recvmsg - receive a message from a socket
cc [ flag ... ] file ... -lxnet [ library ... ] #include <sys/socket.h> ssize_t recvmsg(int socket, struct msghdr *message, int flags);
The recvmsg() function receives 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 receives messages from unconnected or connected sockets and returns the length of the message.
The recvmsg() function returns the total length of the message. For message-based sockets such as SOCK_DGRAM and SOCK_SEQPACKET, the entire message must 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 are discarded, and MSG_TRUNC is 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 is returned to the user as soon as it becomes available, and no data is discarded.
If the MSG_WAITALL flag is not set, data will 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() blocks 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 fails and sets errno to EAGAIN or EWOULDBLOCK.
In the msghdr structure, defined in socket.h(3HEAD), 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_control and msg_controllen members specify a buffer to receive ancillary data sent along with a message. Ancillary data consists of a sequence of pairs. Each pair is composed of a cmsghdr structure followed by a data array. The cmsghdr structure, defined in socket.h(3HEAD), contains descriptive information which allows an application to correctly parse data. The data array contains the ancillary data message.
If ancillary data is not transferred, msg_control is set to NULL and msg_controllen is set to 0.
The msg_iov and msg_iovlen fields of the msghdr structure are used to specify where the received data will be stored. msg_iov points to an array of iovec structures. The msg_iovlen must 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.
If the SO_TIMESTAMP option has been enabled through setsockopt(), then a struct timeval is returned following the cmsghdr, and the cmsg_len field of the cmsghdr indicates the size of the struct timeval.
On successful completion, the msg_flags member of the message header is the bitwise-inclusive OR of all of the following flags that indicate conditions detected for the received message:
The function takes the following arguments:
The select(3C) and poll(2) functions can be used to determine when data is available to be received.
Upon successful completion, recvmsg() returns the length of the message in bytes. If no messages are available to be received and the peer has performed an orderly shutdown, recvmsg() returns 0. Otherwise, -1 is returned and errno is set to indicate the error.
The recvmsg() function will fail if:
The recvmsg() function may fail if:
See attributes(5) for descriptions of the following attributes:
poll(2), recv(3XNET), recvfrom(3XNET), select(3C), send(3XNET), sendmsg(3XNET), sendto(3XNET), setsockopt(3XNET), shutdown(3XNET), socket(3XNET), socket.h(3HEAD), attributes(5), standards(5)
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Created 1996-2021 by Maxim Chirkov
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