setjump() and longjump() in C

In this section, we will see what are the setjmp() and longjmp() functions in C. The setjmp() and longjmp() functions are located in the setjmp.h library and provide a way to perform non-local jumps in C programs.

Syntax

#include <setjmp.h>

int setjmp(jmp_buf env);
void longjmp(jmp_buf env, int val);

Parameters

• env − A buffer of type jmp_buf that stores the calling environment
• val − An integer value to be returned by setjmp() when longjmp() is called

Return Value

• setjmp() − Returns 0 when called directly, or the value specified in longjmp() when returning from a jump
• longjmp() − Does not return; transfers control to the setjmp() call point

How It Works

These functions are used in C for exception handling. The setjmp() can be used as a try block, and longjmp() can be used as a throw statement. The longjmp() transfers control to the point which is set by setjmp().

Example: Printing Without Loops

Here we will see how to print a number 100 times without using recursion, loop, or macro expansion using setjmp() and longjmp() functions −

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

jmp_buf buf;

int main() {
    int x = 1;
    setjmp(buf); // Set the jump position using buf
    printf("5"); // Prints a number
    x++;
    if (x <= 100)
        longjmp(buf, 1); // Jump to the point located by setjmp
    printf("<br>");
    return 0;
}

5555555555555555555555555555555555555555555555555555555555555555555555555555555555555555555555555555

Example: Error Handling

A more practical example showing error handling using setjmp() and longjmp() −

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

jmp_buf error_buf;

void divide(int a, int b) {
    if (b == 0) {
        printf("Error: Division by zero!<br>");
        longjmp(error_buf, 1); // Jump back with error code
    }
    printf("Result: %d / %d = %d<br>", a, b, a/b);
}

int main() {
    if (setjmp(error_buf) == 0) {
        printf("Attempting division...<br>");
        divide(10, 2);  // Valid division
        divide(10, 0);  // This will cause a jump
        printf("This line will not execute<br>");
    } else {
        printf("Caught error and recovered<br>");
    }
    return 0;
}

Attempting division...
Result: 10 / 2 = 5
Error: Division by zero!
Caught error and recovered

Key Points

• setjmp() saves the current execution context in the jmp_buf
• longjmp() restores the saved context and transfers control
• Use with caution as it can make code difficult to understand and debug
• Variables declared after setjmp() may have unpredictable values after longjmp()

Conclusion

The setjmp() and longjmp() functions provide a mechanism for non-local jumps in C, useful for error handling and exceptional situations. However, they should be used sparingly as they can make code flow difficult to follow.