Process exceeding max open files limit

Note that the limit is on the value of newly created file descriptors (as in open()/socket()/pipe() and so on will never return a number greater than n-1 if the limit was set to n, and dup2(1, n or n+1...) will fail), not on the number of currently open files or file descriptors.

In effect, from the instant the limit is set to n, that will prevent that process from opening more than n files, but if file descriptors above n had already been created before the limit was lowered, then the process can possibly have more than n files open.

Example:

$ limit descriptors 10000 # ulimit -n 10000 in bash
$ perl -MBSD::Resource -MPOSIX -e '
    dup2(1,$_) for (2000..4000);
    setrlimit(RLIMIT_NOFILE, 1024, 1024);
    exec zsh'
zsh$ ls /proc/$$/fd | wc -w
2009
zsh$ readlink /proc/$$/fd/3000
/dev/pts/1
zsh$ repeat 100 exec {a}>&1
zsh$ ls /proc/$$/fd | wc -w
2109
zsh$ grep files /proc/$$/limits
Max open files            1024                 1024                 files

At the time zsh was executed, the process had fds 2000 to 4000 open to /dev/pts/1. Yet the fd limit was 1024.

It already had 2009 fds, but was still able to create 100 more.