CODEWITHSUNDEEP
cfor-loopcompiler-warningscomma-operator
narek
narek

Reputation: 43

No compiler warning when there's a comma instead of a semicolon in a for loop

Why doesn't gcc give any warnings about this code?

#include <stdio.h>

void printDigits(int *arr, int arrsize);

int main() {
    int number[] = { 1, 7, 8, 3, 6, 5, 4, 2 };
    size_t arraysize = (sizeof(number) / sizeof(number[0]));

    printDigits(number, arraysize);

    return 0;
}

void printDigits(int *arr, int arrsize) {
    int i;  
    for (i=0; i<arrsize, i++;) {
        printf("%d ", arr[i]);
    }
}

Specifically about the for loop in printDigits function. It's: for(i=0; i<arrsize, i++;) while it really should be for(i=0; i<arrsize; i++)

Gcc didn't give me any warnings and it took me a while to figure out why doesn't the array print out.

Upvotes: 4

Views: 154

Answers (3)

0___________
0___________

Reputation: 67780

It is a valid C. This for loop will execute until i is non zero

Upvotes: -2

Vlad from Moscow
Vlad from Moscow

Reputation: 311068

This for loop

for(i=0; i<arrsize, i++;){
    printf("%d ", arr[i]);
}

is a valid C construction. In the condition of the loop there is used the comma operator

i<arrsize, i++

the value of which is the value of its second operand. So the body of the loop will not be executed because the value of the expression i++ is 0 (that is value of the variable i before its increment).

From the C Standard (6.5.17 Comma operator)

2 The left operand of a comma operator is evaluated as a void expression; there is a sequence point between its evaluation and that of the right operand. Then the right operand is evaluated; the result has its type and value

Pay attention to that any expression in the for loop may be omitted. For example you can even write like

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

Also you should at least change the type of the second function parameter from int to size_t because you are passing an argument of the type size_t.

The function should be declared like

void printDigits( const int *arr, size_t arrsize );

Upvotes: 3

Artyer
Artyer

Reputation: 40891

There is a warning. i<arrsize is computed but then discarded, so gcc warns:

warning: left-hand operand of comma expression has no effect [-Wunused-value]

(Which is enabled by -Wall)

Upvotes: 7

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