Is it common for 0 to mean ‘true’ and 1 to mean ‘false’ in a C API?

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?

Underage

I don't really understand your question.. – Scagliola

Sometimes 0 indicates success and other numbers specify error codes. But I understand why this is confusing since the function returns true/false. – Refectory

0=false, 1=true, and everything else = true (For anyone looking for a quick answer to the question in the title). Source: § 4.12 – Gossett

Only only look as far as something like strcmp (string compare) in the C standard library to see an example where a general "1 = true" assertion doesn't make sense. – Tedric

> Is the coder of the API making an error? No. – Svetlana

Actually it really depends on a language, there's no such a thing that says: 1 has to be true and 0 has to be false. In languages such as js, -1, 1, 'string' and I don't know what else is considered true, on the other hand, 0, undefined, null, NaN, 'empty string' is considered false. – Svetlana

Answer 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).

Misdirection

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. – Underage

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... – Valois

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

Answer 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. :)

Laundress

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.” – Norfolk

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. – Underage

"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 :) – Spirula

@unhammer, hehe, that's great! Thanks – Laundress

Answer 3

Simple Answer

0 = false 
1 = true

Stillman

It is simple to understand that 0 means nothing and 1 means something. – Stillman

Answer 4

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?

Rathbone

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()){}? – Verity

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. – Underage

Answer 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.

Rosenkranz

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

Answer 6

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.

Shorten

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). – Farica

Answer 7

I'm not sure if I'm answering the question right, but here's a familiar example:

The return type of GetLastError() in Windows is nonzero if there was an error, or zero otherwise. The reverse is usually true of the return value of the function you called.

Scagliola

Answer 8

Generally we think that 0 is false and 1 is true

Hoye

This answer does not seem to contain any information that other answers do not already have – Glaser

This does not provide an answer to the question. Once you have sufficient reputation you will be able to comment on any post; instead, provide answers that don't require clarification from the asker. - From Review – Shrubby