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\input{configpres}
\def\lximg{/usr/share/lx/icons/fueller.png}
\subsection{Linux Signals}
\title{Linux Signals}
\maketitle
% stop displaying 'fueller.png' on the following slides
\def\lximg{none}
\begin{frame}
\tableofcontents
\end{frame}
\subsubsection{Introduction}
\begin{frame}
\frametitle{Signal Introduction}
What is a signal:
\begin{itemize}
\item Used to control processes
\item Easy communication instrument between processes
\item A signal is an asynchronous event
\item "Interrupt at process level"
\end{itemize}
\end{frame}
\begin{frame}[fragile]
\frametitle{available signals}
kill -l shows all available signals on the system
\begin{lstlisting}
kill -l
1) SIGHUP 2) SIGINT 3) SIGQUIT 4) SIGILL 5) SIGTRAP
6) SIGABRT 7) SIGBUS 8) SIGFPE 9) SIGKILL 10) SIGUSR1
11) SIGSEGV 12) SIGUSR2 13) SIGPIPE 14) SIGALRM 15) SIGTERM
16) SIGSTKFLT 17) SIGCHLD 18) SIGCONT 19) SIGSTOP 20) SIGTSTP
21) SIGTTIN 22) SIGTTOU 23) SIGURG 24) SIGXCPU 25) SIGXFSZ
26) SIGVTALRM 27) SIGPROF 28) SIGWINCH 29) SIGIO 30) SIGPWR
...
\end{lstlisting}
General description:
\begin{lstlisting}
man 7 signal
\end{lstlisting}
\end{frame}
\begin{frame}
\frametitle{process a signal}
If the process receives a signal, the process has to tell the Kernel what to do with it.
Three possibilities are available how a signal can be disposed:
\begin{itemize}
\item ignore it: we do nothing. Most of the signals can be ignored but signals generated by hardware exceptions like divide by zero, if ignored can have weird consequences.
\item we take care: The process has to registers a function. This function is called by kernel when that signal occurs.
\item we call the default disposition: Every signal has a default action. This could be process terminate, ignore etc.
\end{itemize}
\end{frame}
\begin{frame}
\frametitle{special signals}
Two signals can not be caught:
\begin{itemize}
\item SIGKILL
\item SIGSTOP
\end{itemize}
\end{frame}
\subsubsection{Example}
\begin{frame}[fragile]
\frametitle{simple signal example (2)}
Necessary include files:
\begin{lstlisting}
#include<stdio.h>
#include<signal.h>
#include<unistd.h>
\end{lstlisting}
\end{frame}
\begin{frame}[fragile]
\frametitle{simple signal example (2)}
Signal handler:
\begin{lstlisting}
void sig_handler(int signo)
{
if (signo == SIGINT)
printf("received SIGINT\n");
}
\end{lstlisting}
\end{frame}
\begin{frame}[fragile]
\frametitle{simple signal example (3)}
main() function of our example:
\begin{lstlisting}
int main(void)
{
if (signal(SIGINT, sig_handler) == SIG_ERR)
printf("\ncan't catch SIGINT\n");
// A long long wait so that we can easily issue a signal to this process
while(1)
sleep(1);
return 0;
}
\end{lstlisting}
\end{frame}
\begin{frame}[fragile]
\frametitle{simple signal example (4)}
run our small example
\begin{lstlisting}
./sigfunc
^Creceived SIGINT
^Creceived SIGINT
^Creceived SIGINT
^Creceived SIGINT
\end{lstlisting}
We tried the key combination ctrl+c several times but each time the process didn't terminate. This is because the signal was handled in the code and this was confirmed from the print we got on each line.
\end{frame}
\begin{frame}
\frametitle{send signals}
Two possibilities for sending a signal:
\begin{itemize}
\item kill() - send a signal to a process
\item raise() - send a signal to the caller
\end{itemize}
\end{frame}
\subsubsection{Pitfalls}
\begin{frame}
\frametitle{signal handler must be re-entrant}
Re-entrant means: The function can be stopped in between due to any reason (like due to interrupt or signal) and then can be reentered again safely before its previous invocations complete the execution.
\bigskip
Reason: Signals are asynchronous events. It could be possible the signal occurs again before the signal handler is still called.
\end{frame}
\begin{frame}
\frametitle{system calls return if a signal occur}
If a signal is send to a process, all system calls return. The complete application must be able to handle the return values and check error codes, if the system call was interrupted:
\begin{itemize}
\item EINTR
\end{itemize}
\end{frame}
\begin{frame}
\frametitle{signal handler should not block}
The signal handler is called by the Kernel. It is not allowed to block or wait for a event in the signal handler.
\end{frame}
\begin{frame}
\frametitle{signals and threads}
pthreads(7) describes that POSIX.1 requires all threads in a process share attributes, including: signal dispositions
\bigskip
This mean: it is not clear from which thread the signal handler is called!
\end{frame}
\subsubsection{Alternative}
\begin{frame}
\frametitle{Signals should not be used}
Use other mechanisms like:
\begin{itemize}
\item files
\item pipes
\item pthreads
\end{itemize}
\end{frame}
\input{tailpres}
|
