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    Introduction To Signals: Operating System Lab

    Uploaded by

    Sikandar Khan
    The document discusses signals in Linux operating systems. It defines signals as messages sent by the Linux kernel to processes in response to hardware or software events. It provides examples of common signals like SIGINT (Ctrl-C) and SIGQUIT (Ctrl-\). It demonstrates how to use the kill command to send signals and the trap command to configure how a shell script handles signals. It also shows how to use the signal() function in C to catch signals and define a signal handler function. Exercises demonstrate trapping signals in a shell script and catching SIGINT in a C program.

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    © All Rights Reserved
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    Introduction To Signals: Operating System Lab

    Uploaded by

    Sikandar Khan
    40 views5 pages
    The document discusses signals in Linux operating systems. It defines signals as messages sent by the Linux kernel to processes in response to hardware or software events. It provides examples of common signals like SIGINT (Ctrl-C) and SIGQUIT (Ctrl-\). It demonstrates how to use the kill command to send signals and the trap command to configure how a shell script handles signals. It also shows how to use the signal() function in C to catch signals and define a signal handler function. Exercises demonstrate trapping signals in a shell script and catching SIGINT in a C program.

    Original Description:

    Original Title

    OS-Lab-12

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    The document discusses signals in Linux operating systems. It defines signals as messages sent by the Linux kernel to processes in response to hardware or software events. It provides examples of common signals like SIGINT (Ctrl-C) and SIGQUIT (Ctrl-\). It demonstrates how to use the kill command to send signals and the trap command to configure how a shell script handles signals. It also shows how to use the signal() function in C to catch signals and define a signal handler function. Exercises demonstrate trapping signals in a shell script and catching SIGINT in a C program.

    Copyright:

    © All Rights Reserved
    Download as DOC, PDF, TXT or read online from Scribd
    Download as doc, pdf, or txt
    40 views5 pages

    Introduction To Signals: Operating System Lab

    Uploaded by

    Sikandar Khan
    The document discusses signals in Linux operating systems. It defines signals as messages sent by the Linux kernel to processes in response to hardware or software events. It provides examples of common signals like SIGINT (Ctrl-C) and SIGQUIT (Ctrl-\). It demonstrates how to use the kill command to send signals and the trap command to configure how a shell script handles signals. It also shows how to use the signal() function in C to catch signals and define a signal handler function. Exercises demonstrate trapping signals in a shell script and catching SIGINT in a C program.

    Copyright:

    © All Rights Reserved
    Download as DOC, PDF, TXT or read online from Scribd
    Download as doc, pdf, or txt
    of 5

    Operating System Lab Lab – 13

    Spring- 2019 Signal Handling

    Introduction to Signals

    You already know that you can stop the execution of most running programs by
    interrupting them from the keyboard, usually with Ctrl-C.

    What actually happens is that the part of the Linux kernel software that controls
    the keyboard, sends a message to the running process through CPU. This
    message is called a signal. Infect there are about 30 different signal types that can
    be sent to a Linux process, each of which is generated by some specific hardware
    or software condition. In addition to the normal mechanisms for generating and
    sending signals, the kill command can also be used to send any signal to a process
    at will.
    Each of the signal is given a small integer value which is used as command line
    switch to kill, along with process ID of the process to which the signal should be
    sent. The general format of the kill command is:
    Kill [-sig] pid
    Where sig is the signal to send and pid is the process ID to which it should be
    sent. The square brackets just indicate that the specification of the pid is optional
    – the brackets themselves should not be type. If a signal number is not specified
    to the kill command then the terminate signal is used by default.

    1
    Operating System Lab Lab – 13
    Spring- 2019 Signal Handling

    Some of the more common signals and their numeric values are:
    Signal Value Notes
    SIGHUP 1 Used to kill your processes when you logout
    SIGINT 2 Generated from keyboard (Ctrl-C)
    SIGQUIT 3 Generated from keyboard (Ctrl-\)
    SIGILL 4 Illegal Instruction
    SIGFPE 8 Floating point exception
    SIGKILL 9 Sure process termination – cannot be ignored
    SIGALRM 14 Generated at the end of an alarm() system call
    SIGTERM 15 Default signal sent by kill command

    You can fill single list by typing command kill -l

    When a process receives a signal, it can normally do one of three different things;
    1. Ignore the signal;
    2. Accept the signal’s default action; defined in OS.
    3. Execute some code to deal with the signal.

    Trap a Signal:
    From inside a shell script it is possible to trap a signal and then to get a sequence
    of commands executed when that signal is received. This is done with the trap
    command. There are three basic forms for the trap command, one for each of the
    tree different signal response types:
    Trap "commands" signal_list
    Trap will execute the commands in the quotes when any of the signals
    corresponding to the numbers in the signal_list is received by the script.

    To restore signals to their default action, use the trap command in the form:
    Trap signal_list

    Without any commands specified.


    The third form, which allows signals to be ignored, has the format:
    Trap " " signal_list
    Which just specifies a null commands string.

    2
    Operating System Lab Lab – 13
    Spring- 2019 Signal Handling

    Exercise # 1:
    Please read above first and then try to execute following example;

    This script starts by setting itself up to trap signals 2 and 3 (Ctrl-c and Ctrl-\ from
    the keyboard). If either of these two signals occurs, it will usually be because the
    user is trying to break into the script and abort its operating. After the trap has
    executed, signal 2 and 3 will cause execution of the commands:

    Signal Handling in C:

    The signal() function:


    C++ signal-handling library provides function signal to trap unexpected events.
    Following is the syntax of the signal() function:
    void (*signal (int sig, void (*func)(int)))(int);
    Keeping it simple, this function receives two arguments: first argument as an
    integer which represents signal number and second argument as a pointer to the
    signal-handling function.
    Let us write a simple C++ program where we will catch SIGINT signal using
    signal() function. Whatever signal you want to catch in your program, you must
    register that signal using signal function and associate it with a signal handler.
    Program using signals:

    Exercise 2:
    Type the following and execute it.

    3
    Operating System Lab Lab – 13
    Spring- 2019 Signal Handling

    Press <ctrl-c> while the program is running and see what happens.

    Let’s discuss the program. The first question that comes to mind is… why is there
    a call to the signal function inside the signal handler (this is what we will be
    calling the catcher function)? When the signal is caught once, the function which
    handles the signal is reset, and needs to be set again. If we don’t do this, the
    default action will occur. Try commenting out the function and see what happens.
    The working of the program is simple. It will run normally and print working
    after every second. When the signal is caught, the normal flow of the program is
    interrupted. The catcher function is executed and then the normal flow of the
    program resumes.

    You will notice that commenting the signal line made no difference. Does that
    make everything I just mentioned wrong? No. The original UNIX signal () would
    reset the handler and System V (and the old Linux kernel and libc4,5) do the
    same. On the other hand, BSD does not reset the handler. But remember, that if
    you’re working on UNIX and Linux (before version 6.0, not sure about 6.1), the
    function has to be set again. All our code will assume that this is how the signal
    function works.

    4
    Operating System Lab Lab – 13
    Spring- 2019 Signal Handling

    Exercise 3:
    Execute following code.

    Instructor: Muhammad Zeeshan Sabir,


    FC, Riphah International University
    Email: zeeshan.sabir@riphah.edu.pk

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