They are *user-defined* signals, so they aren&#39;t triggered by any particular action.  You can explicitly send them programmatically:

    #include &lt;signal.h&gt;

    kill(pid, SIGUSR1);

where `pid` is the process id of the receiving process.  At the receiving end, you can register a signal handler for them:

    #include &lt;signal.h&gt;

    void my_handler(int signum)
    {
        if (signum == SIGUSR1)
        {
            printf(&quot;Received SIGUSR1!\n&quot;);
        }
    }

    signal(SIGUSR1, my_handler);


terminal 1

    dd if=/dev/sda of=debian.img

terminal 2  

    killall -SIGUSR1 dd

go back to terminal 1

    34292201+0 records in
    34292200+0 records out
    17557606400 bytes (18 GB) copied, 1034.7 s, 17.0 MB/s

They are signals that application developers use. The kernel shouldn&#39;t ever send these to a process. You can send them using `kill(2)` or using the utility `kill(1)`.

If you intend to use signals for synchronization you might want to check real-time signals (there&#39;s more of them, they are queued, their delivery order is guaranteed etc).

First, please bear with me. I have hard time telling others my problem and this is a long thread...

I am using pythonbrew to run multiple versions of python in Ubuntu 10.10. 
For installing pythonbrew and how it works, please refers to this link below

http://www.howopensource.com/2011/05/how-to-install-and-manage-different-versions-of-python-in-linux/

After reading a couple stackoverflow threads, I finally found the file called Setup under this directory:  **~/.pythonbrew/pythons/Python-2.7.1/lib/python2.7/config**

    In this Setup file I see 
    # Andrew Kuchling&#39;s zlib module.
    # This require zlib 1.1.3 (or later).
    # See http://www.gzip.org/zlib/
    # zlib zlibmodule.c -I$(prefix)/include -L$(exec_prefix)/lib -lz

I uncommented the last line, then I ran **python -v** again. However, I received the same error when I tried **import zlib**, so I guess I have to do something to install zlib into the lib. 

But I am clueless about what I need to do. Can someone please direct me in the right direction??? Thank you very much!

I am doing this because I want to use different version of python in different virtualenv I created. 
When I did  **virtualenv -p python2.7** I received no module named zlib. 

    jwxie518@jwxie518-P5E-VM-DO:~$ virtualenv -p python2.7 --no-site-packages testenv

    Running virtualenv with interpreter /home/jwxie518/.pythonbrew/pythons/Python-2.7.1/bin/python2.7

    Traceback (most recent call last):
      File &quot;/usr/local/lib/python2.6/dist-packages/virtualenv.py&quot;, line 17, in &lt;module&gt;
        import zlib

    ImportError: No module named zlib


---------- 
**EDIT**

I have to install 2.7.1 by appending --force.


I am developing Django, and I need some of these missing modules, for example sqlite3, and to create my virtualenv I definitely need zlib. If I just use the system default (2.6.6), I have no problem.

To do this with system default, all I need to do is

    virtualenv --no-site-packages testenv

Thanks! 


----------
**(2nd edit)**

I installed 3.2 also and I tested it without problem, so I guess my problem comes down to how to install the missing module(s).

    jwxie518@jwxie518-P5E-VM-DO:~$ virtualenv -p python3.2  testenv
    Running virtualenv with interpreter /home/jwxie518/.pythonbrew/pythons/Python-3.2/bin/python3.2
    New python executable in testenv/bin/python3.2
    Also creating executable in testenv/bin/python
    Installing distribute..................................................................................................................................................................................................................................................................................................................................done.
    Installing pip...............done.
    jwxie518@jwxie518-P5E-VM-DO:~$ virtualenv -p python3.2 --no-site-packages testenv
    Running virtualenv with interpreter /home/jwxie518/.pythonbrew/pythons/Python-3.2/bin/python3.2
    New python executable in testenv/bin/python3.2
    Also creating executable in testenv/bin/python
    Installing distribute..................................................................................................................................................................................................................................................................................................................................done.
    Installing pip...............done.



no module named zlib

I turned on Xcode4&#39;s snapshot on replace feature.  I want to turn it off.  How do I do that?

Xcode 4 disable snapshots creation on refactoring

I&#39;m getting acquainted with signals in C. I can&#39;t figure out what kind of signals `SIGUSR1` and `SIGUSR2` are and how can I trigger them. Can anyone please explain it to me?

How to trigger SIGUSR1 and SIGUSR2?

<p>I'm getting acquainted with signals in C. I can't figure out what kind of signals <code>SIGUSR1</code> and <code>SIGUSR2</code> are and how can I trigger them. Can anyone please explain it to me?</p>


I have grown accustomed to `strtod` and variants.
I am wondering why there is no `strtoi` shipped with `&lt;stdlib.h&gt;`.
Why is it that the integer type is left out of this party?

Specifically I am asking why there is not a version of `atoi` with the safety features of `strtod`?

Why is there no strtoi in stdlib.h?

I always think simply `if(p != NULL){..}` will do the job. But after reading [this Stack Overflow question][1], it seems not.

So what&#39;s the canonical way to check for NULL pointers after **absorbing all discussion in that question** which says NULL pointers can have non-zero value?

  [1]: https://stackoverflow.com/questions/2511921/which-of-these-will-create-a-null-pointer


How do we check if a pointer is NULL pointer?

In C, `enums`, internally equates to an integer. Therefore we can treat data types of `enum` as integer also.

How to achieve the same with C#? 

How to get the Enum Index value in C#

A fairly simple question... Compilers these days tend to do a significant amount of optimizations. Do they also remove unused functions from the final output? 

Do unused functions get optimized out?

Question
---------------
In a Windows C application I want to validate a parameter passed into a function to ensure that the specified path exists.*

**How do you check if a directory exists on Windows in C?**

*I understand that you can get into race conditions where between the time you check for the existance and the time you use the path that it no longer exists, but I can deal with that.

Additional Background
----------------
Knowing explicitly that a directory does or does not exist can get tricky when permissions come into play. It&#39;s possible that in attempting to determine if the directory exists, the process doesn&#39;t have permissions to access the directory or a parent directory. *This is OK for my needs*. If the directory doesn&#39;t exist OR I can&#39;t access it, both are treated as an invalid path failure in my application, so I don&#39;t need to differentiate. (Virtual) bonus points if your solution provides for this distinction.

Any solution in the C language, C runtime library, or Win32 API is fine, but ideally I&#39;d like to stick to libraries that are commonly loaded (e.g. kernel32, user32, etc.) and avoid solutions that involve loading non-standard libraries (like [PathFileExists][1] in Shlwapi.dll). Again, (Virtual) bonus points if your solution is cross-platform.

Related
---------------
[How can we check if a file Exists or not using Win32 program?][2]


  [1]: http://msdn.microsoft.com/en-us/library/bb773584%28v=vs.85%29.aspx
  [2]: https://stackoverflow.com/questions/3828835/how-can-we-check-if-a-file-exists-or-not-using-win32-program

How do you check if a directory exists on Windows in C?

I would like to know exactly how the execution of asynchronous signal handlers works on Linux. First, I am unclear as to *which* thread executes the signal handler. Second, I would like to know the steps that are followed to make the thread execute the signal handler.

On the first matter, I have read two different, seemingly conflicting, explanations:

1.  The Linux Kernel, by Andries Brouwer, [&#167;5.2 &quot;Receiving signals&quot; states][1]:

    &gt; When a signal arrives, the process is interrupted, the current registers are saved, and the signal handler is invoked. When the signal handler returns, the interrupted activity is continued.

2.  The [StackOverflow question &quot;Dealing With Asynchronous Signals In Multi Threaded Program&quot;][2] leads me to think that Linux&#39;s behavior is [like SCO Unix&#39;s][3]:

    &gt; When a signal is delivered to a process, if it is being caught, it will be handled by one, and only one, of the threads meeting either of the following conditions:
    &gt; 1. A thread blocked in a [**sigwait**(2)][4] system call whose argument *does* include the type of the caught signal.

    &gt; 2. A thread whose signal mask *does not* include the type of the caught signal.

    &gt; Additional considerations:
    &gt;  * A thread blocked in [**sigwait**(2)][5] is given preference over a thread not blocking the signal type.
    &gt;  * If more than one thread meets these requirements (perhaps two threads are calling [**sigwait**(2)][6]), then one of them will be chosen. This choice is not predictable by application programs.
    &gt;  * If no thread is eligible, the signal will remain \`\`pending&#39;&#39; at the process level until some thread becomes eligible.

    Also, [&quot;The Linux Signals Handling Model&quot; by Moshe Bar states][7] &quot;Asynchronous signals are delivered to the first thread found not blocking the signal.&quot;, which I interpret to mean that the signal is delivered to some thread having its sigmask *not* including the signal.

Which one is correct?

On the second matter, what happens to the stack and register contents for the selected thread?  Suppose the thread-to-run-the-signal-handler *T* is in the middle of executing a `do_stuff()` function. Is thread *T*&#39;s stack used directly to execute the signal handler (i.e. the address of the signal trampoline is pushed onto *T*&#39;s stack and control flow goes to the signal handler)? Alternatively, is a separate stack used? How does it work?


  [1]: http://www.win.tue.nl/~aeb/linux/lk/lk-5.html#ss5.2
  [2]: https://stackoverflow.com/q/6223191/196844
  [3]: http://uw714doc.sco.com/en/SDK_sysprog/PTL_ThdsSigs.html#_Asynch-Generated_Signal
  [4]: http://uw714doc.sco.com/en/man/html.2/sigwait.2.html
  [5]: http://uw714doc.sco.com/en/man/html.2/sigwait.2.html
  [6]: http://uw714doc.sco.com/en/man/html.2/sigwait.2.html
  [7]: http://www.linuxjournal.com/article/3985?page=0,1

How are asynchronous signal handlers executed on Linux?

Can I provide/pass any arguments to signal handler?

    /* Signal handling */
    struct sigaction act;
    act.sa_handler = signal_handler;
    /* some more settings */

Now, handler looks like this:

    void signal_handler(int signo) {
        /* some code */
    }

If I want to do something special i.e. delete temp files, can I provide those files as an argument to this handler?

Edit 0: Thanks for the answers. We generally avoid/discourage use of global variables. And in this case, If you have a huge program, things can go wrong at different places and you might need to do a lot of cleanup. Why was the API designed this way?

Providing/passing argument to signal handler

In the app I&#39;m developing on Android, I keep getting a Fatal Signal 11 error.

I think it&#39;s something to do with the way that I&#39;m accessing the memory but I can&#39;t figure out what is causing it. 

Any help will be much appreciated!

Here&#39;s the LogCat:

    05-02 23:47:17.618: D/dalvikvm(590): GC_FOR_ALLOC freed 68K, 4% free 6531K/6787K, paused 101ms
    05-02 23:47:17.638: I/dalvikvm-heap(590): Grow heap (frag case) to 7.619MB for 1228816-byte allocation
    05-02 23:47:17.738: D/dalvikvm(590): GC_CONCURRENT freed 1K, 4% free 7730K/8007K, paused 5ms+14ms
    05-02 23:47:17.878: D/dalvikvm(590): GC_FOR_ALLOC freed &lt;1K, 4% free 7730K/8007K, paused 37ms
    05-02 23:47:17.888: I/dalvikvm-heap(590): Grow heap (frag case) to 8.790MB for 1228816-byte allocation
    05-02 23:47:17.998: D/dalvikvm(590): GC_CONCURRENT freed &lt;1K, 4% free 8930K/9223K, paused 4ms+4ms
    05-02 23:47:17.998: A/libc(590): Fatal signal 11 (SIGSEGV) at 0x00000000 (code=1)



Android Fatal Signal 11

In Unix we can suspend a process execution temporarily and resume it with signals `SIGSTOP` and `SIGCONT`. How can I suspend a single-threaded process in Windows without programming ? 

How to suspend/resume a process in Windows?

Basically, what signal does &#39;0&#39; represent, because [here][1] I see SIGNAL numbers starting from 1. 


  [1]: http://lxr.linux.no/linux+v2.6.16.60/include/asm-i386/signal.h#L34

Content Type	Original Author	Original Content on Stackoverflow
Question	haunted85	View Question on Stackoverflow
Solution 1 - C	Oliver Charlesworth	View Answer on Stackoverflow
Solution 2 - C	Евгений Кашинский	View Answer on Stackoverflow
Solution 3 - C	cnicutar	View Answer on Stackoverflow

How to trigger SIGUSR1 and SIGUSR2?

C Problem Overview

C Solutions

Solution 1 - C

Solution 2 - C

Solution 3 - C

Xcode 4 disable snapshots creation on refactoring

no module named zlib

Attributions