The longjmp function is defined in the setjmp.h header file in C. It is used to jump to a previously saved execution point (called a "jump buffer") using the setjmp function.

The longjmp function takes two arguments: a jmp_buf object (i.e., a jump buffer) and an int value. The jmp_buf object must have been previously set by calling setjmp. The int value is the return value that will be returned by setjmp when it resumes execution at the saved execution point.

When longjmp is called, the program will immediately jump to the saved execution point, and execution will resume at that point with the return value specified in the value parameter. It is important to note that the longjmp function does not return; instead, it "unwinds" the call stack, jumping directly to the saved execution point.

Here's an example usage of the longjmp function:

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

jmp_buf buf;

void f() {
    printf("f: before longjmp\n");
    longjmp(buf, 1);
    printf("f: after longjmp\n");
}

int main() {
    int value = 0;

    if (setjmp(buf) == 0) {
        printf("main: before f\n");
        f();
    } else {
        printf("main: after longjmp\n");
    }

    return 0;
}

In this example, setjmp is called in the main function to save the current execution point in the jmp_buf object named buf. The f function is then called, and calls longjmp to jump back to the saved execution point. When longjmp is called, the program immediately returns to the point in main where setjmp was called, and the value 1 is returned from setjmp. This value is printed to the console, along with the text "main: after longjmp".

Note that the value passed to longjmp should match the value returned by setjmp, since the value passed to longjmp is the return value that will be passed to the setjmp function when execution resumes at the saved execution point.