Comma in C

September 29, 2023 — 417 words — 3 min

Do you know what this C code produces?

#include <stdio.h>
int main() {
    if (1,0) {
        puts("hello");
    }
}

This code does not print “hello”. When the comma is used in such a case it acts as binary operator. This means the condition in the if statement is evaluated as follows:

evaluate the first expression and discard the result. In this case the evaluation is only 1 or true and the result is discarded.
evaluate the second expression and return the result. In this case, it’s 0 which is equal to false and this result is returned.
That result is used in the if condition and, therefore, it evaluates to false.

You could use more than two expressions separated by a comma. In general, the expressions are elevated from left to right and the rightmost result is returned. If you do this and you turn on -Wunused-value (or -Wall for that matter), your compiler will tell you about this.

$ gcc -Wall main.c && ./a.out
main.c: In function ‘main’:
main.c:3:14: warning: left-hand operand of comma expression has no effect [-Wunused-value]
    3 |         if (1,0)
      |              ^

You can use this in variable initialization as well.

// number would be 42. The number 22 is just discarded.
int number = (22,42);

Or call functions:

#include <stdio.h>

// return 1 on success
int call_grandma() {
    puts("log: call_grandma()");
    // do things
    return 1;
}

int answer_to_everything() {
    return 42;
}

int main() {
    int answer = (call_grandma(), answer_to_everything());
    printf("answer=%d\n", answer);
    // answer is now 42.
    // grandma was also called but the result was discarded.
}

Executing the above program results in this output.

$ gcc main.c && ./a.out
log: call_grandma()
answer=42

Other more common uses that you probably know are the comma as a separator, for example

while assigning values to multiple variables like int a=21, b=42;
in a function call: do_stuff(get_arg1(), get_arg2()). Note that the comma only acts as separator and as far as I know, we don’t know whether get_arg1() is called before get_arg2(). I’m not sure whether the compiler decides based on optimization purposes or if there is other reasoning how to deal with this. If you know more about this, please tell me.
in a for loop: for (int a=0, b=42; a<10; a++) {}

Sometimes, the comma can also be used in place of a semicolon.

#include <stdio.h>
int main() {
    puts("good"),
    puts("morning"),
    puts("world");
}

Yes, this works.

You can find more examples and some more detailed explanations on GeeksForGeeks.

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