int           fdesc;
struct rlimit rl;

/* Retrieve the soft limit for the maximum number of open files */
getrlimit(RLIMIT_NOFILE, &rl);

/* Set all file descriptors except stdin, stdout, and stderr to close-on-exec */
for(fdesc=3; fdesc < rl.rlim_cur; ++fdesc)
    {
    int old_flags = fcntl(fdesc, F_GETFD, 0);
    if(old_flags >= 0)
        fcntl(fdesc, F_SETFD, old_flags | FD_CLOEXEC);
    }

static int fdwalk_close_on_exec(void *unused, int fd)
{
    if (fd >= 3)
        {
        int old_flags = fcntl(fd, F_GETFD, 0);
        if(old_flags >= 0)
            fcntl(fd, F_SETFD, old_flags | FD_CLOEXEC);
        }
    return (0);
}

...

fdwalk(fdwalk_close_on_exec, NULL);