Windows API Reference

Winsock Core Error Handling

Effective error handling is crucial for robust network applications developed using the Windows Sockets API (Winsock). This section covers the mechanisms Winsock provides for detecting, reporting, and diagnosing errors that can occur during socket operations.

Understanding Winsock Error Codes

Winsock errors are typically reported using specific error codes. The most common way to retrieve the last error code for a socket operation is by calling the WSAGetLastError function.

int errorCode = WSAGetLastError();

This function returns a SOCK_ERR value, which is a system-dependent error code. These codes are defined in winsock2.h and can be mapped to human-readable strings using the FormatMessage API with the FORMAT_MESSAGE_FROM_SYSTEM flag.

Common Winsock Error Codes

Here are some frequently encountered Winsock error codes:

Handling Specific Error Scenarios

WSAEINPROGRESS

This error indicates that a blocking Winsock operation is currently in progress. If you are using non-blocking sockets and encounter this error, it means the operation is ongoing and you should retry later.

WSAEWOULDBLOCK

Similar to WSAEINPROGRESS, this error signifies that a non-blocking operation cannot be completed immediately. The socket is not ready for the requested operation. You should typically wait for the socket to become ready (e.g., using select or WSAAsyncSelect) before retrying.

WSAECONNRESET

This is a common error on TCP connections, indicating that the remote end has closed the connection abruptly. Your application should handle this by closing its end of the connection and potentially attempting to re-establish it.

WSAEADDRINUSE

This error occurs when an attempt is made to bind a socket to an address and port that is already in use by another socket. This can happen if a previous instance of your application terminated abnormally without releasing the socket or if another process is already using that address/port combination.

Best Practices for Error Handling

• Always check the return value of Winsock functions. A return value of SOCKET_ERROR (or LB_ERR for some message loop functions) usually indicates an error.
• Immediately call WSAGetLastError after a Winsock function returns an error to get the specific error code.
• Log errors with descriptive messages, including the error code and its symbolic name if available.
• Implement retry logic for transient errors like WSAEINPROGRESS and WSAEWOULDBLOCK, but be mindful of infinite loops.
• Gracefully handle connection-oriented errors like WSAECONNRESET and WSAETIMEDOUT by closing connections and informing the user or retrying.
• For address-related errors like WSAEADDRINUSE, provide informative feedback to the user or implement alternative address/port selection.
• Consider using Winsock's asynchronous notification mechanisms (e.g., WSAAsyncSelect) to avoid blocking your application's main thread while waiting for operations to complete or for network events to occur.