Receive a message from the socket at a specified address
ssize_t recvfrom( int s,
void * buff,
struct sockaddr * from,
socklen_t * fromlen );
- The descriptor for the socket; see
- A pointer to a buffer where the function can store the message.
- The size of the buffer.
- A combination formed by ORing one or more of the values:
- MSG_OOB — process out-of-band data.
This flag requests receipt of out-of-band data that wouldn't be
received in the normal data stream.
You can't use this flag with protocols that place expedited data at the
head of the normal data queue.
- MSG_PEEK — peek at the incoming message.
This flag causes the receive operation to return data from the
beginning of the receive queue without removing that data
from the queue. Thus, a subsequent receive call will return
the same data.
- MSG_WAITALL — wait for full request or error. This
flag requests that the operation block until the full
request is satisfied. But the call may still return less
data than requested if a signal is caught, if an error or
disconnect occurs, or if the next data to be received is of
a different type than that returned.
- NULL, or a pointer to a sockaddr object
where the function can store the source address of the message.
- A pointer to a socklen_t object that specifies the
size of the from buffer.
The function stores the actual size of the address in this object.
Use the -l socket option to
to link against this library.
The recvfrom() routine receives a message from the
socket, s, whether or not it's connection-oriented.
If from is nonzero, and the socket is
connectionless, the source address of the message is filled
in. The parameter fromlen is a value-result
parameter, initialized to the size of the buffer associated
with from, and modified on return to indicate the
actual size of the stored address.
This routine returns the length of the message on
successful completion. If a message is too long for the
supplied buffer, buf, excess bytes may be discarded depending on
the type of socket that the message is received
If no messages are available at the socket, the receive call
waits for a message to arrive, unless the socket is
ioctl()—in which case
recvfrom() returns -1 is returned and sets the external variable
errno to EWOULDBLOCK. Normally, the
receive calls return any data available, up to the requested
amount, rather than wait for the full amount requested; this
behavior is affected by the socket-level options SO_RCVLOWAT
and SO_RCVTIMEO described in
You can use
to determine when more data is to arrive.
The number of bytes received, or -1 if an error occurs
(errno is set).
- Invalid descriptor s.
- The receive buffer pointer(s) point outside the
process's address space.
- The receive was interrupted by delivery of a signal
before any data was available.
- The socket is associated with a connection-oriented
protocol and hasn't been connected; see connect()
- Either the socket is marked nonblocking and the receive
operation would block, or a receive timeout had been set and
the timeout expired before data was received.