Unix max file descriptors

Question

I recently wrote a program to figure out the max amount of file descriptors open per process.

It was essentially

int fd = creat("somefile.dat");
int count = 1;
while(1)
{
 int s = dup(fd);
 if (s == -1)
 break;
 count++;
}
printf("Max fd: %d", s);

Now how would I apply this same program but to find the the max fd amount system-wide instead of per process?

• geekosaur: What would cause you to believe that this technique would work to look at anything beyond the per-process limit?
• wallyk: You would have to create multiple processes which run simultaneously.
• Mat: That only allows you to find the max no. of processes per process for the user that ran that code. Using that technique to push the system-wide limit could be damaging - you'll prevent legitimate processes from opening files while that "thing" runs. Don't. Use whatever OS-specific tool there is to find it out.
• CyberShot: I'm actually running this code on Minix, so it shouldn't be too harmful. I dont know how to change my program to look for a system-wide limit? What does that even mean?
• user823738: why not use just getrlimit() ?

Answer 1

I have multiple ideas for solutions to this:

1 - Multiply the maximum # of processes allowed in MINIX by the max FDs per process

• My only concern is that there is some sort of a hard cap that is under or over the # of processes * FD's per process

2 - Create a program that runs the above program multiple times

• Stipulation is that you have to somehow return count to the original program and sum it
  • Also, I have no idea how to call other processes from within a program and retrieve a return value from it.
  • Additionally, how can you run all the processes simultaneously? and how do you know the MAX # of processes, and how do you know when to stop running processes?

P.S. You must be in my ecs150 class aren't you?