Why this output of this C program?

Question

Program:

   int main( )
    {
    printf("%d",printf("%d %d",5,5)&printf("%d %d",7,7));
    return 0;
    }

Output:

5 57 73

I am new to C, I could guess where the 5 57 7 came from, but no idea where the 3 came from. Can someone explain the output?

Be aware that `7 75 53` would be a valid outcome, too, as `&` is no sqeuence point (c.f. http://stackoverflow.com/questions/4176328/undefined-behavior-and-sequence-points). — Kijewski, Jul 29 '12 at 17:54

score 7 · Accepted Answer · answered Jul 29 '12 at 16:41

7

If you apply binary AND to 3 and 3 (which are the return values of both nested printf calls) you get 3 as result.

Note that the code actually contains undefined behaviour, since the order of the nested calls isn't defined.

answered Jul 29 '12 at 16:41

Šimon Tóth

35,456
20
106
151

2

Do you mean *unspecified behavior* instead of *undefined behavior*? – ouah Jul 29 '12 at 16:48
@ouah To be honest, I'm too lazy to look it up. – Šimon Tóth Jul 29 '12 at 16:50
1

It's unspecified, I think. "An example of unspeciﬁed behavior is the order in which the arguments to a function are evaluated.", I'm pretty sure that operators are like functions in that respect. – Daniel Fischer Jul 29 '12 at 17:30

score 6 · Answer 2 · answered Jul 29 '12 at 16:42

6

The return value of the printf function is the number of characters transmitted, or a negative value if there is an error.

printf("%d %d",5,5) returns 3 if there is no error

printf("%d %d",7,7) also returns 3 if there is no error

So printf("%d %d",5,5) & printf("%d %d",7,7) is 3 & 3 which is evaluated to 3.

answered Jul 29 '12 at 16:42

ouah

142,963
15
272
331

Why does printf("%d %d",5,5) returns 3? – Ashwin Singh Jul 29 '12 at 16:42
1

@AshwinSingh Because that is what `printf` returns. The number of printed characters. – Šimon Tóth Jul 29 '12 at 16:43

Eight · Answer 3 · 2012-07-29T16:54:25.503

2

3 is the Bitwise AND of the values returned by two printf.

printf returns the numbers of characters printed. In your case, printf("%d %d",5,5) has printed three characters that are two 5 and one space, similarly printf("%d %d",7,7) is also printing two 7 and one space. Hence both printf is returning 3.

so, 3 is the result of 3 & 3

edited Jul 29 '12 at 16:54

answered Jul 29 '12 at 16:43

Eight

4,194
5
30
51

score 0 · Answer 4 · answered Jul 29 '12 at 17:51

as you can see here : http://en.wikipedia.org/wiki/Printf_format_string, printf return the number of printed chars, so:

printf("%d",printf("%d %d",5,5)&printf("%d %d",7,7));

is composed of :

printf("%d %d",5,5) return 3 (5 space and 5) and print 5 5

printf("%d %d",7,7) return 3 (7 space and 7) and print 7 7

At this stage we got : 5 57 7

And 3 & 3 = 3, finally you got this output:

5 57 73

Regards.

Why this output of this C program?

4 Answers4