What are undefined reference/unresolved external symbol errors? What are common causes and how to fix/prevent them?

What is an undefined reference/unresolved external symbol error and how do I fix it?

I&#39;m very new to C and I have this code: 

    #include &lt;stdio.h&gt;
    #include &lt;math.h&gt;
    int main(void)
    {
      double x = 0.5;
      double result = sqrt(x);
      printf(&quot;The square root of %lf is %lf\n&quot;, x, result);
      return 0;
    }

But when I compile this with:

    gcc test.c -o test

I get an error like this:

    /tmp/cc58XvyX.o: In function `main&#39;:
    test.c:(.text+0x2f): undefined reference to `sqrt&#39;
    collect2: ld returned 1 exit status


Why does this happen? Is `sqrt()` not in the `math.h` header file? I get the same error with `cosh` and other trigonometric functions. Why?


Why am I getting &quot;undefined reference to sqrt&quot; error even though I include math.h header?

When I try running my Xcode Project it fails with an error stating that I have duplicate symbols. I looked online where the find these duplicates but have had no luck:

![enter image description here][1]


  [1]: http://i.stack.imgur.com/jliti.png

Any ideas how to fix this? 

Duplicate Symbols for Architecture arm64

I have the following code (stripped down to the bare basics for this question):

    #include&lt;stdio.h&gt;
    #include&lt;math.h&gt;
    
    double f1(double x)
    {
    	double res = sin(x);
    	return 0;
    }
    
    /* The main function */
    int main(void)
    {
    	return 0;
    }

When compiling it with `gcc test.c` I get the following error, and I can&#39;t work out why:

    /tmp/ccOF5bis.o: In function `f1&#39;:
    test2.c:(.text+0x13): undefined reference to `sin&#39;
    collect2: ld returned 1 exit status

However, I&#39;ve written various test programs that call `sin` from within the `main` function, and those work perfectly. I must be doing something obviously wrong here - but what is it?


Undefined reference to `sin`

I have installed Rust on windows from [Rust installation page][1]. After installation I tried running the &quot;hello world&quot; program but got the following error.

```none
&gt;cargo run
```

**Error**

```none
Compiling helloworld v0.1.0 (C:\Users\DELL\helloworld)

error: linker `link.exe` not found
note: The system cannot find the file specified. (os error 2)
note: the msvc targets depend on the msvc linker but `link.exe` was not found
note: please ensure that VS 2013, VS 2015 or VS 2017 was installed with the Visual C++ option
error: aborting due to previous error
error: Could not compile `helloworld`.

To learn more, run the command again with --verbose.
```

**Code**:

```rust
fn main() {
    println!(&quot;Hello, world!&quot;);
}
```

  [1]: https://www.rust-lang.org/tools/install



Unable to compile Rust hello world on Windows: linker link.exe not found

Just a simple program, but I keep getting this compiler error. I&#39;m using MinGW for the compiler.

Here&#39;s the header file, **point.h**:

    //type for a Cartesian point
    typedef struct {
      double x;
      double y;
    } Point;
    
    Point create(double x, double y);
    Point midpoint(Point p, Point q);

And here&#39;s **point.c**:

    //This is the implementation of the point type
    #include &quot;point.h&quot;
    
    int main() {
      return 0;
    }
    Point create(double x, double y) {
      Point p;
      p.x = x;
      p.y = y;
      return p;
    }
    
    Point midpoint(Point p, Point q) {
      Point mid;
      mid.x = (p.x + q.x) / 2;
      mid.y = (p.y + q.y) / 2;
      return mid;
    }

And here&#39;s where the compiler issue comes in. I keep getting:

&gt; testpoint.c: undefined reference to &#39;create(double x, double y)&#39;

While it is defined in point.c.

This is a separate file called **testpoint.c**:

    #include &quot;point.h&quot;
    #include &lt;assert.h&gt;
    #include &lt;stdio.h&gt;
    int main() {
      double x = 1;
      double y = 1;
      Point p = create(x, y);
      
      assert(p.x == 1);
      return 0;
    }

I&#39;m at a loss as to what the issue could be.




C error: undefined reference to function, but it IS defined

I am attempting to use the libtommath library. I&#39;m using the NetBeans IDE for my project on Ubuntu linux. I have downloaded and built the library, I have done a &#39;make install&#39; to put the resulting .a file into /usr/lib/ and the .h files into /usr/include

It appears to be finding the files appropriately (since I no longer get those errors, which I did before installing into the /usr directories).

However, when I create a simple main making a call to mp_init (which is in the library), I get the following error when I attempt to make my project:

    mkdir -p build/Debug/GNU-Linux-x86
    rm -f build/Debug/GNU-Linux-x86/main.o.d
    gcc -c -g -MMD -MP -MF build/Debug/GNU-Linux-x86/main.o.d -o build/Debug/GNU-Linux-x86/main.o main.c
    mkdir -p dist/Debug/GNU-Linux-x86
    gcc -o dist/Debug/GNU-Linux-x86/cproj1 build/Debug/GNU-Linux-x86/main.o
    build/Debug/GNU-Linux-x86/main.o: In function &#39;main&#39;:
    /home/[[myusername]]/NetBeansProjects/CProj1/main.c:18: undefined reference to `mp_init&#39;
    collect2: ld returned 1 exit status
    make[2]: *** [dist/Debug/GNU-Linux-x86/cproj1] Error 1

So, it looks like the linker can&#39;t find the function within the library, however it IS there, so I just don&#39;t know what could be causing this.

I get the same error if I type the gcc command directly and skip the makefile, I also made sure the static library got compiled with gcc as well.

Edited to Add:

I get these same errors if I do the compile directly and add the library with -l or -L:

    $ gcc -l /usr/lib/libtommath.a main.c 
    /usr/bin/ld: cannot find -l/usr/lib/libtommath.a
    collect2: ld returned 1 exit status
    
    $ gcc -llibtommath.a main.c 
    /usr/bin/ld: cannot find -llibtommath.a
    collect2: ld returned 1 exit status
    
    $ gcc -Llibtommath.a main.c 
    /tmp/ccOxzclw.o: In function `main&#39;:
    main.c:(.text+0x18): undefined reference to `mp_init&#39;
    collect2: ld returned 1 exit status
    
    $ gcc -Llibtommath.a main.c 
    /tmp/ccOxzclw.o: In function `main&#39;:
    main.c:(.text+0x18): undefined reference to `mp_init&#39;
    collect2: ld returned 1 exit status

I am very rusty on this stuff, so I&#39;m not sure I&#39;m using the right command here, in the -L examples are the libraries being found? If the library isn&#39;t being found how on earth do I get it to find the library? It&#39;s in /usr/lib, I&#39;ve tried it with the .a file in the current directory, etc.  Is there an environment variable I need to set?  If so, how, etc.

I&#39;ve tried a completely different library (GMP) and had the EXACT same problem. This has got to be some kind of Ubuntu environment issue? Anyone have any idea how to fix this?

Trying to include a library, but keep getting &#39;undefined reference to&#39; messages

I copied some bits of code from [apple&#39;s documentation](http://developer.apple.com/library/ios/#documentation/AudioVideo/Conceptual/CameraAndPhotoLib_TopicsForIOS/Articles/PickinganItemfromthePhotoLibrary.html)- and I got these 2 errors: 

    Undefined symbols for architecture i386:
      &quot;_kUTTypeImage&quot;, referenced from:
          -[ImagePicker imagePickerController:didFinishPickingMediaWithInfo:] in ImagePicker.o
    ld: symbol(s) not found for architecture i386
    clang: error: linker command failed with exit code 1 (use -v to see invocation)

What am I doing wrong?

EDIT:
The code:

    - (IBAction) showSavedMediaBrowser {
        [self startMediaBrowserFromViewController: self
                                    usingDelegate: (id)self];
    }
    
    - (BOOL) startMediaBrowserFromViewController: (UIViewController*) controller
                                   usingDelegate: (id &lt;UIImagePickerControllerDelegate,
                                                   UINavigationControllerDelegate&gt;) delegate {
        
        if (([UIImagePickerController isSourceTypeAvailable:
              UIImagePickerControllerSourceTypeSavedPhotosAlbum] == NO)
            || (delegate == nil)
            || (controller == nil))
            return NO;
        
        UIImagePickerController *mediaUI = [[UIImagePickerController alloc] init];
        mediaUI.sourceType = UIImagePickerControllerSourceTypeSavedPhotosAlbum;
        
        // Displays saved pictures and movies, if both are available, from the
        // Camera Roll album.
        mediaUI.mediaTypes =
        [UIImagePickerController availableMediaTypesForSourceType:
         UIImagePickerControllerSourceTypeSavedPhotosAlbum];
        
        // Hides the controls for moving &amp; scaling pictures, or for
        // trimming movies. To instead show the controls, use YES.
        mediaUI.allowsEditing = YES;
        
        mediaUI.delegate = delegate;
        
        [controller presentViewController:mediaUI animated:YES completion:nil];
        return YES;
    }
    
    - (void) imagePickerController: (UIImagePickerController *) picker
     didFinishPickingMediaWithInfo: (NSDictionary *) info {
        
        NSString *mediaType = [info objectForKey: UIImagePickerControllerMediaType];
        UIImage *originalImage, *editedImage, *imageToUse;
        
        // Handle a still image picked from a photo album
        if (CFStringCompare ((__bridge_retained CFStringRef) mediaType, kUTTypeImage, 0)
            == kCFCompareEqualTo) {
            
            editedImage = (UIImage *) [info objectForKey:
                                       UIImagePickerControllerEditedImage];
            originalImage = (UIImage *) [info objectForKey:
                                         UIImagePickerControllerOriginalImage];
            
            if (editedImage) {
                imageToUse = editedImage;
            } else {
                imageToUse = originalImage;
            }
            // Do something with imageToUse
        }
        
        [[picker parentViewController] dismissModalViewControllerAnimated: YES];
    }

        
I think the error is where the last method starts, but I&#39;m not sure.


Your post does not have much context to explain the code sections; please explain your scenario more clearly.


Symbol not found: kUTTypeImage

I have this simple code:

    max = (int) sqrt (number);

and in the header I have:

    #include &lt;math.h&gt;

But application still says undefined reference to `sqrt`. Do you see any problem here? It looks like everything should be okay.


Undefined reference to sqrt (or other mathematical functions)

I got to build static library. I want to use in my iPhone and ipad app. When I try to run the simulator I get linking errrors. I am new to iOS development. kindly help;


&gt; ld: warning: ignoring file
&gt; /Users/valuelabs/Desktop/DruvaProject/libraries/libnetUtils.a, file
&gt; was built for archive which is not the architecture being linked
&gt; (i386) Undefined symbols for architecture i386:  
&gt; &quot;_OBJC_CLASS_$_netUtils&quot;, referenced from:
&gt;       objc-class-ref in ViewController.o ld: symbol(s) not found for architecture i386 clang: error: linker command failed with exit code 1
&gt; (use -v to see invocation)



I tried adding i386 in the Architectures. but no luck

file was built for archive which is not the architecture being linked (i386)

I defined a special file: `config.h`

My project also has files:

    t.c, t.h
    pp.c, pp.h
    b.c b.h
    l.cpp

and #includes:

**in t.c:**

	
        #include &quot;t.h&quot;
    	#include &quot;b.h&quot;
    	#include &quot;pp.h&quot;
        #include &quot;config.h&quot;
	
**in b.c:**	

        #include &quot;b.h&quot;
    	#include &quot;pp.h&quot;
	
**in pp.c:**

        #include &quot;pp.h&quot;
    	#include &quot;config.h&quot;
	
**in l.cpp:**

    	#include &quot;pp.h&quot;
    	#include &quot;t.h&quot;
    	#include &quot;config.h&quot;
	
there are no include directives in my `*.h` files, only in `*.c` files. I defined this in config.h:

    const char *names[i] =
            {
                &quot;brian&quot;, &quot;stefan&quot;, &quot;steve&quot;
            };
        
and need that array in l.cpp, t.c, pp.c but Im getting this error:

    pp.o:(.data+0x0): multiple definition of `names&#39;
    l.o:(.data+0x0): first defined here
    t.o:(.data+0x0): multiple definition of `names&#39;
    l.o:(.data+0x0): first defined here
    collect2: ld returned 1 exit status
    make: *** [link] Error 1

I have include guards in every `*.h` file I use in my project. Any help solving this?




Multiple definition of ... linker error

This is my header:
&lt;!-- language-all: c++ --&gt;

    #ifndef BARELYSOCKET_H
    #define BARELYSOCKET_H
    
    #include &lt;QObject&gt;
    //! The First Draw of the BarelySocket!
    
    class BarelySocket: public QObject
    {
        Q_OBJECT
    
    public:
        BarelySocket();
    public slots:
        void sendMessage(Message aMessage);
    signals:
        void reciveMessage(Message aMessage);
    
    private:
        //   QVector&lt;Message&gt; reciveMessages;
    };
    
    #endif // BARELYSOCKET_H

This is my class:

    #include &lt;QTGui&gt;
    #include &lt;QObject&gt;
    #include &quot;type.h&quot;
    #include &quot;client.h&quot;
    #include &quot;server.h&quot;
    
    #include &quot;barelysocket.h&quot;
    
    BarelySocket::BarelySocket()
    {
        //this-&gt;reciveMessages.clear();
        qDebug(&quot;BarelySocket::BarelySocket()&quot;);
    }
    
    void BarelySocket::sendMessage(Message aMessage)
    {
    }
    
    void BarelySocket::reciveMessage(Message aMessage)
    {
    }

I get a Linker error: 

    undefined reference to &#39;vtable for BarelySocket&#39;

 - This implies that I have a virtual method not implemented. But there
   are no virtual methods in my class.
 - I commented out the vector thinking that it was the cause, but the
   error did not go away.
 - The `Message` is a complex `struct`, but even using `int` instead did
   not fix things.

Qt Linker Error: &quot;undefined reference to vtable&quot;

I&#39;m using Qt Creator 2.0.1 with Qt 4.7.0 (32 bit) on Windows 7 Ultimate 32 bit.

Consider the following code, which is a minimum to produce the error:

    class T : public QObject, public QGraphicsItem
    {
    	Q_OBJECT
    
    public:
    	T() {}
    
    	QRectF		boundingRect() const {return QRectF();}
    	void		paint(QPainter *painter, const QStyleOptionGraphicsItem *option,
                          QWidget *widget) {}
    };

    int main()
    {
        T t;
        return 0;
    }

The above code fragment causes the following linker errors:

&gt; In function `T&#39;:
&gt; 
&gt; undefined reference to `vtable for T&#39;
&gt; 
&gt; undefined reference to `vtable for T&#39;
&gt; 
&gt; In function `~T&#39;:
&gt; 
&gt; undefined reference to `vtable for T&#39;
&gt; 
&gt; undefined reference to `vtable for T&#39;

If I comment out the line that contains `Q_OBJECT`, it compiles fine. I need signal and slots with `QGraphicsItem` so I need `Q_OBJECT`.

What is wrong with the code? Thanks.


Q_OBJECT throwing &#39;undefined reference to vtable&#39; error

I simply want to rid of the warnings that pop up like these:

    ld: warning: directory not found for option &#39;-F/Users/m/Desktop/FacebookSDK&#39;
    ld: warning: directory not found for option &#39;-F/Users/m/Desktop/FacebookSDK/FBAudienceNetwork&#39;
    ld: warning: directory not found for option &#39;-F/Users/gavin/Downloads/shaffiulla11-bat-b86d81d8adc6&#39;
    ld: warning: directory not found for option &#39;-F/Users/gavin/Desktop/shaffiulla11-bat-b86d81d8adc6&#39;
    ld: warning: ignoring file /Users/Gavin/Desktop/FacebookSDK/Bolts.framework/Bolts, missing required architecture x86_64 in file /Users/Gavin/Desktop/FacebookSDK/Bolts.framework/Bolts (2 slices)

Obviously I am using different directories now because of multiple reformats of my drive because of Logic Board issues and just would like to know the most convenient way I can get rid of these errors. Thanks for the time and help.

Apple Mach-O Linker Warning &quot;Directory Not Found For Option...&quot;

The basic Google search [bigobj issue](https://www.google.com/search?q=bigobj+issue) shows that a lot of people are experiencing the [Fatal Error C1128](http://msdn.microsoft.com/en-us/library/8578y171.aspx): &quot;*number of sections exceeded object file format limit : compile with /bigobj*&quot;. The error has more chance to occur if one heavily uses a library of C++ templates, like [Boost libraries](http://www.boost.org) or [CGAL libraries](http://www.cgal.org/).

That error is strange, because it gives the solution to itself: set the [compiler flag `/bigobj`](http://msdn.microsoft.com/en-us/library/ms173499.aspx)!

So here is my question: why is not that flag set by default? There must be a penalty of using that flag, otherwise it would be set by default. That penalty is not documented in MSDN. Does anybody have a clue?

I ask the question because I wonder if the configuration system of [CGAL](http://www.cgal.org) should not set `/bigobj` by default.

Penalty of the MSVS compiler flag /bigobj

First of all, I know this question is all over this site but I have looked at almost all of them and can&#39;t seem to find out what is wrong. This is in VS 2012. Thanks.

    //Socket.h
    #pragma once
    
    #include &lt;iostream&gt;
    #include &lt;WinSock2.h&gt;
    
    using namespace std;
    
    const int STRLEN = 256;
    
    class Socket
    {
        protected:
            WSADATA wsaData;
            SOCKET mySocket;
            SOCKET myBackup;
            SOCKET acceptSocket;
            sockaddr_in myAddress;
        public:
            Socket();
            ~Socket();
            bool SendData( char* );
            bool RecvData( char*, int );
            void CloseConnection();
            void GetAndSendMessage();
    };
    
    class ServerSocket : public Socket
    {
        public:
            void Listen();
            void Bind( int port );
            void StartHosting( int port );
    };
    
    class ClientSocket : public Socket
    {
        public:
            void ConnectToServer( const char *ipAddress, int port );
    };
Here&#39;s Socket.cpp

    //Socket.cpp
 

    #include &quot;stdafx.h&quot;
    #include &quot;Socket.h&quot;

    
    Socket::Socket()
    {
        if( WSAStartup( MAKEWORD(2, 2), &amp;wsaData ) != NO_ERROR )
        {
            cerr&lt;&lt;&quot;Socket Initialization: Error with WSAStartup\n&quot;;
            system(&quot;pause&quot;);
            WSACleanup();
            exit(10);
        }
    
        //Create a socket
        mySocket = socket( AF_INET, SOCK_STREAM, IPPROTO_TCP );
    
        if ( mySocket == INVALID_SOCKET )
        {
            cerr&lt;&lt;&quot;Socket Initialization: Error creating socket&quot;&lt;&lt;endl;
            system(&quot;pause&quot;);
            WSACleanup();
            exit(11);
        }
    
        myBackup = mySocket;
    }
    
    Socket::~Socket()
    {
        WSACleanup();
    }
    
    bool Socket::SendData( char *buffer )
    {
        send( mySocket, buffer, strlen( buffer ), 0 );
        return true;
    }
    
    bool Socket::RecvData( char *buffer, int size )
    {
        int i = recv( mySocket, buffer, size, 0 );
        buffer[i] = &#39;\0&#39;;
        return true;
    }
    
    void Socket::CloseConnection()
    {
        //cout&lt;&lt;&quot;CLOSE CONNECTION&quot;&lt;&lt;endl;
        closesocket( mySocket );
        mySocket = myBackup;
    }
    
    void Socket::GetAndSendMessage()
    {
        char message[STRLEN];
        cin.ignore();//without this, it gets the return char from the last cin and ignores the following one!
        cout&lt;&lt;&quot;Send &gt; &quot;;
        cin.get( message, STRLEN );
        SendData( message );
    }
    
    void ServerSocket::StartHosting( int port )
    {
         Bind( port );
         Listen();
    }
    
    void ServerSocket::Listen()
    {
        //cout&lt;&lt;&quot;LISTEN FOR CLIENT...&quot;&lt;&lt;endl;
        
        if ( listen ( mySocket, 1 ) == SOCKET_ERROR )
        {
            cerr&lt;&lt;&quot;ServerSocket: Error listening on socket\n&quot;;
            system(&quot;pause&quot;);
            WSACleanup();
            exit(15);
        }
        
        //cout&lt;&lt;&quot;ACCEPT CONNECTION...&quot;&lt;&lt;endl;
        
        acceptSocket = accept( myBackup, NULL, NULL );
        while ( acceptSocket == SOCKET_ERROR )
        {
            acceptSocket = accept( myBackup, NULL, NULL );
        }
        mySocket = acceptSocket;
    }
    
    void ServerSocket::Bind( int port )
    {
        myAddress.sin_family = AF_INET;
        myAddress.sin_addr.s_addr = inet_addr( &quot;0.0.0.0&quot; );
        myAddress.sin_port = htons( port );
        
        //cout&lt;&lt;&quot;BIND TO PORT &quot;&lt;&lt;port&lt;&lt;endl;
    
        if ( bind ( mySocket, (SOCKADDR*) &amp;myAddress, sizeof( myAddress) ) == SOCKET_ERROR )
        {
            cerr&lt;&lt;&quot;ServerSocket: Failed to connect\n&quot;;
            system(&quot;pause&quot;);
            WSACleanup();
            exit(14);
        }
    }
    
    void ClientSocket::ConnectToServer( const char *ipAddress, int port )
    {
        myAddress.sin_family = AF_INET;
        myAddress.sin_addr.s_addr = inet_addr( ipAddress );
        myAddress.sin_port = htons( port );
        
        //cout&lt;&lt;&quot;CONNECTED&quot;&lt;&lt;endl;
    
        if ( connect( mySocket, (SOCKADDR*) &amp;myAddress, sizeof( myAddress ) ) == SOCKET_ERROR )
        {
            cerr&lt;&lt;&quot;ClientSocket: Failed to connect\n&quot;;
            system(&quot;pause&quot;);
            WSACleanup();
            exit(13);
        } 

    }

And here&#39;s stdafx.h

    #pragma once
    
    #include &quot;targetver.h&quot;
    
    #define WIN32_LEAN_AND_MEAN             // Exclude rarely-used stuff from Windows headers
    // Windows Header Files:
    #include &lt;windows.h&gt;
    
    // C RunTime Header Files
    #include &lt;stdlib.h&gt;
    #include &lt;malloc.h&gt;
    #include &lt;memory.h&gt;
    #include &lt;tchar.h&gt;
    
    
    // TODO: reference additional headers your program requires here
    #include &quot;Socket.h&quot;

And here are my error messages:

    1&gt;------ Build started: Project: Client, Configuration: Debug Win32 ------
    1&gt;  stdafx.cpp
    1&gt;  Socket.cpp
    1&gt;  Client.cpp
    1&gt;  Generating Code...
    1&gt;Socket.obj : error LNK2019: unresolved external symbol __imp__accept@12 referenced in function &quot;public: void __thiscall ServerSocket::Listen(void)&quot; (?Listen@ServerSocket@@QAEXXZ)
    1&gt;Socket.obj : error LNK2019: unresolved external symbol __imp__bind@12 referenced in function &quot;public: void __thiscall ServerSocket::Bind(int)&quot; (?Bind@ServerSocket@@QAEXH@Z)
    1&gt;Socket.obj : error LNK2019: unresolved external symbol __imp__closesocket@4 referenced in function &quot;public: void __thiscall Socket::CloseConnection(void)&quot; (?CloseConnection@Socket@@QAEXXZ)
    1&gt;Socket.obj : error LNK2019: unresolved external symbol __imp__connect@12 referenced in function &quot;public: void __thiscall ClientSocket::ConnectToServer(char const *,int)&quot; (?ConnectToServer@ClientSocket@@QAEXPBDH@Z)
    1&gt;Socket.obj : error LNK2019: unresolved external symbol __imp__htons@4 referenced in function &quot;public: void __thiscall ServerSocket::Bind(int)&quot; (?Bind@ServerSocket@@QAEXH@Z)
    1&gt;Socket.obj : error LNK2019: unresolved external symbol __imp__inet_addr@4 referenced in function &quot;public: void __thiscall ServerSocket::Bind(int)&quot; (?Bind@ServerSocket@@QAEXH@Z)
    1&gt;Socket.obj : error LNK2019: unresolved external symbol __imp__listen@8 referenced in function &quot;public: void __thiscall ServerSocket::Listen(void)&quot; (?Listen@ServerSocket@@QAEXXZ)
    1&gt;Socket.obj : error LNK2019: unresolved external symbol __imp__recv@16 referenced in function &quot;public: bool __thiscall Socket::RecvData(char *,int)&quot; (?RecvData@Socket@@QAE_NPADH@Z)
    1&gt;Socket.obj : error LNK2019: unresolved external symbol __imp__send@16 referenced in function &quot;public: bool __thiscall Socket::SendData(char *)&quot; (?SendData@Socket@@QAE_NPAD@Z)
    1&gt;Socket.obj : error LNK2019: unresolved external symbol __imp__socket@12 referenced in function &quot;public: __thiscall Socket::Socket(void)&quot; (??0Socket@@QAE@XZ)
    1&gt;Socket.obj : error LNK2019: unresolved external symbol __imp__WSAStartup@8 referenced in function &quot;public: __thiscall Socket::Socket(void)&quot; (??0Socket@@QAE@XZ)
    1&gt;Socket.obj : error LNK2019: unresolved external symbol __imp__WSACleanup@0 referenced in function &quot;public: __thiscall Socket::Socket(void)&quot; (??0Socket@@QAE@XZ)
    1&gt;C:\Users\ajayp_000\documents\visual studio 2012\Projects\Client\Debug\Client.exe : fatal error LNK1120: 12 unresolved externals
    ========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========



Unresolved external symbol LNK2019

I&#39;m having trouble with the linking of my files.

Basically, my program consists of:

 - The main program, `gen1`.
 - `gen1` - receives input sends to `str2value` for
   processing, outputs results `str2value`, breaks input into tokens
   using &quot;tokenizer&quot; determines what sort of processing to do to each
   token, and passes them off to `str2num`, or `str2cmd`. It then returns an
   array of the results.
 - `str2num` - does some processing
 - `str2cmd` - ditto
 - `author.py` - a python script that generates `str2cmd.c` and `str2cmd.h` from a header `cmdTable.h`.


I&#39;m pretty sure I have my includes right, I&#39;ve checked a couple of times. I&#39;ve also checked that there are no conditions `#ifndef` wrong in the headers.


Here is my Makefile:

    #CPP = g++ -lserial
    CPP = g++ -DTESTMODE
    C= gcc
    DEFINES = LURC
    CFLAGS = -Wall -fshort-enums -D$(DEFINES)
    PROJECTFILES = gen1.cpp str2value.o
    
    STR2VALUEFILES = str2value.cpp str2cmd.o str2num.o tokenizer.o str2value.h
    
    gen1 : $(PROJECTFILES)
            $(CPP) $(CFLAGS) -o gen1 $(PROJECTFILES)
    
    
    
    str2value.o : $(STR2VALUEFILES)
    #       echo &quot;str2value&quot;
            $(CPP) $(CFLAGS) -c $(STR2VALUEFILES)
    
    str2num.o: str2num.cpp  str2value.h str2num.hpp
             $(C) $(CFLAGS) -c $^
    
    
    tokenizer.o: tokenizer.cpp tokenizer.hpp
            $(CPP) $(CFLAGS) -c $^
    
    str2cmd.o : authorCMDs.py cmdTable.h
            python authorCMDs.py cmdTable.h str2cmd #this uses the gcc -E cmdTable.h -DLURC
            $(C) $(CFLAGS) -c str2cmd.c str2cmd.h
    
    #TODO: add a thing that checks str2cmd.h/.c has not been modified by hand
   
    
    
    .PHONEY: clean
    clean:
            rm *.o
    
    .PHONEY: all
    all:
            clear
            make clean
            make

Here is the output I recieve from make all:

    make clean
    make[1]: Entering directory `/home/frames/LURC/gen1/gen1Source&#39;
    rm *.o
    make[1]: Leaving directory `/home/frames/LURC/gen1/gen1Source&#39;
    make
    make[1]: Entering directory `/home/frames/LURC/gen1/gen1Source&#39;
    python authorCMDs.py cmdTable.h str2cmd #this uses the gcc -E cmdTable.h -DLURC
    str2cmd.c and str2cmd.h, generated from cmdTable.h
    
    gcc  -Wall -fshort-enums -DLURC -c str2cmd.c str2cmd.h
    gcc  -Wall -fshort-enums -DLURC -c str2num.cpp str2value.h str2num.hpp
    g++ -DTESTMODE -Wall -fshort-enums -DLURC -c tokenizer.cpp tokenizer.hpp
    g++ -DTESTMODE -Wall -fshort-enums -DLURC -c str2value.cpp str2cmd.o str2num.o tokenizer.o str2value.h
    g++: str2cmd.o: linker input file unused because linking not done
    g++: str2num.o: linker input file unused because linking not done
    g++: tokenizer.o: linker input file unused because linking not done
    g++ -DTESTMODE -Wall -fshort-enums -DLURC -o gen1 gen1.cpp str2value.o
    str2value.o: In function `getValue(char*)&#39;:
    str2value.cpp:(.text+0xbd): undefined reference to `str2cmd(char*)&#39;
    str2value.cpp:(.text+0x102): undefined reference to `str2num(char*)&#39;
    str2value.o: In function `getAllValues(char*)&#39;:
    str2value.cpp:(.text+0x164): undefined reference to `tokenizer::tokenizer(char*)&#39;
    str2value.cpp:(.text+0x177): undefined reference to `tokenizer::getNumTokens(char const*)&#39;
    str2value.cpp:(.text+0x1a9): undefined reference to `tokenizer::getNextToken(char const*)&#39;
    str2value.cpp:(.text+0x1e9): undefined reference to `tokenizer::getNumTokens(char const*)&#39;
    str2value.cpp:(.text+0x201): undefined reference to `tokenizer::~tokenizer()&#39;
    str2value.cpp:(.text+0x25b): undefined reference to `tokenizer::~tokenizer()&#39;
    collect2: ld returned 1 exit status
    make[1]: *** [gen1] Error 1
    make[1]: Leaving directory `/home/frames/LURC/gen1/gen1Source&#39;
    make: *** [all] Error 2

Any suggestions about what this is about? `STR2VALUESFILES` has all the object files I need, to define the missing functions.

Linker error: &quot;linker input file unused because linking not done&quot;, undefined reference to a function in that file

I have a XCode4 / iOS project with a regular target and unit test target. Everything works fine, except when I try to #import one of my classes in my test class and try to use it. If I try to build the unit test target, I get the following link error:


    Undefined symbols for architecture i386:
      &quot;_OBJC_CLASS_$_FRRCategory&quot;, referenced from:
          objc-class-ref in CategoryTests.o
    ld: symbol(s) not found for architecture i386
    collect2: ld returned 1 exit status

In CategoryTests.m I&#39;m importing the header file in this way:


    #import &quot;../todoro/FRRCategory.h&quot;


What am I doing wrong?

Link error while building a unit test target

Nearly the final step but still some strange erros....

    bash-3.2$ make
    g++ -Wall -c -g Myworld.cc
    g++ -Wall -g solvePlanningProblem.o Position.o AStarNode.o PRM.o PRMNode.o World.o SingleCircleWorld.o Myworld.o RECTANGLE.o CIRCLE.o -o solvePlanningProblem
    Undefined symbols:
      &quot;vtable for Obstacle&quot;, referenced from:
          Obstacle::Obstacle()in Myworld.o
      &quot;typeinfo for Obstacle&quot;, referenced from:
          typeinfo for RECTANGLEin RECTANGLE.o
          typeinfo for CIRCLEin CIRCLE.o
    ld: symbol(s) not found
    collect2: ld returned 1 exit status
    make: *** [solvePlanningProblem] Error 1

What&#39;s the meaning of vtable and typeinfo?


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