Comma in C

September 29, 2023 — 417 words — 3 min

#programming 

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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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