1 = false and 0 = true?

Question

I came across an is_equals() function in a c API at work that returned 1 for non-equal sql tables (false) and 0 for equal ones (true). I only realized it after running test cases on my code, one for the positive example and one for the negative and they both failed which at first made little sense. The code in the API does not have a bug as the output was recorded correctly in its documentation.

My questions - are there upside down worlds / parallel universes / coding languages where this logical NOTing is normal? Isn't 1 usually true? Is the coder of the API making an error?

Solution 1

It is common for comparison functions to return 0 on "equals", so that they can also return a negative number for "less than" and a positive number for "greater than". strcmp() and memcmp() work like this.

It is, however, idiomatic for zero to be false and nonzero to be true, because this is how the C flow control and logical boolean operators work. So it might be that the return values chosen for this function are fine, but it is the function's name that is in error (it should really just be called compare() or similar).

Solution 2

This upside-down-world is common with process error returns. The shell variable $? reports the return value of the previous program to execute from the shell, so it is easy to tell if a program succeeds or fails:

$ false ; echo $?
1
$ true ; echo $?
0

This was chosen because there is a single case where a program succeeds but there could be dozens of reasons why a program fails -- by allowing there to be many different failure error codes, a program can determine why another program failed without having to parse output.

A concrete example is the aa-status program supplied with the AppArmor mandatory access control tool:

   Upon exiting, aa-status will set its return value to the
   following values:
   0   if apparmor is enabled and policy is loaded.
   1   if apparmor is not enabled/loaded.
   2   if apparmor is enabled but no policy is loaded.
   3   if the apparmor control files aren't available under
       /sys/kernel/security/.
   4   if the user running the script doesn't have enough
       privileges to read the apparmor control files.

(I'm sure there are more widely-spread programs with this behavior, but I know this one well. :)

Solution 3

I suspect it's just following the Linux / Unix standard for returning 0 on success.

Does it really say "1" is false and "0" is true?

Solution 4

It may very well be a mistake on the original author, however the notion that 1 is true and 0 is false is not a universal concept. In shell scripting 0 is returned for success, and any other number for failure. In other languages such as Ruby, only nil and false are considered false, and any other value is considered true, so in Ruby both 1 and 0 would be considered true.

Solution 5

There's no good reason for 1 to be true and 0 to be false; that's just the way things have always been notated. So from a logical perspective, the function in your API isn't "wrong", per se.

That said, it's normally not advisable to work against the idioms of whatever language or framework you're using without a damn good reason to do so, so whoever wrote this function was probably pretty bone-headed, assuming it's not simply a bug.

View more solutions

Comments

[aelveborn]

Seems like it, but, that'd be a pretty brain-dead application of the standard. IsWhatever is obliged to return 0 if it is not Is, otherwise, how are you going to say if(!IsWhatever()){}?

[Ben]

To clarify - it says returns 0 if equal, 1 if not equal. Since it's an is_equals function, I took the two as synonymous.

[MaddTheSane]

grep is a good one. “Normally, exit status is 0 if selected lines are found and 1 otherwise. But the exit status is 2 if an error occurred.”

[Ben]

Thanks, that makes sense. I didn't realize error statuses worked that way. The is_equals case is still weird because it's not associated with any sort of error checking.

[Ben]

Interesting. For this to be the case then there would be two inconsistencies. 1) The is_equals() function should really be called compare(). 2) The function would need to return a negative number. Currently it only returns 0 and 1. Perhaps a previous revision did. Then the question is are 1&2 more likely then the programmer stupidly messing up "C flow control." Perhaps the former since the rest of the functions in the API conventionally return 1 for true and 0 for false.

[Spidey]

Success and failure (error codes) is a very different concept from boolean values. Also, it's only logical for a language that supports boolean types to evaluate valid expressions as true, hence 0 equals true (0 is a valid integer value).

[Chris L]

Many int typed C functions do return 0 when it is a successful result, but that shouldn't be seen as a false result, but rather false error code. There is a logic to it. It is more internally consistent than it might appear. Though, I don't know if seeing it as "error is false" is the intended mental model they were going for, or just how I'm interpreting it...

[unhammer]

"there is a single case where a program succeeds but there could be dozens of reasons why a program fails" – also known as the Anna Karenina principle :)

[sarnold]

@unhammer, hehe, that's great! Thanks

[jinawee]

0+1=1, FALSE or TRUE is TRUE; 0*1=0, FALSE and TRUE is FALSE; etc is the reason for the notation.

[Martien de Jong]

I have encountered many variants, particularly in Pascal code. Things like -1 false 0 true or -1/1