What for (;;) and while(); mean in C

Question

I am looking at some example codes and I saw someone did this

for (;;) {
// ...
}

Is that equivalent to while(1) { } ?

And what does while(condition); do? I don't get the reason behind putting ';' instead of {}

Solution 1

yes,

for(;;){}

is an infinite loop

Solution 2

And what does while(condition); do? I don't get the reason behind putting ';' instead of {}

Well, your question is what happens if you put or you do not put a semicolon after that while condition? The computer identifies the semicolon as an empty statement.

Try this:

#include<stdio.h>

int main(void){
    int a = 5, b = 10;

    if (a < b){
        printf("True");
    }


    while (a < b); /* infinite loop */
        printf("This print will never execute\n");

    return 0;
}

Solution 3

while just loops though a single statement until the condition is false. It doesn't have to be a compound statement (this thing: {}), it can be any statement. ; is a statement that does nothing.

while(getchar() != '\n');

will loop until you hit enter, for example. Though, this is bad practice since it will hog the thread; adding a call to a sleep method in the loop is better.

Comments

[Dmitri]

Right, the empty statement from the semicolon is the loop body. But sometimes that's intentional if the work is done in the condition.

[M.M]

Note that this loop causes undefined behaviour in C11. (In C99 it was unclear whether or not it did). while(1) {} does not cause undefined behaviour, so those two loop constructs are not the same.

[M.M]

The print may execute because while (a < b); causes undefined behaviour. See here for further discussion