What does if (! variable_name) mean in c language [closed]

Question

Here is the problem

int main() {
    int pid = fork();
    if (!pid) {
        // condition 1
    } else {
        // condition 2
    }
    return 0;
}

What does (!pid) do?

Answer 1

It is equivalent to:

if (!pid != 0) /* ... */

And then:

if (pid == 0) /* ... */

C11 (n1570), § 6.5.3.3 Unary arithmetic operators

The result of the logical negation operator ! is 0 if the value of its operand compares unequal to 0, 1 if the value of its operand compares equal to 0. The result has type int. The expression !E is equivalent to (0==E).

Answer 2

if(!pid)

Is as you wrote:

if(pid == 0) {
  /* do something */
}

And then:

if(pid) 

is

if(pid != 0)