Where is OPEN_MAX defined for Linux systems?

Question

OPEN_MAX is the constant that defines the maximum number of open files allowed for a single program.

According to Beginning Linux Programming 4^th

Answer 1

Aside from the link given by cste, I would like to point out that there is a /proc/sys/fs/file-max entry that provides the number of files THE SYSTEM can have open at any given time.

Here's some docs: https://access.redhat.com/knowledge/docs/en-US/Red_Hat_Directory_Server/8.2/html/Performance_Tuning_Guide/system-tuning.html

Note that this is not to say that there's a GUARANTEE you can open that many files - if the system runs out of some resource (e.g. "no more memory available"), then it may well fail.

The FOPEN_MAX indicates that the C library allows this many files to be opened (at least, as discussed), but there are other limits that may happen first. Say for example the SYSTEM limit is 4000 files, and some applications already running has 3990 files open. Then you won't be able to open more than 7 files [since stdin, stdout and stderr take up three slots too]. And if rlimit is set to 5, then you can only open 2 files of your own.

In my opinion, the best way to know if you can open a file is to open it. If that fails, you have to do something else. If you have some process that needs to open MANY files [e.g. a multithreaded search/compare on a machine with 256 cores and 8 threads per core and each thread uses three files (file "A", "B" and "diff") ], then you may need to ensure that your FOPEN_MAX allows for 3 * 8 * 256 files being opened before you start creating threads, as a thread that fails to open its files will be meaningless. But for most ordinary applications, just try to open the file, if it fails, tell the user (log, or something), and/or try again...