recvfrom - receive a message from a socket
cc [ flag ... ] file ... -lxnet [ library ... ]
ssize_t recvfrom(int socket, void *restrict buffer, size_t length,
int flags, struct sockaddr *restrict address,
socklen_t *restrict address_len);
The recvfrom() 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
The function takes the following arguments:
Specifies the socket file descriptor.
Points to the buffer where the message should be
Specifies the length in bytes of the buffer pointed to by
the buffer argument.
Specifies the type of message reception. Values of this
argument are formed by logically OR'ing zero or more of the following values:
Peeks at an incoming message. The data is treated as
unread and the next recvfrom() or similar function will still return
Requests out-of-band data. The significance and semantics
of out-of-band data are protocol-specific.
Requests that the function block until the full amount of
data requested can be returned. The function may return a smaller amount of
data if a signal is caught, if the connection is terminated, if MSG_PEEK was
specified, or if an error is pending for the socket.
A null pointer, or points to a sockaddr structure
in which the sending address is to be stored. The length and format of the
address depend on the address family of the socket.
Specifies the length of the sockaddr structure
pointed to by the address argument.
The recvfrom() function returns the length of the message
written to the buffer pointed to by the buffer argument. 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 buffer, and MSG_PEEK is not set in the
flags argument, the excess bytes are discarded. 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.
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 is
stored in the sockaddr structure pointed to by the address
argument, and the length of this address is 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 will be
If the address argument is not a null pointer and the
protocol does not provide the source address of messages, then 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() blocks until a
message arrives. If no messages are available at the socket and
O_NONBLOCK is set on the socket's file descriptor, recvfrom()
fails and sets errno to EAGAIN or EWOULDBLOCK.
The select(3C) and poll(2) functions can be used to determine when
data is available to be received.
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 the
function returns −1 and sets errno to indicate the error.
In addition to the errors documented below, an asynchronous error generated by
the underlying socket protocol may be returned. For the full list of errors,
please see the corresponding socket protocol manual page. For example, for a
list of TCP errors, please see tcp(4P).
The recvfrom() function will 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
The socket argument is not a valid file
A connection was forcibly closed by a peer.
The buffer, address or address_len
parameter can not be accessed or written.
A signal interrupted recvfrom() before any data
The MSG_OOB flag is set and no out-of-band data is
A receive is attempted on a connection-mode socket that
is not connected.
The socket argument does not refer to a
The specified flags are not supported for this socket
The connection timed out during connection establishment,
or due to a transmission timeout on active connection.
The recvfrom() function may fail if:
An I/O error occurred while reading from or writing to
the file system.
Insufficient resources were available in the system to
perform the operation.
Insufficient memory was available to fulfill the
There were insufficient STREAMS resources available for
the operation to complete.
See attributes(7) for descriptions of the following attributes:
poll(2), sockaddr(3SOCKET), recv(3XNET),
recvmsg(3XNET), select(3C) send(3XNET),
sendmsg(3XNET), sendto(3XNET), shutdown(3XNET),
socket(3XNET), tcp(4P), attributes(7),