Is it reasonable to expect that in Linux, fd < maximum number of open file descriptors?

Question

I'm writing a server that needs to handle many open sockets, so I use setrlimit() to set the maximum number of open file descriptors (as root, before dropping privileges) like so:

#include <sys/resource.h>
#define MAX_FD_C 9001

if (setrlimit(
      RLIMIT_NOFILE, &(struct rlimit){.rlim_cur = MAX_FD_C, .rlim_max = MAX_FD_C}
    ) == -1) {
    perror("Failed to set the maximum number of open file descriptors");
    return EXIT_FAILURE;
}

Now, I realize there probably won't be any guarantees and that I'm at the mercy of whatever method the Linux kernel uses to implements file descriptor tables; but in practice, is it reasonable to assume that any fd this program receives from the Linux kernel will have a value less than the MAX_FD_C I set above?

I'd like to keep per socket data as compact as possible which could mean simply using an array like static struct client clients[MAX_FD_C] = {{0}}; and using the fd as the index to the client struct (which would basically be my own version of the FDT).

Answer 1

There are functions in the POSIX standard which assume this already. Have a look at FD_SETSIZE, select(), FD_SET.