ioctlsocket

ioctlsocket controls the mode of the socket. It can be used to enable or disable non‑blocking mode, retrieve the number of bytes pending in the receive buffer, and more.

Syntax

int ioctlsocket(
    SOCKET s,
    long   cmd,
    u_long *argp
);

Parameters

Name	Description
`s`	Descriptor identifying the socket.
`cmd`	Control operation to perform. Common values: `FIONBIO`: Enable/disable non‑blocking mode. `FIONREAD`: Return number of bytes pending.
`argp`	Pointer to a variable that holds the argument or receives the result, depending on `cmd`.

Return Value

Returns 0 on success. On failure, returns -1 and sets WSAGetLastError() to indicate the error.

Remarks

When cmd is FIONBIO, *argp is non‑zero to enable non‑blocking mode, zero to disable.
When cmd is FIONREAD, *argp receives the number of bytes that can be read without blocking.
Use WSAGetLastError() to retrieve error codes such as WSAEINVAL, WSAENOTSOCK, etc.
The function is thread‑safe.

Example

C++

#include <winsock2.h>
#include <stdio.h>

int main() {
    WSADATA wsa;
    SOCKET s;
    u_long mode = 1; // non‑blocking

    WSAStartup(MAKEWORD(2,2), &wsa);
    s = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
    if (s == INVALID_SOCKET) return 1;

    // enable non‑blocking mode
    if (ioctlsocket(s, FIONBIO, &mode) != 0) {
        printf("ioctlsocket failed: %d\n", WSAGetLastError());
        return 1;
    }

    // ... use the socket ...

    closesocket(s);
    WSACleanup();
    return 0;
}

#include <winsock2.h>
#include <iostream>

int main() {
    WSADATA wsa;
    SOCKET s;
    u_long mode = 1; // non‑blocking

    if (WSAStartup(MAKEWORD(2,2), &wsa) != 0) {
        std::cerr << "WSAStartup failed\n";
        return 1;
    }

    s = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
    if (s == INVALID_SOCKET) {
        std::cerr << "socket failed: " << WSAGetLastError() << '\n';
        return 1;
    }

    // enable non‑blocking mode
    if (ioctlsocket(s, FIONBIO, &mode) != 0) {
        std::cerr << "ioctlsocket failed: " << WSAGetLastError() << '\n';
        return 1;
    }

    // ... socket operations ...

    closesocket(s);
    WSACleanup();
    return 0;
}