printf conversion specifier for _Bool?

Question

With printf() , I can use %hhu for unsigned char , %hi for a short int , %zu for a size_t , %tx for a ptrdiff_t , etc.

What conversion format specifier do I use for a _Bool ? Does one exist in the standard?

Or do I have to cast it like this:

_Bool foo = 1;
printf("foo: %i\n", (int)foo);

Answer 1

There is no specific conversion length modifier for _Bool type.

_Bool is an unsigned integer type large enough to store the values 0 and 1 . You can print a _Bool this way:

_Bool b = 1;
printf("%d\n", b);

Because of the integer promotions rules, _Bool is guaranteed to promote to int .

Answer 2

Until C99, bool was not apart of standard C, and thus did not exist as a printf modifier. C99, defined _Bool (which you are using) to be an integer, thus you should be fine casting it as an integer to display (at least from a machine perspective). Alternatively, you could do something like:

printf("%d",foo?1:0);

Answer 3

As you stated in a comment to @Jack, "6.3.1.1p1 says that the conversion rank of _Bool is less than the rank of all other standard integer types".

In a call to printf , a char or short will be promoted and passed on the stack as an int (or an unsigned int ), so I would think that using %d as a format specifier would be fine. That also means that you don't need the explicit cast to int , because that will happen automatically.

The only possible issue would be with how the compiler represents a _Bool , something that it probably implementation defined and could vary from one compiler to another. I see two likely implementations -- 0 and 1 or 0 and -1.

For the ultimate in portability, follow @user325181's answer and use a ternary to choose between two options. Either integers (which the compiler may optimize away) or strings.

Edit: As reported in other answers, a _Bool is defined as an unsigned integral type that can store either 0 or 1. Because of that, and the fact that it will be promoted to an unsigned int when passed to printf() , I would say that %u %d is the most appropriate specifier.

Answer 4

There is no. Just handling it like an int by using %d or %i specifier.

_Bool

In C99, a new keyword, _Bool, is introduced as the new boolean type. In many aspects, it behaves much like an unsigned int, but conversions from other integer types or pointers always constrained to 0 and 1. Other than for other unsigned types, and as one would expect for a boolean type, such a conversion is 0 if and only if the expression in question evaluates to 0 and it is 1 in all other cases. The header stdbool.h provides macros bool, true and false that are defined as _Bool, 1 and 0, respectively.

The first way to implement it that come from into mind is by using a char or( int8_t ) an enum and with bit fields . But actually, it depends. It can be a typedef for an int (as I've mentioned, it's used, but is not recommend, subject to bugs) or char or unsigned int or an enum and #define that's commonly used.

For exampe, Apple's implementation uses int ,as you can see:

#ifndef _STDBOOL_H_
#define _STDBOOL_H_ 

#define __bool_true_false_are_defined   1

#ifndef __cplusplus

#define false   0
#define true    1

#define bool    _Bool
#if __STDC_VERSION__ < 199901L && __GNUC__ < 3
typedef int _Bool;
#endif

#endif /* !__cplusplus */

#endif /* !_STDBOOL_H_ */

Others implementations:

typedef int8_t _Bool;

typedef enum { false = 0, true = 1 } bool;

typedef unsigned char Boolean; typedef _Bool Boolean;

Answer 5

I use %b for a Java printf() conversion specifier. I tried it recently and to my surprise it worked.

Here is the line of code:

System.out.printf("firstRobot == secondRobot: %b",firstRobot == secondRobot);

Here is the output:

firstRobot == secondRobot: false