Error codes -- errno, h_errno & WSAGetLastError()

Error codes set by the Windows Sockets implementation are NOT made available via the errno variable. Additionally, for the getXbyY() class of functions, error codes are NOT made available via the h_errno variable. Instead, error codes are accessed by using the WSAGetLastError() API. This function is provided in Windows Sockets as a precursor (and eventually an alias) for the Win32 function GetLastError(). This is intended to provide a reliable way for a thread in a multi-threaded process to obtain per-thread error information.

For compatibility with BSD, an application may choose to include a line of the form:

#define errno WSAGetLastError()

This will allow networking code which was written to use the global errno to work correctly in a single-threaded environment. There are, obviously, some drawbacks. If a source file includes code which inspects errno for both socket and non-socket functions, this mechanism cannot be used. Furthermore, it is not possible for an application to assign a new value to errno. (In Windows Sockets the function WSASetLastError() may be used for this purpose.)

TYPICAL BSD STYLE:

r = recv(...);
if (r == -1
    && errno == EWOULDBLOCK)
        {...}

PREFERRED STYLE:

r = recv(...);
if (r == -1       /* (but see below) */
    && WSAGetLastError() == EWOULDBLOCK)
        {...}

Although error constants consistent with 4.3 Berkeley Sockets are provided for compatibility purposes, applications should, where possible, use the "WSA" error code definitions. For example, a more accurate version of the above source code fragment is:

r = recv(...);
if (r == -1
    && WSAGetLastError() == WSAEWOULDBLOCK)
        {...}