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NAME
recvfrom - receive a message from a socket
SYNOPSIS
#include <sys/socket.h>
ssize_t recvfrom(int socket, void *restrict buffer, size_t length,
int flags, struct sockaddr *restrict address,
socklen_t *restrict address_len);
DESCRIPTION
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:
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 OR'ing zero or more of the following values:
MSG_PEEK
Peeks at an incoming message. The data is treated as unread and the next recvfrom()
or similar function shall still return this data.
MSG_OOB
Requests out-of-band data. The significance and semantics of out-of-band data are
protocol-specific.
MSG_WAITALL
On SOCK_STREAM sockets this requests that the function block until the full amount
of data can be returned. The function may return the smaller amount of data if the
socket is a message-based socket, if a signal is caught, if the connection is
terminated, if MSG_PEEK was specified, or if 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.
address_len
Specifies the length of the sockaddr structure pointed to by the address argument.
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].
RETURN VALUE
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.
ERRORS
The recvfrom() function shall fail if:
EAGAIN or EWOULDBLOCK
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.
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.
EINVAL The MSG_OOB flag is set and no out-of-band data is available.
ENOTCONN
A receive is attempted on a connection-mode 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 establishment, or due to a transmission
timeout on active connection.
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.
The following sections are informative.
EXAMPLES
None.
APPLICATION USAGE
The select() and poll() functions can be used to determine when data is available to be
received.
RATIONALE
None.
FUTURE DIRECTIONS
None.
SEE ALSO
poll() , read() , recv() , recvmsg() , select() , send() , sendmsg() , sendto() ,
shutdown() , socket() , write() , the Base Definitions volume of IEEE Std 1003.1-2001,
<sys/socket.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std
1003.1, 2003 Edition, Standard for Information Technology -- Portable Operating System
Interface (POSIX), The Open Group Base Specifications Issue 6, Copyright (C) 2001-2003 by
the Institute of Electrical and Electronics Engineers, Inc and The Open Group. In the
event of any discrepancy between this version and the original IEEE and The Open Group
Standard, the original IEEE and The Open Group Standard is the referee document. The
original Standard can be obtained online at http://www.opengroup.org/unix/online.html .