ssize_t recvfrom(int socket, void *restrict buffer,
int flags, struct sockaddr *restrict address,
socklen_t *restrict address_len);
The recvfrom() 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 recvfrom() function takes the following arguments:
The recvfrom() function shall return the length of the message written to the buffer pointed to by the buffer argument. For message-based sockets, such as SOCK_RAW, 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 buffer, and MSG_PEEK is not set in the flags argument, the excess bytes shall be discarded. 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.
Not all protocols provide the source address for messages. If the address argument is not a null pointer and the protocol provides the source address of messages, the source address of the received message shall be stored in the sockaddr structure pointed to by the address argument, and the length of this address shall be stored in the object pointed to by the address_len argument.
If the actual length of the address is greater than the length of the supplied sockaddr structure, the stored address shall be truncated.
If the address argument is not a null pointer and the protocol does not provide the source address of messages, the value stored in the object pointed to by address is unspecified.
If no messages are available at the socket and O_NONBLOCK is not set on the socket's file descriptor, recvfrom() 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, recvfrom() shall fail and set errno to [EAGAIN] or [EWOULDBLOCK].
Upon successful completion, recvfrom() 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, recvfrom() shall return 0. Otherwise, the function shall return -1 and set errno to indicate the error.
The recvfrom() 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 recvfrom() 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() , read() , recv() , recvmsg() , select() , send() , sendmsg() , sendto() , shutdown() , socket() , write() , the Base Definitions volume of IEEE Std 1003.1-2001, <sys/socket.h>