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

I suggest to use the magic of grep to find this constant on /usr/include:

grep -rn --col OPEN_MAX /usr/include

...
...
/usr/include/stdio.h:159:   FOPEN_MAX   Minimum number of files that can be open at once.
...
...

Hope it helps you

Answer 2

For what it's worth, the 4th edition of Beginning Linux Programming was published in 2007; parts of it may be a bit out of date. (That's not a criticism of the book, which I haven't read.)

It appears that OPEN_MAX is deprecated, at least on Linux systems. The reason appears to be that the maximum number of file that can be opened simultaneously is not fixed, so a macro that expands to an integer literal is not a good way to get that information.

There's another macro FOPEN_MAX that should be similar; I can't think of a reason why OPEN_MAX and FOPEN_MAX, if they're both defined, should have different values. But FOPEN_MAX is mandated by the C language standard, so system's don't have the option of not defining it. The C standard says that FOPEN_MAX

expands to an integer constant expression that is the minimum number of files that the implementation guarantees can be open simultaneously

(If the word "minimum" is confusing, it's a guarantee that a program can open at least that many files at once.)

If you want the current maximum number of files that can be opened, take a look at the sysconf() function; on my system, sysconf(_SC_OPEN_MAX) returns 1024. (The sysconf() man page refers to a symbol OPEN_MAX. This is not a count, but a value recognized by sysconf(). And it's not defined on my system.)

I've searched for OPEN_MAX (word match, so excluding FOPEN_MAX) on my Ubuntu system, and found the following (these are obviously just brief excerpts):

/usr/include/X11/Xos.h:

# ifdef __GNU__
#  define PATH_MAX 4096
#  define MAXPATHLEN 4096
#  define OPEN_MAX 256 /* We define a reasonable limit.  */
# endif

/usr/include/i386-linux-gnu/bits/local_lim.h:

/* The kernel header pollutes the namespace with the NR_OPEN symbol
   and defines LINK_MAX although filesystems have different maxima.  A
   similar thing is true for OPEN_MAX: the limit can be changed at
   runtime and therefore the macro must not be defined.  Remove this
   after including the header if necessary.  */  
#ifndef NR_OPEN
# define __undef_NR_OPEN
#endif
#ifndef LINK_MAX
# define __undef_LINK_MAX
#endif
#ifndef OPEN_MAX
# define __undef_OPEN_MAX
#endif
#ifndef ARG_MAX
# define __undef_ARG_MAX
#endif

/usr/include/i386-linux-gnu/bits/xopen_lim.h:

/* We do not provide fixed values for 

   ARG_MAX      Maximum length of argument to the `exec' function
                including environment data.

   ATEXIT_MAX   Maximum number of functions that may be registered
                with `atexit'.

   CHILD_MAX    Maximum number of simultaneous processes per real
                user ID. 

   OPEN_MAX     Maximum number of files that one process can have open
                at anyone time.

   PAGESIZE
   PAGE_SIZE    Size of bytes of a page.

   PASS_MAX     Maximum number of significant bytes in a password.

   We only provide a fixed limit for

   IOV_MAX      Maximum number of `iovec' structures that one process has
                available for use with `readv' or writev'.

   if this is indeed fixed by the underlying system.
*/

Answer 3

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...