Is true == 1 && false == 0 always true in C?

Asked May 19 '15 at 12:22

Active May 19 '15 at 12:32

Viewed 4,693 times

There are many questions of this kind on SO so I apologize for this additional one.

stdbool.h defines this:

#define true  (1)
#define false (0)

However I've seen many other declarations in bad code such as

#define true  (0xFF)
#define false (!true)

Or even worse

#define true  (0xAAAAAAAA)
#define false (~true)

In a real context we could imagine a variable int istrue used somewhere in a bad C program:

istrue = 1;

Because the code I am looking at is pretty ugly, I cannot assume I won't see things like:

if (istrue == 1)

That will make me refrain from replacing istrue = 1 with istrue = true.

So my question is:

In C, is true always equal to 1 and false always equal to 0 for any C compiler, and should this assumption always be valid in any case?

edited May 19 '15 at 12:28

jwodder

asked May 19 '15 at 12:22

nowox

6

Every non zero value is TRUE. – Dayal rai May 19 '15 at 12:24
http://stackoverflow.com/questions/6627178/c99-why-are-false-and-true-defined-as-0-and-1-and-not-as-bool0-and-bool – Dmitry Bychenko May 19 '15 at 12:24
Actually not. in C bool are integer, then all number != 0 are true – LPs May 19 '15 at 12:25
@Dayalrai Perfectly right, but it doesn't answer my question – nowox May 19 '15 at 12:25
@DmitryBychenko This link answer the case for C99 not for any C compiler or programming methodology – nowox May 19 '15 at 12:26
2

In C, it is idiomatic to simply use `if (!x)` or `if (x)`. The only reasonable use for `true/false` is in loop conditions like `while (true) {}` or `do {} while (false);`. – EOF May 19 '15 at 12:29
@EOF: note that `while (true) {}` is UB in C. – Bathsheba May 19 '15 at 12:37
2

@Bathsheba: *sigh*. This was a *comment*. So I really need to write `while (true) {...}` to show that there is supposed to be code there? If I didn't want the loop to do anything (indeed being undefined), I'd write `while (true);`. – EOF May 19 '15 at 12:39
@EOF - what about bool returnValue = false; ? – Spalteer May 19 '15 at 13:32
@LPs In C - for the past 16 years, boolean variables are type `_Bool`, not `int`. – chux - Reinstate Monica May 19 '15 at 14:37
@chux Yes, you right. Probably is the sign I'm a little bit seasoned... – LPs May 19 '15 at 14:48
But is it safe to say "c += (x > 0);" as opposed to "if (x > 0) c++;" – Samuel Danielson Dec 28 '15 at 02:00

0 Answers0