This C macro undoes the effect of the FD_SET macro. Assuming a socket c once again, if the calling program wants to remove this descriptor from the set, it can perform the following:
int c; /* Client socket */
fd_set read_socks; /* Read set */
. . .
The FD_CLR macro has the effect that it zeros the corresponding bit representing the file descriptor. Note that this macro differs from FD_ZERO in that it only clears one specific file descriptor.