Lost at C: Bool

One of the most common data types in any programming language is the boolean, giving you the basic ability to say whether something is true or false. C is special though, in that there isn't one single "correct" way to use booleans. Let's take a look at the many different ways you will see booleans represented in C programs.

Here is a PDF copy of the C99 standard, for reference below.

In older versions of C (and even in C programs today), people used int instead of an actual boolean type. A boolean is essentially a one bit integer, so storing a boolean value in an int isn't terrible, but it doesn't do a good job of explaining intent: For example, returning 2 from a function that uses an int instead of a boolean means that if you are checking for 1 explicitly (ie, the true case), your check will fail, despite 1 and 2 being truthy.

Here are a few ways you might find bool/true/false being defined:

#define bool int
// or
typedef int bool;

#define true 1
#define false 0
// or
enum { false, true };

If you are stuck with something like C89 or ANSI C, then you have no other option then to define your own bool type. If you are using C99 and up, use the following method:

In C99, a boolean type was added, called _Bool. You can define boolean data types, and all is well:

_Bool isProgrammer = 0;

Note that we can't use true or false. We will get into that later.

Why did they name it _Bool, and not bool? The reason is that other programmers had already defined their own bool types, like in the last section. If the C committee decided to add the bool type globally, it could cause issues with people who had already defined bool to be something else.

Also, the C99 standard states that:

"All identifiers that begin with an underscore and either an uppercase letter or another underscore are always reserved for any use." - ISO/IEC 9899:1999 § 7.1.3

With that in mind, it is possible for the C standard to add in new data types (and other identifiers) using a reserved name. So long as a user isn't already using the reserved name, everything should just work.

To make our lives easier, C99 also made the bool/true/false keywords available via the stdbool.h header:

#include <stdbool.h>

bool isProgrammer = true;

Although it is somewhat annoying that you need to include a header just to have access to these, it is better then the alternative (defining your own bool type).

Although not directly related to C, when you are dealing with header files which might be included in both C and C++ code, it might be important to note a few things:

• bool is a built-in data type (no need to include anything)
• _Bool is only available if you include stdbool.h or cstdbool
• Different C versions might not support the stdbool.h header (pre C99)

That's it! Although it is somewhat minor, this is something that you will see all over the place in C code, and is something to be on the lookout for.