Getting count of current used file descriptors from C code

Question

Is there a C API to get the:

1. Current used file descriptors system wide
2. Current used file descriptors of the current process

Answer 1

For the current process count, you can use getrlimit to get the file descriptor limit, then iterate over all integers from 0 to that limit and try calling fcntl with the F_GETFD command. It will succeed only on the file descriptors which are actually open, letting you count them.

Edit: I now have a better way to do it. After getting the rlimit, make a large array of struct pollfd (as large as the limit if possible; otherwise you can break it down into multiple runs/calls) with each fd in the range and the events member set to 0. Call poll on the array with 0 timeout, and look for the POLLNVAL flag in the revents for each member. This will tell you which among a potentially-huge set of fds are invalid with a single syscall, rather than one syscall per fd.