Re: stty and tcgetattr/tcsetattr do not match
From: Jim Alexander (jalex_at_cis.upenn.edu)
Date: 09/22/05
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Date: Thu, 22 Sep 2005 15:57:07 +0000 (UTC)
In article <barmar-B3C7BB.21061521092005@comcast.dca.giganews.com>,
Barry Margolin <barmar@alum.mit.edu> wrote:
]In article <dgsk9o$9fmm$1@netnews.upenn.edu>,
] jalex@cis.upenn.edu (Jim Alexander) wrote:
]
]> I'm trying to write a program that reads from the serial port, and one
]> of the things I would like it to do is flip CLOCAL on and off from within
]> the program. My problem is that the attributes I get back from tcgetattr
]> do not match what I see in stty, and any changes I make with tcsetattr
]> can be read back with tcgetattr, but again the changes do not appear
]> when I query the port with stty, and the port behaves according to
]> what stty says the settings are, not what tcgetattr says they are.
]> It's like tcgetattr and tcsetattr maintain a parallel set of terminal
]> properties, which don't actually do anything.
]>
]> Here's a code fragment that tries to clear CLOCAL, if it is set:
]>
]> fd = open_port_noblock();
]> if (tcgetattr(fd, termSettings) == -1){
]> perror("monitor: Couldn't check tty settings - ");
]> exit(1);
]> }
]> printf("%08o\n", termSettings->c_cflag);
]> if(termSettings->c_cflag&CLOCAL){
]> termSettings->c_cflag &= ~CLOCAL;
]> if (tcsetattr(fd, TCSANOW, termSettings) == -1){
]> perror("monitor: Couldn't change tty settings - ");
]> }
]> }
]> tcgetattr(fd, termSettings);
]> printf("%08o\n", termSettings->c_cflag);
]> close(fd);
]
]The problem is with close(fd). When tty devices are closed, they revert
]back to their default settings. If there are multiple descriptors
]attached to the device (either in the same or different processes), this
]happens when the last one is closed.
]
]You need to open the device, set the attributes, do all the things that
]depend on these attributes, and only then can you close the descriptor.
That isn't the case on the Linux systems I'm working with. I suppose it
could be true of serial ports under to control of something like ttymon.
As Murphy's Law dictates, I found my problem shortly after posting,
and cancelled the article, but of course a lot of news servers ignore
cancel control messages.
For posterity, the problem was actually in the open_port_noblock(), whose
code I didn't post, arising from an incorrect cut-and-paste. It was always
returning 0 for the file descriptor, which means I was actually manipulating
the pty I was running the program in. So I really was manipulating a
different set of terminal properties!
-- ________ Jim Alexander __________________ jalex@cis.upenn.edu ________________ I have yet to see a problem, however complicated, which, when you looked at it in the right way, did not become still more complicated. -- Poul Anderson
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