Question

I am coding an application for linux in C, it will receive thousands of UDP messages which may be of variable size. Somehow, I have to get the message size, before reading it into the buffer with recvfrom() syscall. I can not allocate memory for the maximum possible message, because since I use MTU of 9,000 and get thousands of messages a lot of memory will be wasted.
I checked in google, it is possible to find out what is the size of the message with SO_NREAD option for getsockopt syscall, however this works only on BSD. I know the message is located somewhere in Linux kernel because my sockets are working in non-blocking mode and I am being notified by kernel events that the data is available, so, somewhere it has to store the message length, but how can I get it ?

Thanks in advance

Answer 1

You probably want to use ioctl with FIONREAD. It’s available for both Linux and BSD.

FIONREAD int Get the number of bytes that are immediately available for reading.

if(ioctl(s, FIONREAD, &bytes) != -1)
    printf("%d bytes available", bytes);