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
The recvfrom() 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
Peeks at an incoming message. The data is
treated as unread and the next recvfrom() or similar function shall
still return this data.
Requests out-of-band data. The significance
and semantics of out-of-band data are protocol-specific.
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.
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 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
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
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 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 socket argument is not a valid file
A connection was forcibly closed by a peer.
A signal interrupted recvfrom() before any data was
The MSG_OOB flag is set and no out-of-band data is
A receive is attempted on a connection-mode socket that is
The socket argument does not refer to a socket.
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
Insufficient resources were available in the system to
perform the operation.
Insufficient memory was available to fulfill the request.
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