What is the max opened files limitation on Linux?

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.

It is a number of open file descriptors per process. They can all refer to the same file, or different files.

You can see the current limits with ulimit -a in the shell, or programatically with getrlimit. The system-wide limits are set in /etc/security/limits.conf.

The filesystem object model on Linux is:

file descriptor -> file description -> dentry -> inode

1. file descriptor is the integer number used by the application.
2. file description is the kernel data structure to which one or more file descriptors refer to.
3. dentry is the filename. One file can have many names (hard links).
4. inode is the file contents.

dup creates a new file descriptor to the same file description. open creates a new file descriptor and a file description.