8.2 recvfrom
and sendto Functions
These two functions are similar to the standard
read and write functions, but three additional
arguments are required.
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#include <sys/socket.h>
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ssize_t recvfrom(int sockfd, void *buff, size_t nbytes, int flags, struct sockaddr *from, socklen_t *addrlen);
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ssize_t sendto(int sockfd, const void *buff, size_t nbytes, int flags, const struct sockaddr
*to, socklen_t
addrlen);
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Both return: number of bytes read or written if
OK, 鈥? on error
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The first three arguments, sockfd, buff, and nbytes, are identical to the first three
arguments for read and write: descriptor, pointer
to buffer to read into or write from, and number of bytes to read
or write.
We will describe the flags argument in Chapter 14 when we discuss the
recv, send, recvmsg, and
sendmsg functions, since we do not need them with our
simple UDP client/server example in this chapter. For now, we will
always set the flags to 0.
The to argument
for sendto is a socket address structure containing the
protocol address (e.g., IP address and port number) of where the
data is to be sent. The size of this socket address structure is
specified by addrlen. The
recvfrom function fills in the socket address structure
pointed to by from with the
protocol address of who sent the datagram. The number of bytes
stored in this socket address structure is also returned to the
caller in the integer pointed to by addrlen. Note that the final argument to
sendto is an integer value, while the final argument to
recvfrom is a pointer to an integer value (a value-result
argument).
The final two arguments to recvfrom are
similar to the final two arguments to accept: The contents
of the socket address structure upon return tell us who sent the
datagram (in the case of UDP) or who initiated the connection (in
the case of TCP). The final two arguments to sendto are
similar to the final two arguments to connect: We fill in
the socket address structure with the protocol address of where to
send the datagram (in the case of UDP) or with whom to establish a
connection (in the case of TCP).
Both functions return the length of the data
that was read or written as the value of the function. In the
typical use of recvfrom, with a datagram protocol, the
return value is the amount of user data in the datagram
received.
Writing a datagram of length 0 is acceptable. In
the case of UDP, this results in an IP datagram containing an IP
header (normally 20 bytes for IPv4 and 40 bytes for IPv6), an
8-byte UDP header, and no data. This also means that a return value
of 0 from recvfrom is acceptable for a datagram protocol:
It does not mean that the peer has closed the connection, as does a
return value of 0 from read on a TCP socket. Since UDP is
connectionless, there is no such thing as closing a UDP
connection.
If the from
argument to recvfrom is a null pointer, then the
corresponding length argument (addrlen) must also be a null pointer, and this
indicates that we are not interested in knowing the protocol
address of who sent us data.
Both recvfrom and sendto can
be used with TCP, although there is normally no reason to do
so.
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