Taken from the original at http://www.ecst.csuchico.edu/~beej/guide/net/ ... see end of writeup for Copyright statement.
sendto() and recvfrom()--Talk to me, DGRAM-style
"This is all fine and dandy," I hear you saying, "but where does this leave me with unconnected datagram sockets?" No problemo, amigo. We have just the thing.
Since datagram sockets aren't connected to a remote host, guess which piece of information we need to give before we send a packet? That's right! The destination address! Here's the scoop:
int sendto(int sockfd, const void *msg, int len, unsigned int flags,
const struct sockaddr *to, int tolen);
As you can see, this call is basically the same as the call to send() with the addition of two other pieces of information. to is a pointer to a struct sockaddr (which you'll probably have as a struct sockaddr_in and cast it at the last minute) which contains the destination IP address and port. tolen can simply be set to sizeof(struct sockaddr).
Just like with send(), sendto() returns the number of bytes actually sent (which, again, might be less than the number of bytes you told it to send!), or -1 on error.
Equally similar are recv() and recvfrom(). The synopsis of recvfrom() is:
int recvfrom(int sockfd, void *buf, int len, unsigned int flags
struct sockaddr *from, int *fromlen);
Again, this is just like recv() with the addition of a couple of fields. from is a pointer to a local struct sockaddr that will be filled with the IP address and port of the originating machine. fromlen is a pointer to a local int that should be initialized to sizeof(struct sockaddr). When the function returns, fromlen will contain the length of the address actually stored in from.
recvfrom() returns the number of bytes received, or -1 on error (with errno set accordingly.)
Remember, if you connect() a datagram socket, you can then simply use send() and recv() for all your transactions. The socket itself is still a datagram socket and the packets still use UDP, but the socket interface will automatically add the destination and source information for you.
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Copyright © 1995, 1996 by Brian "Beej" Hall. This guide may be reprinted in any medium provided that its content is not altered, it is presented in its entirety, and this copyright notice remains intact. Contact firstname.lastname@example.org for more information.