Nonlocal Jumps

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TOC: Exceptional Control Flow

• a form of user level exception control flow that transfers control directly from one function to another currently executing function w/o having to go through the normal call and return sequence.
• It's provided by setjmp and longjmp.

#include <setjmp.h>

int setjmp(jmp_buf env);
int sigsetjmp(sigjmp_buf env, int savesigs);
								Returns: 0 from setjmp, nonzero from longjmps

• it saves the current calling environment in the env buffer for later use by longjmp and returns 0.
• The calling env includes program counter, stack pointer, gen purpose registers etc.
• And we should not assign the value returned from setjmp to our a variable cuz when coming from longjmp, our registers will be restored to previous value, and having one of the stuff changed (from prev 0 to now something else) could cause crash (not full reason but smthing similar).
• But we can use the expression itself as a test in swtich or condn statement

#include <setjmp.h>

void longjmp(jmp_buf env, int retval);
void siglongjmp(sighmp_buf env, int retval);
														Never returns

• longjmp restores the calling environment from the env buffer and then triggers a return from the most recent setjmp call that initialized env.
• the setjmp then returns w non-zero return value retvalue.
• setjmp is called once and can return multiple times (with multiple longjmp calls), but longjmp is called once and never returns.
• A longjmp can only jump down a stack that currently exists and is in the call stack up somewhere, if it hasn't been called yet or worse has been returned the behaviour is undefined.
• siglongjmp and sigsetjmp are signal handling versions of a functions
• Example usage:

#include "csapp.h"

sighmp_buf buf;

void hanlder(int sig){
	siglongjmp(buf, 1);
}
int main(){
	if(!sigsetjmp(buf, 1)){
		Signal(SIGINT, handler);
		Sio_put("starting\n");
	}
	else
		Sio_put("restarting\n");
	while(1){
		sleep(1);
		Sio_puts("processing--\n");
	}
	exit(0);
}

• here to avoid race we put handler AFTER we put sigsetjmp, that way there is no possible of having undefined behaviour where SIGINT leads to siglongjmp to some undefined place.
• note that the second arg to sigsetjmp (which is non existent in setjmp) is to tell the system whether or not to remember the current signal mask.

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TOC: Exceptional Control Flow