Why is scanf working abnormally for bool input in C?

Question

I am trying to get input for 3 bool variables and 1 int variable. Even though I give input correctly, it is not behaving right.

I am using %d as format specifier for bool in stdbool.h as suggested by @taufique in Format specifier in scanf for bool datatype in C

Here is my code and its behaviour:

#include <stdio.h>
#include <stdbool.h>
int main( )
{
    bool health,sex,living;
    int age;
    scanf("%d%d%d%d",&sex,&health,&living,&age);
    printf("\n%d %d %d %d\n",sex,health,living,age);
}

Console:

0 1 0 25
0 0 0 25

For some other Input:

1 0 0 26
0 0 0 26

But when using temporary integer variables to get input as suggested by @ouah in the same Format specifier in scanf for bool datatype in C , it works fine.

So why is scanf behaving improperly ?

PS : It does work correctly for some input:

0 0 1 26
0 0 1 26

Because for `%d` you have to pass an `int*` and nothing else. Passing anything else is undefined behavior. If you're not sure what to pass for each format specifier, check the table here: http://www.cplusplus.com/reference/cstdio/scanf/ — Blaze, Jun 12 '19 at 07:31
You do not know what is behind the bool object and you cannot assume that it is int. My advice: less useless forums, more books — 0___________, Jun 12 '19 at 07:35
3 is not many. 3 is users who don't know any better. 20 is many. And did you miss the comment on that answer that explains *exactly* what's wrong with it? — StoryTeller - Unslander Monica, Jun 12 '19 at 07:35
@StoryTeller Sorry for misleading with that. Is there any way to use scanf to directly get input for bool ? — Alagusankar, Jun 12 '19 at 07:37
@Alagusankar - There is no standard format specifier for booleans. But you don't need one. The extra integer is not going to be the performance bottleneck in your program. — StoryTeller - Unslander Monica, Jun 12 '19 at 07:38
@StoryTeller Thanks for the mention. I'll better change it to int then. — Alagusankar, Jun 12 '19 at 07:39
FYI, taufique's answer is now deleted. Hopefully no more people will be misled by it. — StoryTeller - Unslander Monica, Jun 12 '19 at 07:53
Possible duplicate of [Format specifier in scanf for bool datatype in C](https://stackoverflow.com/questions/12920694/format-specifier-in-scanf-for-bool-datatype-in-c) — vlad_tepesch, Jun 12 '19 at 08:03

score 1 · Answer 1 · answered Jun 12 '19 at 08:00

There is no format specifier for bool and having one doesn't make much sense. What would the user type, "true"? You can't use %d for any other type than int.

If you for some reason need to take boolean input from stdin, use int values 1 or 0, then convert it to bool later. For example:

int living; 
scanf("%d", &living);
bool is_living = living;

Conversions from int to bool will automatically convert any non-zero value to true and zero to false.

score 0 · Answer 2 · answered Jun 12 '19 at 08:09

There is no format specifier for bool. Because for bool it not that clear how it should look like.

There are many possibilities:

yes/no
y/n
true/false
t/f
any different case version of the above
what about locals?
1/0

Unlike for numbers and their different formats the above possibilities have different semantics. So support them all with a single specifier is not desirable.

So if you want to support it, you have to chose and implement it by yourself.

Why is scanf working abnormally for bool input in C?

2 Answers2