[illumos-Developer] Review for 89: Bug uu_list_find() leaves error unset when successfully returning NULL

[Joerg Schilling]

Eric Schrock <eric.schrock at delphix.com> wrote:

> Ugh, not the most pleasant API design, but nothing that can really be
> changed, now.  A simple test case might be:
>
> 1. Create two lists, one with a NULL comparison routine, and a normal one.
> 2. Try to search in the one with a NULL comparison routine (sets libuutil
> error)
> 3. Try to search for a non-existent entry in the normal one (should have a
> zero errno)
>
> Otherwise looks OK to me.  This reminds me of strtol() and friends, where
> most consumers are forced to preface it with 'errno = 0' to distinguish a
> zero value from an invalid entry.  This seems like a better solution than
> exporting that complexity to the user.

The idea behind this concept is to never change errno in case there was no error.

This allows to call any such function from inside other functions without being 
forced to save and restore the previous errno. Note that there are many 
situations where you like to return a previous "better" errno in case something 
fails.

If you ever see a function where you believe you need to clear errno before 
calling the function, you found an ill-designed function. Functions should be 
designed in a way that allows to recognize whether there was an error or not 
without looking at errno. Errno is only needed to find out _which_ error 
happened.

Unfortunately, there is no documentation for uu_list_find(), so it is hard to 
judge....

Jörg

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