[gnutls-dev] Gnutls4win: Problem with custom push/pull functions, errno and Visual Studio (Workaround included)

[Tim Kosse]

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I encountered some problems while trying to use custom push/pull
functions in a program compiled with Visual Studio.

Just setting errno to EAGAIN and returning -1 caused gnutls to fail with
GNUTLS_E_PUSH_ERROR.

I found out that libgnutls-13.dll is linked against msvcrt.dll, whereas
visual studio programs use a different runtime library (msvcr71.dll for
example). These libraries bring along their own version of errno.

So the errno inside the program is different from the one used in GnuTLS.

A workaround is to load the correct errno from msvcrt.dll:

        HMODULE crtdll = LoadLibrary(_T("msvcrt.dll"));
        typedef int* (*t_errno)(void);
        t_errno pErrno = (t_errno)GetProcAddress(crtdll, "_errno");
        *pErrno() = EAGAIN;
        FreeLibrary(crtdll);

Would it be possible to add a function like for example void
gnutls_transport_set_errno(int), which does nothing else than setting
errno to the provided value?

Regards,
Tim Kosse
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