From best to worse:

Option 1 (C99 and newer)

#include <stdbool.h>

Option 2

typedef enum { false, true } bool;

Option 3

typedef int bool;enum { false, true };

Option 4

typedef int bool;#define true 1#define false 0

Explanation

• Option 1 will work only if you use C99 (or newer) and it's the "standard way" to do it. Choose this if possible.
• Options 2, 3 and 4 will have in practice the same identical behavior. #2 and #3 don't use #defines though, which in my opinion is better.

If you are undecided, go with #1!

A few thoughts on booleans in C:

I'm old enough that I just use plain ints as my boolean type without any typedefs or special defines or enums for true/false values. If you follow my suggestion below on never comparing against boolean constants, then you only need to use 0/1 to initialize the flags anyway. However, such an approach may be deemed too reactionary in these modern times. In that case, one should definitely use <stdbool.h> since it at least has the benefit of being standardized.

Whatever the boolean constants are called, use them only for initialization. Never ever write something like

if (ready == TRUE) ...while (empty == FALSE) ...

These can always be replaced by the clearer

if (ready) ...while (!empty) ...

Note that these can actually reasonably and understandably be read out loud.

Give your boolean variables positive names, ie full instead of notfull. The latter leads to code that is difficult to read easily. Compare

if (full) ...if (!full) ...

with

if (!notfull) ...if (notfull) ...

Both of the former pair read naturally, while !notfull is awkward to read even as it is, and becomes much worse in more complex boolean expressions.

Boolean arguments should generally be avoided. Consider a function defined like this

void foo(bool option) { ... }

Within the body of the function, it is very clear what the argument means since it has a convenient, and hopefully meaningful, name. But, the call sites look like

foo(TRUE);foo(FALSE):

Here, it's essentially impossible to tell what the parameter meant without always looking at the function definition or declaration, and it gets much worse as soon if you add even more boolean parameters. I suggest either

typedef enum { OPT_ON, OPT_OFF } foo_option;void foo(foo_option option);

or

#define OPT_ON true#define OPT_OFF falsevoid foo(bool option) { ... }

In either case, the call site now looks like

foo(OPT_ON);foo(OPT_OFF);

which the reader has at least a chance of understanding without dredging up the definition of foo.

A boolean in C is an integer: zero for false and non-zero for true.

See also Boolean data type, section C, C++, Objective-C, AWK.