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Exercise 4.1: Examining Signal Priorities and Execution


I was hoping that someone could assist me with this lab.
I have no idea how to do this.
I have installed gcc on my Ubuntu 18.04 but have never performed such a task.
I am fairly new to LINUX still and am willing to read / study up even if someone could point me in the correct direction.


  • Hi,

    Have you downloaded file signals.c ? Just put the file into a directory, saying "/home/your_user/Documents/LFS201/Labs/".

    Then, in the terminal, go to /home/your_user/Documents/LFS201/Labs/, by doing "cd /home/your_user/Documents/LFS201/Labs/ ". Once in there, just do what's in the lab file:

    gcc -o signals signals.c

    The first line will compile the source file, and generate an executable file, called "signals". The second line will run the executable and will give an output.

    I hope that helps.


  • Thanks Luis,
    I didn't think about having to be in the same directory as the downloaded signals.c file.
    It worked perfectly. I am still very new to Linux so I will have to examine the outcome to understand what it did some more now but so happy that it worked.
    Thank you once again for your time.

  • It's a pleasure!

    Enjoy the course :)


  • Where do I get the download you are talking about? Thanks in advance!

  • coop
    coop Posts: 913


    use the user name and password in the book

  • I do not have a book. The username/password I use to access the class is not working for the download. Sorry to be so much trouble!

  • fcioanca
    fcioanca Posts: 1,766

    If you are taking the online course, the username and password for online resources is found in the intro chapter, under Course Resources.

  • TFulz
    TFulz Posts: 3
    edited August 2020

    So, what are the answers to the questions at the end of the lab? I don't want to assume anything. Especially for "One signal, SIGCONT(18 on x86) may not get through; can you figure out why?"

    Here are the questions:

    If more than one of a given signal is raised while the process has blocked it, does the process receive it multiple times?
    Does the behavior of real time signals differ from normal signals?
    Are all signals received by the process, or are some handled before they reach it?
    What order are the signals received in?

  • luisviveropena
    luisviveropena Posts: 1,101
    edited August 2020

    Hi @TFulz ,

    I think you can find very useful information in the man 7 signal page. There are sections for the following items that you can find interesting:

    Queueing and delivery semantics for standard signals

    Real-time signals

    And yes, both kind of signals are managed in a different way, as you may see in the associated man page. Also not everything is fixed:

    If both standard and real-time signals are pending for a process,
    POSIX leaves it unspecified which is delivered first. Linux, like
    many other implementations, gives priority to standard signals in
    this case.


  • TFulz
    TFulz Posts: 3

    Thank you sir!

  • I'd like to check my understanding of the question "One signal, SIGCONT (18 on x86 ) may not get through; can you figure out why?"
    If I'm reading the manpage right, it is because the operating system handles the signal?

  • coop
    coop Posts: 913

    yes. On x86 18=SIGCONT (Continue if stopped). So the OS handles it before it even reaches the kernel directly


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