Using boolean values in C

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 false
void 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.

Here is the version that I used:

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

Because false only has one value, but a logical true could have many values, but technique sets true to be what the compiler will use for the opposite of false.

This takes care of the problem of someone coding something that would come down to this:

if (true == !false)

I think we would all agree that that is not a good practice, but for the one time cost of doing "true = !false" we eliminate that problem.

[EDIT] In the end I used:

typedef enum { myfalse = 0, mytrue = !myfalse } mybool;

to avoid name collision with other schemes that were defining true and false. But the concept remains the same.

[EDIT] To show conversion of integer to boolean:

mybool somebool;
int someint = 5;
somebool = !!someint;

The first (right most) ! converts the non-zero integer to a 0, then the second (left most) ! converts the 0 to a myfalse value. I will leave it as an exercise for the reader to convert a zero integer.

[EDIT] It is my style to use the explicit setting of a value in an enum when the specific value is required even if the default value would be the same. Example: Because false needs to be zero I use false = 0, rather than false,

If you are using a C99 compiler it has built-in support for bool types:

#include <stdbool.h>
int main()
{
  bool b = false;
  b = true;
}

http://en.wikipedia.org/wiki/Boolean_data_type

First things first. C, i.e. ISO/IEC 9899 has had a boolean type for 19 years now. That is way longer time than the expected length of the C programming career with amateur/academic/professional parts combined when visiting this question. Mine does surpass that by mere perhaps 1-2 years. It means that during the time that an average reader has learnt anything at all about C, C actually has had the boolean data type.

For the datatype, #include <stdbool.h>, and use true, false and bool. Or do not include it, and use _Bool, 1 and 0 instead.

There are various dangerous practices promoted in the other answers to this thread. I will address them:

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

This is no-no, because a casual reader - who did learn C within those 19 years - would expect that bool refers to the actual bool data type and would behave similarly, but it doesn't! For example

double a = ...;
bool b = a;

With C99 bool/ _Bool, b would be set to false iff a was zero, and true otherwise. C11 6.3.1.2p1

1. When any scalar value is converted to _Bool, the result is 0 if the value compares equal to 0; otherwise, the result is 1. 59)

Footnotes

59) NaNs do not compare equal to 0 and thus convert to 1.

With the typedef in place, the double would be coerced to an int - if the value of the double isn't in the range for int, the behaviour is undefined.

Naturally the same applies to if true and false were declared in an enum.

What is even more dangerous is declaring

typedef enum bool {
    false, true
} bool;

because now all values besides 1 and 0 are invalid, and should such a value be assigned to a variable of that type, the behaviour would be wholly undefined.

Therefore iff you cannot use C99 for some inexplicable reason, for boolean variables you should use:

• type int and values 0 and 1 as-is; and carefully do domain conversions from any other values to these with double negation !!
• or if you insist you don't remember that 0 is falsy and non-zero truish, at least use upper case so that they don't get confused with the C99 concepts: BOOL, TRUE and FALSE!