recvfrom(3N-xs) (X/OPEN SOCKETS) recvfrom(3N-xs)
NAME
recvfrom - receive a message from a socket
SYNOPSIS
cc [flag ...] file ... -lxnet [library ...]
#include <sys/socket.h>
ssizet recvfrom(int socket, void *buffer, sizet length, int flags,
struct sockaddr *address, sizet *addresslen);
DESCRIPTION
The recvfrom function receives a message from a connection-oriented or
connectionless socket. It is normally used with connectionless sockets
because it permits the application to retrieve the source address of
received data.
The function takes the following arguments:
socket Specifies the socket file descriptor.
buffer Points to the buffer where the message should be stored.
length Specifies the length in bytes of the buffer pointed to by
the buffer argument.
flags Specifies the type of message reception. Values of this
argument are formed by logically ORing zero or more of
the following values:
MSGPEEK
Peeks at an incoming message. The data is treated as
unread and the next recvfrom or similar function
will still return this data.
MSGOOB
Requests out-of-band data. The significance and
semantics of out-of-band data are protocol-specific.
MSGWAITALL
Requests that the function block until the full
amount of data requested can be returned. The func-
tion may return a smaller amount of data if a signal
is caught, the connection is terminated, or an error
is pending for the socket.
address 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.
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recvfrom(3N-xs) (X/OPEN SOCKETS) recvfrom(3N-xs)
addresslen 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 SOCKDGRAM and SOCKSEQPACKET, the entire message must be read
in a single operation. If a message is too long to fit in the supplied
buffer, and MSGPEEK is not set in the flags argument, the excess
bytes are discarded. For stream-based sockets such as SOCKSTREAM,
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 MSGWAITALL 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 argu-
ment, and the length of this address is stored in the object pointed
to by the addresslen argument.
If the actual length of the address is greater than the length of the
supplied sockaddr structure, the stored address will be truncated.
If the address argument is not a null pointer and the protocol does
not provide the source address of messages, the the value stored in
the object pointed to by address is unspecified.
If no messages are available at the socket and ONONBLOCK is not set
on the socket's file descriptor, recvfrom blocks until a message
arrives. If no messages are available at the socket and ONONBLOCK is
set on the socket's file descriptor, recvfrom fails and sets errno to
EWOULDBLOCK or EAGAIN.
RETURN VALUE
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 func-
tion returns -1 and sets errno to indicate the error.
ERRORS
The recvfrom function will fail if:
EBADF The socket argument is not a valid file descriptor.
ECONNRESET A connection was forcibly closed by a peer.
EINTR A signal interrupted recvfrom before any data was
available.
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recvfrom(3N-xs) (X/OPEN SOCKETS) recvfrom(3N-xs)
EINVAL The MSGOOB flag is set and no out-of-band data is
available.
ENOTCONN A receive is attempted on a connection-oriented socket
that is not connected.
ENOTSOCK The socket argument does not refer to a socket.
EOPNOTSUPP The specified flags are not supported for this socket
type.
ETIMEDOUT The connection timed out during connection establish-
ment, or due to a transmission timeout on active con-
nection.
EWOULDBLOCK or EAGAIN
The socket's file descriptor is marked ONONBLOCK and
no data is waiting to be received; or MSGOOB is set
and no out-of-band data is available and either the
socket's file descriptor is marked ONONBLOCK or the
socket does not support blocking to await out-of-band
data.
The recvfrom function may fail if:
EIO An I/O error occurred while reading from or writing to
the file system.
ENOBUFS Insufficient resources were available in the system to
perform the operation.
ENOMEM Insufficient memory was available to fulfill the
request.
ENOSR There were insufficient STREAMS resources available
for the operation to complete.
APPLICATION USAGE
The select and poll functions can be used to determine when data is
available to be received.
SEE ALSO
poll(2), read(2), write(2), select(3C), recv(3N-xs), recvmsg(3N-xs),
send(3N-xs), sendmsg(3N-xs), sendto(3N-xs), shutdown(3N-xs),
socket(3N-xs).
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