Problem with TIOCMGET ioctl

[EvilGuru]

Hi. I am currently porting an application which interfaces with a serial port from Windows to Linux. The two main things done are the setting of DTR and the reading of DSR. I am having a problem, however, reading off the value of DSR from the serial port.

My current code does it like so:

bool getDSR (void)
{
	int controlBits;
	
	ioctl(serialFD, TIOCMGET, &controlBits);
	
	return controlBits & TIOCM_DSR;
}

Problem is that ioctl keeps returning "Bad address" as the error. Does anyone have any idea why this is? Setting the value of DTR works perfectly. I have tried it under two systems, a PIII-era Debian system (2.6.15) and a 2.6.23 Gentoo system.

Regards, Freddie.

[brewbuck]

TIOCMGET is no longer supported.

http://www.linuxjournal.com/node/7000/print

[EvilGuru]

TIOCMGET is no longer supported.

http://www.linuxjournal.com/node/7000/print

I found the article as well during my googling-frenzy but it seems to be for kernel modules by the looks of it as opposed to user-space applications.

After a good few hours searching I did find the root cause of the problem, and as usual it was down to my stupidity. I made a dangerous assumption that ioctl returned 0 on failure, so was detecting that the wrong part of the code was failing.

Once I worked this out it was just a simple error where I forgot an & in front of a variable so was passing it as opposed to its address to ioctl. Why my compiler did not pick up on this I am unsure.

Sadly my application still has teething trouble, no doubt due to the subtle differences in Linux/Windows serial ports.

Regards, Freddie.

[dwks]

Once I worked this out it was just a simple error where I forgot an & in front of a variable so was passing it as opposed to its address to ioctl. Why my compiler did not pick up on this I am unsure.

It's likely that you didn't include the header file that ioctl is in (in which case you should have gotten a warning anyway), or perhaps the prototype for ioctl is K&R-style, not specifying the parameters, something like this.

int ioctl();

In which case there's nothing you can do about this lack of errors or warnings.

[brewbuck]

It's likely that you didn't include the header file that ioctl is in (in which case you should have gotten a warning anyway), or perhaps the prototype for ioctl is K&R-style, not specifying the parameters, something like this.

int ioctl();

It's actually

int ioctl(int d, int request, ...);

. ioctl() is a variadic function and way too complex for the compiler to check parameters.

Yeah -- nearly all POSIX/UNIX functions return 0 on success, -1 on failure, setting errno appropriately. And errno is only set when an error occurs, which means that if an error does NOT occur, that's no guarantee that errno isn't clear.

Generally you should only check errno IMMEDIATELY after a failed call.