Question

I have an open device descriptor, where I don’t know the device name and the options
passed to open(…).
I want to open a new device descriptor with the same options passed to open.

int newFd = copy(referenceFd);

Where copy would do the job. dup() is certainly the wrong choice as a further ioctl() on newFd would also alter the referenceFd, therefore I want to open a new descriptor.

Is there a system call which provides such functionality?
I have not been able to find something yet.

Answer 1

First, get the descriptor flags of file descriptor fd using

    int flags = fcntl(fd, F_GETFL);

Then, reopen the descriptor using the Linux-specific /proc/self/fd/fd entry:

    int  newfd;
    char buffer[32];

    if (snprintf(buffer, sizeof buffer, "/proc/self/fd/%d", fd) < sizeof buffer) {
        do {
            newfd = open(buffer, flags & ~(O_TRUNC | O_EXCL), 0666);
        } while (newfd == -1 && errno == EINTR);
        if (newfd == -1) {
            /* Error: Cannot reopen file. Error in errno. */
        }
    } else {
        /* Error: the path does not fit in the buffer. */
    }

The reopened file descriptor is then in newfd.

Note that you most likely want to make sure O_TRUNC (truncate the file) and O_EXCL (fail if exists) are not in the flags, to avoid the cases where reopening using the exact original flags would cause unwanted results.

You do not want to use lstat(), because that opens up a race condition with respect to rename — a user renaming the target file between the lstat() and the open() in above code. The above avoids that completely.

If you don’t want to be Linux-specific, add code that tries the same with /dev/fd/fd if the above fails. It is supported in some Unix systems (and most Linux distros too).