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

[Xin, Peixing]

Thanks for your explanation, Christian. Actually my question is not about thread safe or the original value 0 on errno. Probably I didn't express the point clearly. To be more clear, let me take expm1 as an example below.

On certain platform, expm1() is implemented as exp() minus 1. To calculate expm1(-1420.0), that will call exp(-1420.0) then substract 1. You know, exp(-1420.0) will underflow to zero and errno is set to ERANGE. As a consequence the errno keeps set there when expm1() returns the correct result -1. So for this situation, CPthon's api is_error() will raise overflow unexpectedly. Whose bug should it be scoped to? A bug of the platform? Isn't errno allowed to be set when calculation gets normal result? 

Thanks,
Peixing


-----Original Message-----
From: Christian Heimes [mailto:christian at python.org] 
Sent: Thursday, April 11, 2019 8:24 PM
To: Xin, Peixing; python-dev at python.org; Mark Dickinson
Subject: Re: checking "errno" for math operaton is safe to determine the error status?

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.

Christian