Re: Best Practices For Thread-Safe errno

Bart van Ingen Schenau <>
Tue, 13 Oct 2009 12:48:09 CST
Le Chaud Lapin wrote:

On Oct 9, 9:31 am, Anders Dalvander <> wrote:

On Oct 8, 1:10 am, Le Chaud Lapin <> wrote:

I would like to know if there is an portable or semi-portable
equivalent of Microsoft's SetLastError():

1. Exceptions.
    throw std::runtime_error("bad things happened")

2. Thread-Local
    __declspec(thread) int myerrno = 0;
    // or __thread int myerrno = 0; in some compilers.
    void seterror(int e)
       myerrno = e;
    int geterror()
       return myerrno;

I am responding to Alan McKenney, Daniel Kr?gler, and Anders Dalvander
here since my response is the same.

While walking in the woods, I was trying to think of a way to obviate
exceptions [since they are not in C] and thread-local storage [since a
CPU might not support multi-threading] for return error.

And then you come up with a proposal that is almost certainly
unacceptable for C and very likely as well for C++.

Don't forget that C is also used on systems that have very severe memory
limitations and adding a hidden cost of sizeof(int) bytes to every
function call will most definitely be a show-stopper for such systems (I
recently worked with a system where I had as much as 125 words of stack
memory available).

And besides that, not everybody agrees with you that if a function
returns a success/failure indication that then the reason for the
failure should be communicated separately, so your proposal also goes
against the spirit of "you don't pay for what you don't use".


This model would be universally consistent, and would allow us to have
a portable "SetLastError()" facility while simultaneously avoiding the
overhead of a function call.

Requiring in the standard that the value of errno is not affected by
what happens in other threads would allow an equally consistent coding
style with an equally portable "SetLastError()".

-Le Chaud Lapin-

Bart v Ingen Schenau
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