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Date: Sun, 19 Feb 2023 08:56:54 +0100
To: "Levi Morrison" <internals@lists.php.net>
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Subject: Re: [PHP-DEV] What's the purpose of zend_result?
From: nikita.ppv@gmail.com ("Nikita Popov")

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On Sun, Feb 19, 2023, at 08:31, Max Kellermann wrote:
> Hi,
> 
> I've done a bit of refactoring work around code using "zend_result",
> but I keep wondering why it even exists.
> 
> It was added in 1999 by commit 573b46022c46 in a huge 16k line commit
> (as macros, converted to enum in 2012 by commit e3ef84c59bf).
> 
> In 1999, C99 was brand new, and the "bool" type had just been
> introduced to the C language - yes, C was 18 years old when a native
> boolean type was added - but PHP didn't switch to C99 for another 19
> years later (b51a99ae358b).
> 
> C had a long history of abusing "int" as boolean type, wasting 2 or 4
> bytes for something that could have easily fit in 1 byte.  As if that
> wasn't obscure enough, POSIX used 0 for success and -1 for error (and
> missed the chance to return error codes, and instead added a global
> variable for that which caused more headaches).  (And don't get me
> started on the horrible strcmp() return value.)  Returning errors in C
> is a huge obscure and inconsistent mess; every function does it
> differently.
> 
> This is PHP's original definition:
> 
> #define SUCCESS 0
> #define FAILURE -1 /* this MUST stay a negative number, or it may effect functions! */
> 
> This appears to follow the POSIX school of bad error return values.
> There's a comment which makes the thing even more obscure.
> 
> Really, why does it need to be negative?
> 
> Why does it imitate POSIX instead of using a boolean? (i.e. "int" and
> non-zero for success for old-schoolers)
> 
> The obvious way to indicate success or failure (without giving details
> about the nature of the failure) would be to just return "bool".  With
> C99, any discussion on "0 or 1" vs "-1 or 0" is obsolete - there is
> now a canonical boolean type that should be used.
> 
> Of course, I know already that getting rid of "zend_result" in favor
> of "bool" would be a major API breakage that requires careful
> refactoring of almost all extensions.
> 
> I just want to understand why "zend_result" was ever invented and why
> it uses those surprising integer values.  The commit message and that
> code comment doesn't explain it.
> 
> Rephrased: do you consider it a worthy goal to eventually get rid of
> "zend_result", or do you believe it's good API design that should stay
> forever?
> 
> (Yet again I wish PHP was fully C++ - unlike C, C++ has strongly-typed
> enums which cannot be casted implicitly to "int" or "bool"; that makes
> refactoring a lot easier and safer.)
> 
> Max

Any type that has only two values is isomorphic to a boolean. However, for us humans, not all two-valued types are semantically equivalent.

If you have a function like zend_hash_exists(), true and false are directly meaningful values.

If you have a function like zend_stream_open_function(), SUCCESS and FAILURE are directly meaningful values.

Now, if you make zend_stream_open_function() return a boolean instead, it's no longer clear what the return value means. Does a true return value mean that the stream was opened successfully, or that it failed?

For a PHP programmer, that might sound silly -- of course true means success. However, in C the common error reporting convention is actually the other way around: Non-zero return values indicate failure. This means that false indicates success and true indicates failure. (I'm not kidding you -- I'm literally working on a project that uses boolean return values with this convention right now.)

The current guideline for use of bool and zend_result in php-src is that bool is an appropriate return value for "is" or "has" style functions, which return a yes/no answer. zend_result is an appropriate return value for functions that perform some operation that may succeed or fail.

I think that's a pretty reasonable state of things, and don't think there is a need to change it.

Regards,
Nikita
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