Locking Linux Serial Port

Question

I have an issue that I'm trying to solve regarding the serial port in Linux. I'm able to open, read from, and close the port just fine. However, I want to ensure that I am the only person reading/writing from the port at any given time.

I thought that this was already done for me after I make the open() function call. However, I am able to call open() multiple times on the same port in my program. I can also have two threads which are both reading from the same port simultaneously.

I tried fixing this issue with flock() and I still had the same problem. Is it because both systems calls are coming from the same pid, even though there are different file descriptors involved with each set of opens and reads? For the record, both open() calls do return a valid file descriptor.

As a result, I'm wondering if there's any way that I can remedy by problem. From my program's perspective, it's not a big deal if two calls to open() are successful on the same port since the programmer should be aware of the hilarity that they are causing. However, I just want to be sure that when I open a port, that I am the only process with access to it.

Thanks for the help.

I've asked a question about the ["best practice" method for locking serial ports and other devices in Linux](http://stackoverflow.com/questions/30316722/what-is-the-best-practice-for-locking-serial-ports-and-other-devices-in-linux). — Craig McQueen, May 19 '15 at 23:44

score 17 · Accepted Answer · answered Aug 01 '13 at 01:17

In Linux, you can use the TIOCEXCL TTY ioctl to stop other open()s to the device from succeeding (they'll return -1 with errno==EBUSY, device or resource busy). This only works for terminals and serial devices, but it does not rely on advisory locking.

For example:

#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <sys/ioctl.h>
#include <termios.h>
#include <fcntl.h>
#include <errno.h>

int open_device(const char *const device)
{
    int descriptor, result;

    if (!device || !*device) {
        errno = EINVAL;
        return -1;
    }

    do {
        descriptor = open(device, O_RDWR | O_NOCTTY);
    } while (descriptor == -1 && errno == EINTR);
    if (descriptor == -1)
        return -1;

    if (ioctl(descriptor, TIOCEXCL)) {
        const int saved_errno = errno;
        do {
            result = close(descriptor);
        } while (result == -1 && errno == EINTR);
        errno = saved_errno;
        return -1;
    }

    return descriptor;
}

Hope this helps.

I'm not getting any EBUSY error under root for the ports already opened. — Mehmet Fide, May 06 '21 at 14:16

score 0 · Answer 2 · answered Jul 31 '13 at 21:17

0

I was able to fix the issue with use of the flock() function. Use of the structure and fcntl() wasn't working for me for some reason. With use of flock() I was able to add two lines of code and solve my issue.

answered Jul 31 '13 at 21:17

It'sPete

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This answer is confusing because in your question you said you tried to use `flock()` but it didn't work. Now here you're saying `flock()` is the solution. What? – Craig McQueen May 06 '15 at 23:48
Is it possible that in your original question you meant to say `fcntl()` didn't work, rather than `flock()`? – Craig McQueen Jan 22 '16 at 00:08

Locking Linux Serial Port

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