Confusion with usage of ! operator in if statment

See example below:

Code:

---


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

static jmp_buf buf;

void second(void) {
    printf("second\n");        // prints
    longjmp(buf,1);            // jumps back to where setjmp was called - making setjmp now return 1
}

void first(void) {
    second();
    printf("first\n");          // does not print
}

int main() { 
    if ( ! setjmp(buf) ) {
        first();                // when executed, setjmp returns 0
    } else {                    // when longjmp jumps back, setjmp returns 1
        printf("main\n");      // prints
    }
    return 0;
}

---

I don't understand this part "if ( ! setjmp(buf) )" does. Can anyone explain?

Place the code in code tags like this [code]/*YOUR CODE*/ [/code] :)

NOT (!) just flips Boolean results. Try reading the relevent section of this page.
If Statements in C++ - Cprogramming.com

Then that leads to other question. I don't understand the sythax of this part. It sounds like an uncompleted sentence "If not setjmp(buf), then run function first()."

Well to fully understand the condition, you need to understand what setjmp returns and why. Then calling first when setjmp returns 0 should make sense.

Well we normally write what you have there, when the result is zero.

Example.

Code:

---

int a = 0;
if( a == 0)
{
    //it's zero
}
else
{
    // it's not zero
}

---

Many people write this code we just read like this (which is equivalent)

Code:

---

int a = 0;
if( !a )
{
    //it's zero
}
else
{
    // it's not zero
}

---

a is zero, so we have as condition of if this: !0 which is 1 which results to true.

It's nice to use this style when you have variables names found, etc. ...

But notice that you have a .h file that defines what the function set returns.

I see! The numbers are being treated as boolean TRUE and FALSE. Thanks for clearing up.

Remember: 0 results to false, anything else in true.

You are welcome :)

yes that menas if setjmp(buf) returns 0, then the if block will execute, saying this the setjmp() must be a function returning some kind of integer value..

Other question: does longjmp() cause a frame in the middle of the stack to be deallocated?

For example:

Code:

---


#include <stdio.h>
#include <setjmp.h>
 
static jmp_buf buf;
 
void second(void) {
    longjmp(buf,1);   
    printf("second\n");                   

}
 
void first(void) {
    second();
    setjmp(buf);
    printf("first\n");         

}
 
int main() { 
    first();
    return 0;
}

---

This is something that had to do with the implementation of the function. This is something of the standard. You created setjmp.h or is it something that everybody around you use it?

No I did not create setjmp.h, it is already included in the library. Check out setjmp.h - Wikipedia, the free encyclopedia.

Quote:

---

Originally Posted by vxs8122

Other question: does longjmp() cause a frame in the middle of the stack to be deallocated?

---

The reason why your example doesn't work is that you call longjmp() before you call setjmp() thus the value of "buf" is garbage.

Bye, Andreas