Linux: Getting the kernel buffer size for a socket

Question

I have a C application which receives a lot of data over a TCP socket. Is it somehow possible to get the kernel buffer size for that file descriptor / socket? I would like to know how much data is still in the kernel for the file-descriptor I have.

Thanks a lot

Answer 1

You can use the SIOCINQ/FIONREAD ioctl:

int bytes_left;
ioctl(sock, FIONREAD, &bytes_left);

This “returns the amount of queued unread data in the receive buffer.” SIOCINQ is defined in linux/sockios.h (on Linux), FIONREAD in sys/ioctl.h.

To retrieve the buffer capacity for the socket, use getsockopt with SO_RCVBUF.