recvmsg

The recvmsg function receives a message from a socket and returns the length of the message.

It is normally used with connection-less sockets because it permits the application to retrieve the source address of received data.

For message-based sockets, such as SOCK_DGRAM , the entire message is read in a single operation. If a message is too long to fit in the supplied buffers, and MSG_PEEK is not set in the flags argument, the excess bytes are discarded, and MSG_TRUNC shall be set in the msg_flags member of the msghdr structure.

For stream-based sockets, such as SOCK_STREAM, message boundaries are ignored. In this case, data are returned to the user as soon as it becomes available, and no data are discarded.

If the MSG_WAITALL flag is not set, data are returned only up to the end of the first message.

ssize_t recvmsg(

int socket,

struct msghdr *msg,

int flags

);

Points to a msghdr structure, containing both the buffer to store the source address and the buffers for the incoming message. The length and format of the address depend on the address family of the socket. The msg_flags member is ignored on input, but may contain meaningful values on output.

In the msghdr structure, the msg_name and msg_namelen members specify the source address if the socket is unconnected. If the socket is connected, the msg_name and msg_namelen members are ignored. The msg_name member may be a null pointer if no names are desired or required. The msg_iov and msg_iovlen fields are used to specify where the received data shall be stored. msg_iov points to an array of iovec structures; msg_iovlen is set to the dimension of this array. In each iovec structure, the iov_base field specifies a storage area and the iov_len field gives its size in bytes. Each storage area indicated by msg_iov is filled with received data in turn until all of the received data is stored or all of the areas have been filled.

Specifies the type of message reception. Values of this argument are formed by logically OR'ing zero or more of the following values:

Type	Meaning
MSG_PEEK	Peeks at an incoming message. The data is treated as unread and next recv or similar function shall still return this data.
MSG_OOB	Requests out-of-band data. The significance and semantics of out-of-band data are protocol-specific.
MSG_WAITALL	On SOCK_STREAM sockets this requests that the function block until the full amount of data can be returned. This function may return the smaller amount of data if the socket is a message-based socket, if the connection is terminated, if MSG_PEEK was specified, or if an error is pending for the socket.

Upon successful completion, recvfrom returns the length of the message in bytes. If no messages are available to be received and the peer has performed an orderly shutdown, recvfrom returns 0. Otherwise, -1 is returned and errno set to indicate the error.