WSAEWOULDBLOCK
The WSAEWOULDBLOCK error, defined in the Winsock API, indicates that a non-blocking socket operation would block if it were performed in a blocking mode. This is a common and expected condition when working with non-blocking sockets.
Meaning
When you set a socket to be non-blocking using ioctlsocket with FIONBIO or by using setsockopt with SO_NONBLOCK, operations like connect, send, or recv might return immediately even if the operation cannot be completed at that exact moment. Instead of blocking the thread until the operation can proceed, these functions return a specific error code:
WSAEWOULDBLOCK(10035): The operation would block.
This error signifies that the requested operation is valid, but the underlying network subsystem cannot fulfill it immediately because it would require waiting. For example, a send operation might return WSAEWOULDBLOCK if the transmit buffer for the socket is full.
Handling WSAEWOULDBLOCK
Proper handling of WSAEWOULDBLOCK is crucial for developing efficient, non-blocking network applications. The typical strategy involves:
- Retrying the Operation: When you receive
WSAEWOULDBLOCK, you should not treat it as a fatal error. Instead, you should queue the operation to be retried later. - Using Event Notification Mechanisms: The most common and efficient way to know when to retry is by using Winsock's event notification mechanisms:
select(): This function allows a process to monitor multiple file descriptors (sockets) for readiness to read, write, or exceptional conditions. Whenselect()indicates that a socket is ready for writing (e.g., its transmit buffer has space), you can retry yoursendoperation. Similarly, readiness for reading implies data is available to be received.WSAEventSelect(): This function associates a Winsock event object with a socket and specifies the network events for which you want to be notified. When an event occurs, the associated event object is signaled, and you can then perform the appropriate operation (e.g., retry asendif the event isFD_WRITE).- Overlapped I/O with Completion Ports: For high-performance scenarios, asynchronous I/O using
WSAOVERLAPPEDstructures and I/O Completion Ports (IOCP) is the preferred method. In this model, the system notifies you via a completion port when an I/O operation (which can be initiated as non-blocking) has completed, allowing you to process the results.
Example Scenario (Conceptual)
Consider a non-blocking send operation:
int bytesSent = send(socket, buffer, bufferSize, 0);
if (bytesSent == SOCKET_ERROR) {
int error = WSAGetLastError();
if (error == WSAEWOULDBLOCK) {
// The buffer is full. Don't block.
// Schedule this send operation to be retried later
// when the socket becomes writable.
printf("Send would block. Retrying later.\n");
} else {
// Handle other potential errors.
printf("Send failed with error: %d\n", error);
}
} else {
// Data sent successfully.
printf("%d bytes sent.\n", bytesSent);
}
In a real application, the "Schedule this send operation..." part would involve using select(), WSAEventSelect(), or IOCP to monitor the socket's writability.
Related Functions
connect()send()recv()ioctlsocket()setsockopt()select()WSAEventSelect()WSAOVERLAPPEDWSAGetLastError()