ssize_t recvmsg(int socket, struct msghdr *message,
The recvmsg() function shall receive a message from a connection-mode or connectionless-mode socket. It is normally used with connectionless-mode sockets because it permits the application to retrieve the source address of received data.
The recvmsg() function takes the following arguments:
The recvmsg() function shall receive messages from unconnected or connected sockets and shall return the length of the message.
The recvmsg() function shall return the total length of the message. For message-based sockets, such as SOCK_DGRAM and SOCK_SEQPACKET, the entire message shall be 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 shall be 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 shall be ignored. In this case, data shall be returned to the user as soon as it becomes available, and no data shall be discarded.
If the MSG_WAITALL flag is not set, data shall be returned only up to the end of the first message.
If no messages are available at the socket and O_NONBLOCK is not set on the socket's file descriptor, recvmsg() shall block until a message arrives. If no messages are available at the socket and O_NONBLOCK is set on the socket's file descriptor, the recvmsg() function shall fail and set errno to [EAGAIN] or [EWOULDBLOCK].
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 shall be 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 shall be 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.
Upon successful completion, the msg_flags member of the message header shall be the bitwise-inclusive OR of all of the following flags that indicate conditions detected for the received message:
Upon successful completion, recvmsg() shall return the length of the message in bytes. If no messages are available to be received and the peer has performed an orderly shutdown, recvmsg() shall return 0. Otherwise, -1 shall be returned and errno set to indicate the error.
The recvmsg() function shall fail if:
The socket's file descriptor is marked O_NONBLOCK and no data is waiting to be received; or MSG_OOB is set and no out-of-band data is available and either the socket's file descriptor is marked O_NONBLOCK or the socket does not support blocking to await out-of-band data.
The recvmsg() function may fail if:
The following sections are informative.
The select() and poll() functions can be used to determine when data is available to be received.
poll() , recv() , recvfrom() , select() , send() , sendmsg() , sendto() , shutdown() , socket() , the Base Definitions volume of IEEE Std 1003.1-2001, <sys/socket.h>