boolean expressions

Section 9. Boolean Expressions

9.1: What is the right type to use for Boolean values in C? Why
isn't it a standard type? Should I use #defines or enums for
the true and false values?

A: C does not provide a standard Boolean type, in part because
picking one involves a space/time tradeoff which can best be
decided by the programmer. (Using an int may be faster, while
using char may save data space. Smaller types may make the
generated code bigger or slower, though, if they require lots of
conversions to and from int.)

The choice between #defines and enumeration constants for the
true/false values is arbitrary and not terribly interesting (see
also questions 2.22 and 17.10). Use any of

#define TRUE 1 #define YES 1
#define FALSE 0 #define NO 0

enum bool {false, true}; enum bool {no, yes};

or use raw 1 and 0, as long as you are consistent within one
program or project. (An enumeration may be preferable if your
debugger shows the names of enumeration constants when examining
variables.)

Some people prefer variants like

#define TRUE (1==1)
#define FALSE (!TRUE)

or define "helper" macros such as

#define Istrue(e) ((e) != 0)

These don't buy anything (see question 9.2 below; see also
questions 5.12 and 10.2).

9.2: Isn't #defining TRUE to be 1 dangerous, since any nonzero value
is considered "true" in C? What if a built-in logical or
relational operator "returns" something other than 1?

A: It is true (sic) that any nonzero value is considered true in C,
but this applies only "on input", i.e. where a Boolean value is
expected. When a Boolean value is generated by a built-in
operator, it is guaranteed to be 1 or 0. Therefore, the test

if((a == b) == TRUE)

would work as expected (as long as TRUE is 1), but it is
obviously silly. In general, explicit tests against TRUE and
FALSE are inappropriate, because some library functions (notably
isupper(), isalpha(), etc.) return, on success, a nonzero value
which is *not* necessarily 1. (Besides, if you believe that
"if((a == b) == TRUE)" is an improvement over "if(a == b)", why
stop there? Why not use "if(((a == b) == TRUE) == TRUE)"?) A
good rule of thumb is to use TRUE and FALSE (or the like) only
for assignment to a Boolean variable or function parameter, or
as the return value from a Boolean function, but never in a
comparison.

The preprocessor macros TRUE and FALSE (and, of course, NULL)
are used for code readability, not because the underlying values
might ever change. (See also questions 5.3 and 5.10.)

On the other hand, Boolean values and definitions can evidently
be confusing, and some programmers feel that TRUE and FALSE
macros only compound the confusion. (See also question 5.9.)

References: K&R1 Sec. 2.6 p. 39, Sec. 2.7 p. 41; K&R2 Sec. 2.6
p. 42, Sec. 2.7 p. 44, Sec. A7.4.7 p. 204, Sec. A7.9 p. 206;
ANSI Sec. 3.3.3.3, Sec. 3.3.8, Sec. 3.3.9, Sec. 3.3.13,
Sec. 3.3.14, Sec. 3.3.15, Sec. 3.6.4.1, Sec. 3.6.5; ISO
Sec. 6.3.3.3, Sec. 6.3.8, Sec. 6.3.9, Sec. 6.3.13, Sec. 6.3.14,
Sec. 6.3.15, Sec. 6.6.4.1, Sec. 6.6.5; H&S Sec. 7.5.4 pp. 196-7,
Sec. 7.6.4 pp. 207-8, Sec. 7.6.5 pp. 208-9, Sec. 7.7 pp. 217-8,
Sec. 7.8 pp. 218-9, Sec. 8.5 pp. 238-9, Sec. 8.6 pp. 241-4;
"What the Tortoise Said to Achilles".

9.3: Is if(p), where p is a pointer, a valid conditional?

A: Yes. See question 5.3.