Question

What is the difference between goto and long jmp( ) and setjmp()?

Answer 1

A goto statement implements a local jump of program execution, and the longjmp() and setjmp() functions implement a nonlocal, or far, jump of program execution. Generally, a jump in execution of any kind should be avoided because it is not considered good programming practice to use such statements as goto and longjmp in your program.

A goto statement simply bypasses code in your program and jumps to a predefined position. To use the goto statement, you give it a labeled position to jump to. This predefined position must be within the same function. You cannot implement gotos between functions. Here is an example of a goto statement:

void bad_programmers_function(void)
{
     int x;
     printf("Excuse me while I count to 5000...\n");
     x = 1;
     while (1)
     {
          printf("%d\n", x);
          if (x == 5000)
               goto all_done;
          else
               x = x + 1;
     }
all_done:
     printf("Whew! That wasn't so bad, was it?\n");
}

This example could have been written much better, avoiding the use of a goto statement. Here is an example of an improved implementation:

void better_function(void)
{
     int x;
     printf("Excuse me while I count to 5000...\n");
     for (x=1; x<=5000; x++)
          printf("%d\n", x);
     printf("Whew! That wasn't so bad, was it?\n");
}

As previously mentioned, the longjmp() and setjmp() functions implement a nonlocal goto. When your program calls setjmp(), the current state of your program is saved in a structure of type jmp_buf. Later, your program can call the longjmp() function to restore the program's state as it was when you called setjmp(). Unlike the goto statement, the longjmp() and setjmp() functions do not need to be implemented in the same function.

However, there is a major drawback to using these functions: your program, when restored to its previously saved state, will lose its references to any dynamically allocated memory between the longjmp() and the setjmp(). This means you will waste memory for every malloc() or calloc() you have implemented between your longjmp() and setjmp(), and your program will be horribly inefficient. It is highly recommended that you avoid using functions such as longjmp() and setjmp() because they, like the goto statement, are quite often an indication of poor programming practice.

Here is an example of the longjmp() and setjmp() functions:

#include <stdio.h>
#include <setjmp.h>
jmp_buf saved_state;
void main(void);
void call_longjmp(void);
void main(void)
{
     int ret_code;
     printf("The current state of the program is being saved...\n");
     ret_code = setjmp(saved_state);
     if (ret_code == 1)
     {
          printf("The longjmp function has been called.\n");
          printf("The program's previous state has been restored.\n");
          exit(0);
     }
     printf("I am about to call longjmp and\n");
     printf("return to the previous program state...\n");
     call_longjmp();
}
void call_longjmp(void)
{
     longjmp(saved_state, 1);
}