[PATCH 0/3] Fix wrong assumption about errno

[Carlos O'Donell]

On 12/21/2017 07:40 PM, Zack Weinberg wrote:
> On Thu, Dec 21, 2017 at 2:54 PM, Aurelien Jarno <aurelien@aurel32.net> wrote:
>> These 3 patches fixes the wrong assumption that a successful function
>> does not change errno. POSIX explicitly says that applications should
>> check errno only after failure (or if the function specification
>> provides additional scenarios where it has a defined value).
> 
> I like these changes in general, but there are a few specific cases
> where functions _are_ guaranteed not to change the value of errno on
> success, and application code relies on that.  The one I remember off
> the top of my head is the strto* family, where all possible return
> values _could_ be the result of a successful parse, so you have to set
> errno to 0 beforehand and then check it afterward.  Please make sure
> that your changes do not imply this is not the case.

The CERT coding rules[1] cover the list. What you mention is called in-band
error return, where the function error return cannot be disambiguated from a
correct result.

The manual clarification is more correct than we had before. We say that
no function will set errno to zero, but may set it to non-zero when they
succeed. This does not preclude what you are talking about with functions
that have in-band error return.

-- 
Cheers,
Carlos.

https://wiki.sei.cmu.edu/confluence/pages/viewpage.action?pageId=87152351