Re: strerror vs. threads

[Eric Blake]

On 05/24/2011 12:52 PM, Simon Josefsson wrote:
>> POSIX explicitly allows strerror to use a static buffer, and that's
>> _exactly_ what gnulib's implementation does on out-of-range input.
>> Which means that "Unknown error (-1)" of thread 1 and "Unknown error
>> (-2)" of thread 2 are calling sprintf on the same memory at the same
>> time, and you will get indeterminate results.
> 
> Which begs the question why the error messages needs to be different for
> different unknown errors?  Is that required by POSIX?

Not required, but heavily recommended, and guaranteed by several common
platforms.  And since "Error: " is much more confusing than "Error:
Unknown error -1", especially at tracking down a bug at why errno is set
to some weird value, it was worth documenting that we guarantee that
behavior as one of the points of the gnulib strerror replacement.

If you don't ever pass out-of-range errno values to strerror, then you
probably don't care about thread-safety of the out-of-range buffer, but
there is still the question of whether all existing implementations are
thread-safe even on in-range errno values.

-- 
Eric Blake   address@hidden    +1-801-349-2682
Libvirt virtualization library http://libvirt.org

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