Launch a child process on OSX/Unix that doesn't inherit files/ports

Question

I want to launch a child process on OS X such that the child process does not inherit the current process's open files/ports.

Any suggestions on how to accomplish this? Shell commands that I can invoke using the system() function would work too - I just don't know of a good shell command that enables this.

Thanks!

Answer 1

You didn't specify what language you are developing in, but since you mentioned system(), I assume you mean C or C++.

Typically this is accomplished by using fcntl() to set the close-on-exec flag on those file descriptors that you do not want to be inherited:

int fd = open("somefile", O_RDONLY);
fcntl(fd, F_SETFD, FD_CLOEXEC);

You could also do it the brute force way by iterating through all possible fd's in the child process after you fork but before you exec. This is a bit trickier because it requires that you know what the maximum possible fd value is, and it is also less efficient because you'll end up iterating through and "closing" a bunch of unused fd's. Something like this:

pid_t pid = fork();
switch (pid) {
    case 0:
        // In the child process.  Iterate through all possible file descriptors
        // and explicitly close them.

        long maxfd = sysconf(OPEN_MAX);
        for (long i = 0; i < maxfd; ++i) {
            close(i);
        }

        // Now exec the new program, file-handle free.

        execlp("someprogram", "arg1", "arg2");
        break;
    case -1:
        // TODO: handle errors.

        break;
    default:
        // Parent process.

        int status;
        pid_t finished = waitpid(pid, &status, 0);
        break;
}

Hope that helps,

Eric Melski

Answer 2

On Mac OS X you can use read the directory /dev/fd/ with opendir()/readdir() to determine the set of open file descriptors. With this information you can either call fcntl(fd, F_SETFD, FD_CLOEXEC) on each open file descriptor, or alternatively you could call close(fd) on each file descriptor after forking.

See http://stackoverflow.com/questions/899038/getting-the-highest-allocated-file-descriptor/918469#918469 for some additional details, including approaches for other systems.

Answer 3

After some experimenting, I found that the 'open' shell command on Mac creates a process that does not inherit the parents files/ports. Invoking 'open foo.app' as arguments to the system function does the trick. Of course, this is mac only. For Unix, one of the other approaches suggested here makes a lot of sense.

Answer 4

@Eric M: Your answer got me most of the way there, but I found that it still left some file descriptors open. I replaced sysconf(OPEN_MAX) in your code above with getdtablesize() and it works great. Thanks for the help!