1);   
          case 2:  printf("There are %d even and %d odd random numbers\n",
                           even, odd); 
     }
     exit(0);

LOOP:                              
     even = odd = count = 0;
     srand((unsigned int) time(NULL));

     if (setjmp(Counting) == 0)    
          longjmp(Return, 1);    
     else {                      
          number = rand();       
          printf("  Random number = %d ", number);
          if ((number % 2) == 0) {
               printf("(even)\n");
               even++;
          }
          else {
               printf("(odd)\n");
               odd++;
          }
          if (even + odd != MaxCount)
               longjmp(Return, 1);  
          else
               longjmp(Return, 2); 
     }
}

This program requires some explanation.

• This program generates a specific number of random numbers and counts the number of even and odd random numbers.
• It has an "internal" subroutine marked by the label LOOP. This "internal" subroutine has to be initialized. The initialization includes clearing all counters to zero, initializing the random number seed, and setting up an entering point Counting using setjmp(). After this, it jumps back to the return point Return in the "main" program setup by setjmp(). The next call to this "internal" subroutine is done by a longjmp() rather than by a goto.
• The "main" program uses setjmp() to setup a return point Return. If the returned value of this call to setjmp() is 0, this is the first time that Return is setup. Thus, a goto sends the control to the "internal" subroutine for initialization. The "internal" subroutine will send the control back and return a 1. Thus, case 1 is executed on this return. Once we are in case 1, we know that the "internal" subroutine has been setup and counting can start. Consequently, we "call" the "internal" subroutine by executing longjmp(Counting,1).
• The second time the control jumps back to the "internal" subroutine, the else part is executed. In this section, a random number is generated and counted. Finally, the control is returned to the "main" program's Return.
• Thus, as you can see, the "main" program "calls" the "internal" subroutine to generate and count one random number at a time. This continues until the count is a full count. In this case, the "internal" subroutine sends the control back to Return and returns 2 as the second argument. Once the control returns to the "main" program, case 2 is executed. The result is printed and the program exits.