Maximum buffer length for sendto?

Question

How do you get the maximum number of bytes that can be passed to a sendto(..) call for a socket opened as a UDP port?

Answer 1

Use getsockopt(). This site has a good breakdown of the usage and options you can retrieve.

In Windows, you can do:

int optlen = sizeof(int);
int optval;
getsockopt(socket, SOL_SOCKET, SO_MAX_MSG_SIZE, (int *)&optval, &optlen);

For Linux, according to the UDP man page, the kernel will use MTU discovery (it will check what the maximum UDP packet size is between here and the destination, and pick that), or if MTU discovery is off, it'll set the maximum size to the interface MTU and fragment anything larger. If you're sending over Ethernet, the typical MTU is 1500 bytes.