How to set errno in Linux device driver?

Question

I am designing a Linux character device driver. I want to set errno when error occurs in ioctl() system call.

long my_own_ioctl(struct file *file, unsigned int req, unsigned long arg)
{
    long ret = 0;
    BOOL isErr = FALSE;

    // some operation
    // ...

    if (isErr) {
        // set errno
        // ...                 <--- What should I do?
        ret = -1;
    }

    return ret;
}

What should I do to achieve that? Thank you at advance!

---

Please allow me to explain my application with more detail.

My device is located in /dev/myCharDev. My user space application is like this:

#define _COMMAND                    (1)
#define _ERROR_COMMAND_PARAMETER    (-1)

int main()
{
    int fd = open("/dev/myCharDec", O_RDONLY);
    int errnoCopy;

    if (fd) {
        if (ioctl(fd, _COMMAND, _ERROR_COMMAND_PARAMETER) < 0) {      // should cause error in ioctl()
            errnoCopy = errno;
            printf("Oops, error occurred: %s\n", strerr(errnoCopy));  // I want this "errno" printed correctly
        }

        close(fd);
    }

    return 0;
}

As I mentioned in the comments above, How should I set the "errno" in my own device driver codes and make it readable by user space application?

Answer 1

if (isErr)
    {
        printk(KERN_ALERT "Error %d: your description\n", errno);
        ret = errno;
    }

where, errno is the return value of some function.

Your device driver should always return a status for a request it received. It is advocated you always use enumerated return codes as well as normal return codes. Returning 0 = pass 1 or -1 = failed is vague and could be misleading.

Read section 3.1 Efficient error handling, reporting and recovery: for more information

Answer 2

Return the negative error number from the ioctl. The c library interprets this and gives a -1 return code and sets errno to the positive error. For instance your original example will set errno to 1.

As an aside your prototype for an ioctl function in the kernel looks wrong. Which kernel version are you using?