[Python-Dev] checking "errno" for math operaton is safe to determine the error status?

Christian Heimes christian at python.org
Thu Apr 11 08:23:48 EDT 2019

On 11/04/2019 11.45, Xin, Peixing wrote:
> Hi, Math experts:
> Looking at the codes below, for many math operations, CPython is checking errno to determine the error status even though the math function returns normal value back. Is it a safe solution? From the description here http://man7.org/linux/man-pages/man3/errno.3.html and https://wiki.sei.cmu.edu/confluence/pages/viewpage.action?pageId=87152351, it looks apis probably set the errno when normal result is returned. Or being a side effect by calling other APIs in the implementation. In this situation, CPython's math operation might raise exceptions however in fact the result is correct.
> https://github.com/python/cpython/blob/master/Modules/mathmodule.c#L956
> https://github.com/python/cpython/blob/master/Modules/mathmodule.c#L864

This is safe because all places first set errno to 0. Errno is a thread
local variable, so other threads cannot influence the variable during
the calls.

This is one of the many quirks that Mark has implemented for platforms
bugs in various libm.


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