The Windows Sockets ioctlsocket function controls the mode of a socket.

int ioctlsocket (


long cmd,

u_long FAR* argp




[in] A descriptor identifying a socket.


[in] The command to perform on the socket s.


[in/out] A pointer to a parameter for cmd.


This routine can be used on any socket in any state. It is used to get or retrieve operating parameters associated with the socket, independent of the protocol and communications subsystem. Here are the supported commands and their semantics:


Enable or disable nonblocking mode on socket s. argp points at an unsigned long, which is nonzero if nonblocking mode is to be enabled and zero if it is to be disabled. When a socket is created, it operates in blocking mode (that is, nonblocking mode is disabled). This is consistent with BSD sockets.

The WSAAsyncSelect or WSAEventSelect routine automatically sets a socket to nonblocking mode. If WSAAsyncSelect or WSAEventSelect has been issued on a socket, then any attempt to use ioctlsocket to set the socket back to blocking mode will fail with WSAEINVAL. To set the socket back to blocking mode, an application must first disable WSAAsyncSelect by calling WSAAsyncSelect with the lEvent parameter equal to zero, or disable WSAEventSelect by calling WSAEventSelect with the lNetworkEvents parameter equal to zero.


Determine the amount of data which can be read atomically from socket s. argp points to an unsigned long in which ioctlsocket stores the result. If s is stream oriented (for example, type SOCK_STREAM), FIONREAD returns an amount of data which can be read in a single recv; this may or may not be the same as the total amount of data queued on the socket. If s is message oriented (for example, type SOCK_DGRAM), FIONREAD returns the size of the first datagram (message) queued on the socket.


Determine whether or not all out-of-band data has been read. (See section Windows Sockets 1.1 Blocking Routines & EINPROGRESS for a discussion of this topic.) This applies only to a socket of stream style (for example, type SOCK_STREAM) which has been configured for in-line reception of any out-of-band data (SO_OOBINLINE). If no out-of-band data is waiting to be read, the operation returns TRUE. Otherwise, it returns FALSE, and the next recv or recvfrom performed on the socket will retrieve some or all of the data preceding the "mark"; the application should use the SIOCATMARK operation to determine whether any remains. If there is any normal data preceding the "urgent" (out of band) data, it will be received in order. (Note that a recv or recvfrom will never mix out-of-band and normal data in the same call.) argp points to an unsigned long in which ioctlsocket stores the boolean result.


This function is a subset of ioctl as used in Berkeley sockets. In particular, there is no command which is equivalent to FIOASYNC, while SIOCATMARK is the only socket-level command which is supported.

Return Values

Upon successful completion, the ioctlsocket returns zero. Otherwise, a value of SOCKET_ERROR is returned, and a specific error code can be retrieved by calling WSAGetLastError.

Error Codes

A successful WSAStartup must occur before using this function.
The network subsystem has failed.
The descriptor s is not a socket.
The argp argument is not a valid part of the user address space.

See Also

, setsockopt, socket, WSAAsyncSelect, WSAEventSelect, WSAIoctl

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