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I know about lsof and ls /proc/*/fd but none of them are atomic AFAIK. Because in the latter case I would need to get all pids for user and then filter by them and by that time some of the file descriptors could be closed.
Maybe there is some system call for that or something, because obviously OS tracks that number as it would refuse to create FD if max limit for user is exhausted.
I haven't made an intensive search, but I don't think what you're looking for exists on Linux. Opening a file descriptor doesn't take any global lock, only a per-process lock, so on a multicore machine whatever you'd be using to count the number of open file descriptors could be running literally at the same time that other threads is opening or closing files on other cores.
Linux doesn't have a global limit on the total number of open files. There's no explicit per-user limit either. There's a per-user limit on processes, and a per-process limit on file descriptor numbers, which indirectly imposes a limit on open files per user, but that isn't explicitly tracked.
Exploring /proc (which is what lsof does under the hood) is as good as it gets. /proc is the Linux API to get information about processes.
limits.conf defines different values for each user, but what that means is that each process run by the user has this limit, not that the limit is for the total number of files open by a process run by the user.
Apr 28, 2016 at 11:48
it would refuse to create FD if max limit for user is exhausted? The setrlimit/getrlimit system calls work aper-processbase. diskquota works on aper filesystembase. AFAIK, there is no API that works on aper userbase.lsof(with non atomic constraint) or read kernel sources and build your own utility. A good question anyway.RLIMIT_NOFILEset to 100, she'll can have two processes with 99 open files (198 in total).