Re: Best Practices For Thread-Safe errno

Le Chaud Lapin <>
Sun, 11 Oct 2009 14:56:40 CST
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 Storage.
    __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.

I realized that every stack-based machine already has "thread-local"
storage: the stack itself.

We might propose a new C/C++ keyword:

_error: "an int l-value accessible at anytime"

Each call frame would include space for _error.

When stack expands due to function-call, _error would be copied into
the correct position in the activated frame. When the stack contracts
due to function-return, _error would be copied from its position in
the deactivating frame into the corresponding position of the
activating frame.

Platforms where space/time overhead for _error is too great could have
compiler option to disable it, just as we do for exceptions, etc.

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.

-Le Chaud Lapin-

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