R – Linux: multiple-SIGIO problem (async serial communication)

asynchronouslinuxserial-portsignals

I'm esperiencing a SIGIO-related problem in a C++ program I'm working on.

Platform: Linux 2.6.30, x86 Arch
Scenario: async serial communication.

I followed this async-communication example from the Linux Serial Programming HowTo and it worked flawlessy.

Then I removed the "sleep+check wait_flag" thing and now I handle the read() part directly in signal_handler_IO(). That is, when some input is available, the signal handler is called and it forces a read on the serial port (*).

My code seems to work but unfortunately when new input is available the SIGIO signal is raised several times so I get spurious/incomplete reads (each SIGIO received forces a read).

I changed VMIN and VTIME serial options to control the read buffer (i.e. VMIN=255/VTIME=15, VMIN=50/VTIME=0, …). I tried setting SA_SIGINFO flag (as suggested by some), but no success.

So:

what am I missing about SIGIO raising/handling?
is there a way to avoid this "SIGIO storm"?
is there any way to control SIGIO raising policies?

Thanks in advance.

Bye.

(*): actually I'm following this C++ FAQ Lite hint so the signal handler calls a member function of an object that encapsulates all my serial handling stuff. Unfortunately the problem still happens, even if I invoke read() in the signal handler itself.

Best Answer

You are calling read while in a signal handler? This is bad. Very few functions are async-signal safe. The SIGIO storm might be caused by read sending SIGIO recursively. Your signal handler should just set a flag and return immediately.

Related Solutions

Linux – How to prompt for Yes/No/Cancel input in a Linux shell script

The simplest and most widely available method to get user input at a shell prompt is the read command. The best way to illustrate its use is a simple demonstration:

while true; do
    read -p "Do you wish to install this program?" yn
    case $yn in
        [Yy]* ) make install; break;;
        [Nn]* ) exit;;
        * ) echo "Please answer yes or no.";;
    esac
done

Another method, pointed out by Steven Huwig, is Bash's select command. Here is the same example using select:

echo "Do you wish to install this program?"
select yn in "Yes" "No"; do
    case $yn in
        Yes ) make install; break;;
        No ) exit;;
    esac
done

With select you don't need to sanitize the input – it displays the available choices, and you type a number corresponding to your choice. It also loops automatically, so there's no need for a while true loop to retry if they give invalid input.

Also, Léa Gris demonstrated a way to make the request language agnostic in her answer. Adapting my first example to better serve multiple languages might look like this:

set -- $(locale LC_MESSAGES)
yesptrn="$1"; noptrn="$2"; yesword="$3"; noword="$4"

while true; do
    read -p "Install (${yesword} / ${noword})? " yn
    if [[ "$yn" =~ $yesexpr ]]; then make install; exit; fi
    if [[ "$yn" =~ $noexpr ]]; then exit; fi
    echo "Answer ${yesword} / ${noword}."
done

Obviously other communication strings remain untranslated here (Install, Answer) which would need to be addressed in a more fully completed translation, but even a partial translation would be helpful in many cases.

Finally, please check out the excellent answer by F. Hauri.

Linux – Merge / convert multiple PDF files into one PDF

Considering that pdfunite is part of poppler it has a higher chance to be installed, usage is also simpler than pdftk:

pdfunite in-1.pdf in-2.pdf in-n.pdf out.pdf

Just make sure you remember to provide out.pdf, or else it will overwrite the last file in your command

Related Topic