Thread: bytes lost with partial read in UDP

With a UDP (DGRAM) socket
if i read() n bytes and the packet size is m bytes > n bytes
do i loose the m-n bytes?

I tried and i think as a default the answre is yes

But is there an option I can set that prevents this?

Here pread(3): read from file - Linux man page I found

A read() from a STREAMS file can read data in three different modes: byte-stream mode, message-nondiscard mode, and message-discard mode. The default shall be byte-stream mode. This can be changed using the I_SRDOPT ioctl() request, and can be tested with I_GRDOPT ioctl(). In byte-stream mode, read() shall retrieve data from the STREAM until as many bytes as were requested are transferred, or until there is no more data to be retrieved. Byte-stream mode ignores message boundaries.

In STREAMS message-nondiscard mode, read() shall retrieve data until as many bytes as were requested are transferred, or until a message boundary is reached. If read() does not retrieve all the data in a message, the remaining data shall be left on the STREAM, and can be retrieved by the next read() call. Message-discard mode also retrieves data until as many bytes as were requested are transferred, or a message boundary is reached. However, unread data remaining in a message after the read() returns shall be discarded, and shall not be available for a subsequent read(), getmsg(), or getpmsg() call.

but it seems valid only for TCP sockets and not for UDP sockets
isn't it?

You can use the MSG_PEEK option which allows to read the reception buffer without discarding it (man recv for details)

The MSG_PEEK 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.

In this sort of situation, it is common to use a buffer size of 1500 bytes. This is the MTU of the ethernet protocol, so you can be sure that you will be able to read in all the data for each packet.

Originally Posted by bithub

In this sort of situation, it is common to use a buffer size of 1500 bytes. This is the MTU of the ethernet protocol, so you can be sure that you will be able to read in all the data for each packet.

you mean 9000 as i am working with jumbo frames?