C: How to tell the status of a socket

If you are trying to check if a socket is empty but you cannot use select(), because what you need is to check the write buffers instead of the read ones just to be sure that you have successfully managed to send all data to your peers before terminating, or if you want to tell if a socket buffer is full.

You can use the ioctl to find out.

To check a write buffer if it empty (assuming you have already put data there and want to check if they were consumed):

ioctl(fd, SIOCOUTQ, &pending);

Where fd is the socket’s file descriptor and pending the variable were the remaining size of data will be returned.

To check a read buffer if it empty (assuming someone has already put data there and you want to check if they there is any without consuming them):

ioctl(fd, SIOCINQ, &pending);
*note the difference on the second parameter, where we change the flag from SIOCOUTQ to SIOCINQ

Among other includes be sure to add these:

#include <sys/ioctl.h>
#include <linux/sockios.h>

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