The extern keyword declares a variable or function and specifies that it has external linkage (its name is visible from files other than the one in which it&#39;s defined). When modifying a variable, extern specifies that the variable has static duration (it is allocated when the program begins and deallocated when the program ends). The variable or function may be defined in another source file, or later in the same file. Declarations of variables and functions at file scope are external by default.

You can find a [more complete description here][1].


  [1]: http://www.geeksforgeeks.org/understanding-extern-keyword-in-c/

`extern` gives a name _external linkage_. This means that the object or function is accessible through this name from other translation units in the program. For functions, this is the default linkage in any case so its usage (in this context) is usually redundant.


For Beginners,

Initially I was confused to learn that, &quot;**the extern keyword declares a variable or function and specifies that it has external linkage** &quot; by @Romain Hippeau.


Now I understood that, we will be able to share our variables with other classes through extern keyword.

For Example:
Notification.h

     #import &lt;Foundation/Foundation.h&gt;
     extern const NSString* notificationConstant;

Notification.m

     #import &quot;Notification.h&quot;
     const NSString* notificationConstant = @&quot;NotificationConstant&quot;;


By importing notification.h in any of my other classes, I can read the value of string NotificationConstant. 

Without extern keyword For Notification constant will create following error.
[![enter image description here][3]][3]  


  [1]: https://i.stack.imgur.com/HS2jx.png
  [2]: https://i.stack.imgur.com/nb7Dy.png
  [3]: https://i.stack.imgur.com/asW4q.png

Refer to [this post][1] for discussions on previous versions.

Asking the question again since Visual Studio 2010 has been officially released. Does anyone have any visibility this early on as to which is better? What about any other products that may be out there? This one will probably be on my dime, so I&#39;d like the wisdom of those with experience before pulling the trigger on a decision.


  [1]: https://stackoverflow.com/questions/24216/resharper-vs-coderush

Resharper vs. Coderush - 2010 remake

I am using the Mac OS X Apple Script Editor and (while debugging) instead of writing a lot of `display dialog` statements, I&#39;d like to write the results of some calculation in the window below, called &quot;Result&quot; (I have the German UI here, so the translation is a guess). So is there a write/print statement that I can use for putting messages in the &quot;standard out&quot; window? I am not asking to put the messages in a logfile on the file system, it is purely temporary.

AppleScript Editor, write message to the &quot;Result&quot; window

What does the `extern` keyword mean? I&#39;ve seen that in front of an function declaration like

    extern void DoFoo ...

What does the extern keyword mean?

What does the <code>extern</code> keyword mean? I've seen that in front of an function declaration like
<pre><code class="hljs language-arduino">extern void DoFoo ...
</code></pre>

If I create a variable within a new set of curly braces, is that variable popped off the stack on the closing brace, or does it hang out until the end of the function? For example:

    void foo() {
       int c[100];
       {
           int d[200];
       }
       //code that takes a while
       return;
    }

Will `d` be taking up memory during the `code that takes a while` section?

In C, do braces act as a stack frame?

In C (or C++ for that matter), pointers are special if they have the value zero: I am adviced to set pointers to zero after freeing their memory, because it means freeing the pointer again isn&#39;t dangerous; when I call malloc it returns a pointer with the value zero if it can&#39;t get me memory; I use `if (p != 0)` all the time to make sure passed pointers are valid, etc.

But since memory addressing starts at 0, isn&#39;t 0 just as a valid address as any other? How can 0 be used for handling null pointers if that is the case? Why isn&#39;t a negative number null instead?

***

Edit:

A bunch of good answers. I&#39;ll summarize what has been said in the answers expressed as my own mind interprets it and hope that the community will correct me if I misunderstand.

* Like everything else in programming it&#39;s an abstraction. Just a constant, not really related to the address 0. C++0x emphasizes this by adding the keyword `nullptr`.

* It&#39;s not even an address abstraction, it&#39;s the constant specified by the C standard and the compiler can translate it to some other number as long as it makes sure it never equals a &quot;real&quot; address, and equals other null pointers if 0 is not the best value to use for the platform.

* In case it&#39;s not an abstraction, which was the case in the early days, the address 0 is used by the system and off limits to the programmer.

* My negative number suggestion was a little wild brainstorming, I admit. Using a signed integer for addresses is a little wasteful if it means that apart from the null pointer (-1 or whatever) the value space is split evenly between positive integers that make valid addresses and negative numbers that are just wasted.

* If any number is always representable by a datatype, it&#39;s 0. (Probably 1 is too. I think of the one-bit integer which would be 0 or 1 if unsigned, or just the signed bit if signed, or the two bit integer which would be [-2, 1]. But then you could just go for 0 being null and 1 being the only accessible byte in memory.)

Still there is something that is unresolved in my mind. The Stack&amp;nbsp;Overflow question *https://stackoverflow.com/questions/2389251* tells me that even if 0 for null pointer is an abstraction, other pointer values aren&#39;t necessarily. This leads me to post another Stack&amp;nbsp;Overflow question, *https://stackoverflow.com/questions/2761360*.


Why is address zero used for the null pointer?

The constant 0 is used as the null pointer in C and C++. But as in the question [*&quot;Pointer to a specific fixed address*&quot;][1] there seems to be some possible use of assigning fixed addresses. Is there ever any conceivable need, in any system, for whatever low level task, for accessing the address 0?

If there is, how is that solved with 0 being the null pointer and all?

If not, what makes it certain that there is not such a need?


  [1]: https://stackoverflow.com/questions/2389251/pointer-to-a-specific-fixed-address

Could I ever want to access the address zero?

What order should include files be specified, i.e. what are the reasons for including one header before another?

For example, do the system files, STL, and Boost go before or after the local include files?



C/C++ include header file order

I’m reading up on `pthread.h`; the condition variable related functions (like `pthread_cond_wait(3)`) require a mutex as an argument. Why? As far as I can tell, I’m going to be creating a mutex *just* to use as that argument? What is that mutex supposed to do?

Why do pthreads’ condition variable functions require a mutex?

I usually create my projects without IB-stuff. The first thing I do is to strip off all references to xibs, outlets updated plist, etc and so forth. No problems, works great (in my world)!

Now, I just installed 3.2 and tried to develop my first iPad app. Following same procedure as before, I created a UISplitView-based application project and stripped off all IB-stuff. Also, I followed the section in Apple&#39;s reference docs: [Creating a Split View Controller Programmatically][1], but nevertheless, the Master-view is never shown, only the Detail-view is (no matter what the orientation is). I really have tried to carefully look this through but I cannot understand what I have missed.

Is there a working example of a UISplitViewController without the nibs floating around somewhere? I have googled but could not find any. Or do you know what I probably have missed?

  [1]: http://developer.apple.com/library/ios/#documentation/iPhone/Conceptual/iPhoneOSProgrammingGuide/Introduction/Introduction.html

UISplitViewController programmatically without nib/xib

I need to display an email address on the left side of a `UIButton`, but it is being positioned to the centre.

Is there any way to set the alignment to the left side of a `UIButton`?

This is my current code:

    UIButton* emailBtn = [[UIButton alloc] initWithFrame:CGRectMake(5,30,250,height+15)];
    emailBtn.backgroundColor = [UIColor clearColor];
    [emailBtn setTitle:obj2.customerEmail forState:UIControlStateNormal];
    emailBtn.titleLabel.font = [UIFont systemFontOfSize:12.5];
    [emailBtn setTitleColor:[[[UIColor alloc]initWithRed:0.121 green:0.472 blue:0.823 alpha:1]autorelease] forState:UIControlStateNormal];
    [emailBtn addTarget:self action:@selector(emailAction:) forControlEvents:UIControlEventTouchUpInside];
    [elementView addSubview:emailBtn];
    [emailBtn release];

How can I set the title of a UIButton as left-aligned?

Building an NSCharacter set to restrict a UITextField for entering user names. I want the user to be able to also enter an underscore (so [A-Za-z0-9_]) but alphanumericCharacterSet does not include it. Is there a way to specify a range like that in short form? I see `+ (id)characterSetWithRange:(NSRange)aRange`, but I&#39;m not really understanding how that would work.

I&#39;ve got a simple UITextField sub-class that I pass the character set to. The restriction works fine and doesn&#39;t allow the user to enter anything but alpha numeric. Just need to add the &quot;_&quot; to those allowances.

    NSCharacterSet *characterSet = [NSCharacterSet alphanumericCharacterSet];
    [textField setAllowed:characterSet];
    [textField setFrame:frame];


NSCharacterSet: How do I add &quot;_&quot; to alphanumericCharacterSet text restriction?

I have a `UITableView` that is populated with cells of a variable height. I would like the table to scroll to the bottom when the view is pushed into view.

I currently have the following function

    NSIndexPath *indexPath = [NSIndexPath indexPathForRow:[log count]-1 inSection:0];
    [self.table scrollToRowAtIndexPath:indexPath atScrollPosition:UITableViewScrollPositionBottom animated:NO];

log is a mutable array containing the objects that make up the content of each cell.

The above code works fine in `viewDidAppear` however this has the unfortunate side effect of displaying the top of the table when the view first appears and then jumping to the bottom. I would prefer it if the `table view` could be scrolled to the bottom before it appears.

I tried the scroll in `viewWillAppear` and `viewDidLoad` but in both cases the data has not been loaded into the table yet and both throw an exception.

Any guidance would be much appreciated, even if it&#39;s just a case of telling me what I have is all that is possible.

How to scroll to the bottom of a UITableView on the iPhone before the view appears

Currently, we are defining ourselves an extended log mechanism to print out the class name and the source line number of the log. 

    #define NCLog(s, ...) NSLog(@&quot;&lt;%@:%d&gt; %@&quot;, [[NSString stringWithUTF8String:__FILE__] lastPathComponent], \
        __LINE__, [NSString stringWithFormat:(s), ##__VA_ARGS__])

For example, when I call NCLog(@&quot;Hello world&quot;);
The output will be:

    &lt;ApplicationDelegate:10&gt;Hello world

Now I also want to log out the method name like:

    &lt;ApplicationDelegate:applicationDidFinishLaunching:10&gt;Hello world


So, this would make our debugging become easier when we can know which method is getting called. I know that we also have Xcode debugger but sometimes, I also want to do debugging by logging out.

NSLog the method name with Objective-C in iPhone

Why does `extern int n` not compile when n is declared (in a different file) `static int n`, but works when declared `int n`?  (Both of these declarations were at file scope.)  

Basically, why is `int n` in file scope not the same as `static int n` in the same scope?  Is it only in relation to extern?  If so, what about extern am I missing?

Why won&#39;t extern link to a static variable?

Whenever I look deeply enough into reflector I bump into `extern` methods with no source.  I read the msdn documentation at http://msdn.microsoft.com/en-us/library/e59b22c5(v=vs.80).aspx.  What I got from that article is that methods with the `extern` modifier have to be injected.  I interpreted this to mean it works something like an abstract factory pattern.  I also noticed that I&#39;ve never seen a non-static extern method.  Is static declaration a requirement (I could see how this would make sense)?   I&#39;m still guessing here and I&#39;m not sure how it actually works.  It seems to me like the compiler must recognize certain attributes that mitigate processing, but I don&#39;t know what the attributes are other than ones I&#39;ve come across like `MethodImplAttribute` and `DllImportAttribute` from the MSDN example.  How does someone leverage the `extern` attribute?  It said that in many instances this can increase performance.  Also, how would I go about looking into the source of `extern` methods like `Object.InternalGetEquals()`? 

How does extern work in C#?

If I define a global variable in a `.c` file, how can I use the same variable in another `.c` file?

`file1.c`:

    #include&lt;stdio.h&gt;
    
    int i=10;
    
    int main()
    {
        printf(&quot;%d&quot;,i);
        return 0;
    }

`file2.c`:

    #include&lt;stdio.h&gt;
    
    int main()
    {
        //some data regarding i
        printf(&quot;%d&quot;,i);
        return 0;
    }

How can the second file `file2.c` use the value of `i` from the first file `file1.c`?


How do I share a global variable between c files?

## Figure 1: _function templates_

TemplHeader.h

    template&lt;typename T&gt;
    void f();


TemplCpp.cpp

    template&lt;typename T&gt;
    void f(){
       //...
    }    
    //explicit instantation
    template void f&lt;T&gt;();

Main.cpp

    #include &quot;TemplHeader.h&quot;
    extern template void f&lt;T&gt;(); //is this correct?
    int main() {
        f&lt;char&gt;();
        return 0;
    }

Is this the correct way to use `extern template`, or do I use this keyword only for class templates as in Figure 2?

## Figure 2: _class templates_

TemplHeader.h

    template&lt;typename T&gt;
    class foo {
        T f();
    };

TemplCpp.cpp

    template&lt;typename T&gt;
    void foo&lt;T&gt;::f() {
        //...
    }
    //explicit instantation
    template class foo&lt;int&gt;;

Main.cpp

    #include &quot;TemplHeader.h&quot;
    extern template class foo&lt;int&gt;();
    int main() {
        foo&lt;int&gt; test;
        return 0;
    }

I know it is good to put all of this in one header file, but if we instantiate templates with the same parameters in multiple files, then we got multiple same definitions and the compiler will remove them all (except one) to avoid errors. How do I use `extern template`? Can we use it only for classes, or can we use it for functions too?

Also, Figure 1 and Figure 2 may be expanded to a solution where templates are in a single header file . In that case, we need to use the `extern template` keyword to avoid multiple same instantations. Is this only for classes or functions too?

using extern template (C++11)

I would like to ask what&#39;s the reason behind using `FOUNDATION_EXPORT` instead of `extern` in _Objective C_ projects.

I&#39;ve checked [this question][1] and using `FOUNDATION_EXPORT` has earned whopping 340 points (1st place) whereas using `extern` only 74 points (2nd place). 

Could anybody explain why? Is there any practical reason for using `FOUNDATION_EXPORT` instead of `extern`?

Thanks!

  [1]: https://stackoverflow.com/questions/538996/constants-in-objective-c

Content Type	Original Author	Original Content on Stackoverflow
Question	dontWatchMyProfile	View Question on Stackoverflow
Solution 1 - C	Romain Hippeau	View Answer on Stackoverflow
Solution 2 - C	CB Bailey	View Answer on Stackoverflow
Solution 3 - C	Sujananth	View Answer on Stackoverflow

What does the extern keyword mean?

C Problem Overview

C Solutions

Solution 1 - C

Solution 2 - C

Solution 3 - C

AppleScript Editor, write message to the "Result" window

Resharper vs. Coderush - 2010 remake

Attributions