Getting count of current used file descriptors from C code

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.

You can read /proc/sys/fs/file-nr to find the total number of allocated and free file system handles as well as the maximum allowed.

[root@box proc]# cat /proc/sys/fs/file-nr
3853    908     53182
|       |       |
|       |       |
|       |       max: maximum open file descriptors
|       free: total free allocated file descriptors
allocated: total allocated file descriptors since boot

To calculate the number that are currently in use, just do allocated - free. You could also calculate a percentage of used descriptors by doing ((allocated - free) / max) * 100

As for per-process, I'm not sure of any programmatic way you can do it.

Here's a tutorial on how to do it with lsof anyway:

Since you say you are on Linux, you can open the folder /proc/self/fd/ which should contain symbolic links to all open file descriptors.