How to increase the limit of "maximum open files" in C on Mac OS X

Question

The default limit for the max open files on Mac OS X is 256 (ulimit -n) and my application needs about 400 file handlers.

I tried to change the limit with setrlimit() but even if the function executes correctly, i'm still limited to 256.

Here is the test program I use:

#include <stdio.h>
#include <sys/resource.h>

main()
{
  struct rlimit rlp;

  FILE *fp[10000];
  int i;

  getrlimit(RLIMIT_NOFILE, &rlp);
  printf("before %d %d\n", rlp.rlim_cur, rlp.rlim_max);

  rlp.rlim_cur = 10000;
  setrlimit(RLIMIT_NOFILE, &rlp);

  getrlimit(RLIMIT_NOFILE, &rlp);
  printf("after %d %d\n", rlp.rlim_cur, rlp.rlim_max);

  for(i=0;i<10000;i++) {
    fp[i] = fopen("a.out", "r");
    if(fp[i]==0) { printf("failed after %d\n", i); break; }
  }

}

and the output is:

before 256 -1
after 10000 -1
failed after 253

I cannot ask the people who use my application to poke inside a /etc file or something. I need the application to do it by itself.

Answer 1

rlp.rlim_cur = 10000;

Two things.

1st. LOL. Apparently you have found a bug in the Mac OS X' stdio. If I fix your program up/add error handling/etc and also replace fopen() with open() syscall, I can easily reach the limit of 10000 (which is 240 fds below my 10.6.3' OPEN_MAX limit 10240)

2nd. RTFM: man setrlimit. Case of max open files has to be treated specifically regarding OPEN_MAX.