How to check amount of data available for a socket in C and Linux

Question

I have a server that receives a continuous stream of data. As opposed to reading multiple times from a socket, I would like to read the entire data in socket receive buffer with one system call to read().

Of course I can pass a large buffer and read() will try to fill it with all available data. But this would waste a lot of memory as most of the times the malloc'ed buffer would be bigger than actual data available on socket. Is there a way to query the available data on a socket?

Answer 1

Yes:

#include <sys/ioctl.h>  ...  int count; ioctl(fd, FIONREAD, &count); 

Answer 2

No, there is not. Even if there were a way to do this, any answer you get would be immediately out of date (because new data may arrive at any time).

Note that when you pass a buffer to read(), the function will return when there is any amount of data to read (at least one byte), instead of waiting for the buffer to completely fill.