What is the max opened files limitation on Linux?

Question

On Linux, when a process opens a file, the OS will check the max opened files limitation.

If the limitation was 1024, what's the number mean?

Does it represent

• the number of files opened by the process
• the number of files opened by the user who owns the process
• the number of all the opened files in OS currently?

Answer 1

You can check the Soft limits and hard limits of your system by ulimit -a command.

1. soft limits are simply the currently enforced limits.
2. hard limits mark the maximum value which cannot be exceeded by setting a soft limit.

Soft limits could be set by any user while hard limits are changeable only by root. Limits are a property of a process. They are inherited when a child process is created so system-wide limits should be set during the system initialization in init scripts and user limits should be set during user login for example by using pam_limits.

There are often defaults set when the machine boots. So, even though you may reset your ulimit in an individual shell, you may find that it resets back to the previous value on reboot. You may want to grep your boot scripts for the existence ulimit commands if you want to change the default.

If the limitation was 1024, means you/process can open maximum 1024 files. if you exceed this limit means open, pipe and dup system calls will fail:

RLIMIT_NOFILE:

Specifies a value one greater than the maximum file descriptor number that can be opened by this process. Attempts (open(2), pipe(2), dup(2), etc.) to exceed this limit yield the error EMFILE.