C Dynamic Shared Library on Linux

Question

This is a follow-up to Dynamic Shared Library compilation with g     I m trying to create a shared class library in C   on Linux   I m able to get the library to compile  and I can call some of the  non-class  functions using the tutorials that I found here and here   My problems start when I try to use the classes that are defined in the library   The second tutorial that I linked to shows how to load the symbols for creating objects of the classes defined in the library  but stops short of using those objects to get any work done   Does anyone know of a more complete tutorial for creating shared C   class libraries that also shows how to use those classes in a separate executable   A very simple tutorial that shows object creation  use  simple getters and setters would be fine   and deletion would be fantastic   A link or a reference to some open source code that illustrates the use of a shared class library would be equally good     Although the answers from codelogic and nimrodm do work  I just wanted to add that I picked up a copy of Beginning Linux Programming since asking this question  and its first chapter has example C code and good explanations for creating and using both static and shared libraries   These examples are available through Google Book Search in an older edition of that book

User · Answer

Basically  you should include the class  header file in the code where you want to use the class in the shared library   Then  when you link  use the  -l  flag to link your code with the shared library   Of course  this requires the  so to be where the OS can find it   See 3 5  Installing and Using a Shared Library  Using dlsym is for when you don t know at compile time which library you want to use   That doesn t sound like it s the case here   Maybe the confusion is that Windows calls the dynamically loaded libraries whether you do the linking at compile or run-time  with analogous methods    If so  then you can think of dlsym as the equivalent of LoadLibrary   If you really do need to dynamically load the libraries  i e   they re plug-ins   then this FAQ should help

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myclass h   ifndef   MYCLASS H    define   MYCLASS H    class MyClass   public    MyClass          use virtual otherwise linker will try to perform static linkage      virtual void DoSomething     private    int x       endif   myclass cc   include  myclass h   include  lt iostream gt   using namespace std   extern  C  MyClass  create object       return new MyClass     extern  C  void destroy object  MyClass  object       delete object     MyClass  MyClass       x   20     void MyClass  DoSomething       cout lt  lt x lt  lt endl      class user cc   include  lt dlfcn h gt   include  lt iostream gt   include  myclass h   using namespace std   int main int argc  char   argv         on Linux  use    myclass so       void  handle   dlopen  myclass so   RTLD LAZY      MyClass    create       void   destroy  MyClass       create    MyClass        dlsym handle   create object      destroy    void     MyClass   dlsym handle   destroy object       MyClass  myClass    MyClass  create      myClass- gt DoSomething      destroy  myClass        On Mac OS X  compile with   g   -dynamiclib -flat namespace myclass cc -o myclass so g   class user cc -o class user   On Linux  compile with   g   -fPIC -shared myclass cc -o myclass so g   class user cc -ldl -o class user   If this were for a plugin system  you would use MyClass as a base class and define all the required functions virtual  The plugin author would then derive from MyClass  override the virtuals and implement create object and destroy object  Your main application would not need to be changed in any way

User · Answer

Basically  you should include the class  header file in the code where you want to use the class in the shared library   Then  when you link  use the  -l  flag to link your code with the shared library   Of course  this requires the  so to be where the OS can find it   See 3 5  Installing and Using a Shared Library  Using dlsym is for when you don t know at compile time which library you want to use   That doesn t sound like it s the case here   Maybe the confusion is that Windows calls the dynamically loaded libraries whether you do the linking at compile or run-time  with analogous methods    If so  then you can think of dlsym as the equivalent of LoadLibrary   If you really do need to dynamically load the libraries  i e   they re plug-ins   then this FAQ should help

User · Answer

Basically  you should include the class  header file in the code where you want to use the class in the shared library   Then  when you link  use the  -l  flag to link your code with the shared library   Of course  this requires the  so to be where the OS can find it   See 3 5  Installing and Using a Shared Library  Using dlsym is for when you don t know at compile time which library you want to use   That doesn t sound like it s the case here   Maybe the confusion is that Windows calls the dynamically loaded libraries whether you do the linking at compile or run-time  with analogous methods    If so  then you can think of dlsym as the equivalent of LoadLibrary   If you really do need to dynamically load the libraries  i e   they re plug-ins   then this FAQ should help

User · Answer

myclass h   ifndef   MYCLASS H    define   MYCLASS H    class MyClass   public    MyClass          use virtual otherwise linker will try to perform static linkage      virtual void DoSomething     private    int x       endif   myclass cc   include  myclass h   include  lt iostream gt   using namespace std   extern  C  MyClass  create object       return new MyClass     extern  C  void destroy object  MyClass  object       delete object     MyClass  MyClass       x   20     void MyClass  DoSomething       cout lt  lt x lt  lt endl      class user cc   include  lt dlfcn h gt   include  lt iostream gt   include  myclass h   using namespace std   int main int argc  char   argv         on Linux  use    myclass so       void  handle   dlopen  myclass so   RTLD LAZY      MyClass    create       void   destroy  MyClass       create    MyClass        dlsym handle   create object      destroy    void     MyClass   dlsym handle   destroy object       MyClass  myClass    MyClass  create      myClass- gt DoSomething      destroy  myClass        On Mac OS X  compile with   g   -dynamiclib -flat namespace myclass cc -o myclass so g   class user cc -o class user   On Linux  compile with   g   -fPIC -shared myclass cc -o myclass so g   class user cc -ldl -o class user   If this were for a plugin system  you would use MyClass as a base class and define all the required functions virtual  The plugin author would then derive from MyClass  override the virtuals and implement create object and destroy object  Your main application would not need to be changed in any way

User · Answer

The following shows an example of a shared class library shared  h cpp  and a main cpp module using the library  It s a very simple example and the makefile could be made much better  But it works and may help you   shared h defines the class   class myclass      int myx     public       myclass     myx 0        void setx int newx       int  getx         shared cpp defines the getx setx functions    include  shared h   void myclass  setx int newx    myx   newx    int  myclass  getx     return myx      main cpp uses the class    include  lt iostream gt   include  shared h   using namespace std   int main int argc  char  argv        myclass m     cout  lt  lt  m getx    lt  lt  endl    m setx 10     cout  lt  lt  m getx    lt  lt  endl      and the makefile that generates libshared so and links main with the shared library   main  libshared so main o       CXX  -o main  main o -L  -lshared  libshared so  shared cpp       CXX  -fPIC -c shared cpp -o shared o       CXX  -shared  -Wl -soname libshared so -o libshared so shared o  clean       rm   o   so   To actual run  main  and link with libshared so you will probably need to specify the load path  or put it in  usr local lib or similar    The following specifies the current directory as the search path for libraries and runs main  bash syntax    export LD LIBRARY PATH     main   To see that the program is linked with libshared so you can try ldd   LD LIBRARY PATH   ldd main   Prints on my machine       prj test shared  LD LIBRARY PATH   ldd main     linux-gate so 1   gt    0xb7f88000      libshared so   gt    libshared so  0xb7f85000      libstdc   so 6   gt   usr lib libstdc   so 6  0xb7e74000      libm so 6   gt   lib libm so 6  0xb7e4e000      libgcc s so 1   gt   usr lib libgcc s so 1  0xb7e41000      libc so 6   gt   lib libc so 6  0xb7cfa000       lib ld-linux so 2  0xb7f89000

User · Answer

myclass h   ifndef   MYCLASS H    define   MYCLASS H    class MyClass   public    MyClass          use virtual otherwise linker will try to perform static linkage      virtual void DoSomething     private    int x       endif   myclass cc   include  myclass h   include  lt iostream gt   using namespace std   extern  C  MyClass  create object       return new MyClass     extern  C  void destroy object  MyClass  object       delete object     MyClass  MyClass       x   20     void MyClass  DoSomething       cout lt  lt x lt  lt endl      class user cc   include  lt dlfcn h gt   include  lt iostream gt   include  myclass h   using namespace std   int main int argc  char   argv         on Linux  use    myclass so       void  handle   dlopen  myclass so   RTLD LAZY      MyClass    create       void   destroy  MyClass       create    MyClass        dlsym handle   create object      destroy    void     MyClass   dlsym handle   destroy object       MyClass  myClass    MyClass  create      myClass- gt DoSomething      destroy  myClass        On Mac OS X  compile with   g   -dynamiclib -flat namespace myclass cc -o myclass so g   class user cc -o class user   On Linux  compile with   g   -fPIC -shared myclass cc -o myclass so g   class user cc -ldl -o class user   If this were for a plugin system  you would use MyClass as a base class and define all the required functions virtual  The plugin author would then derive from MyClass  override the virtuals and implement create object and destroy object  Your main application would not need to be changed in any way

User · Answer

On top of previous answers  I d like to raise awareness about the fact that you should use the RAII  Resource Acquisition Is Initialisation  idiom to be safe about handler destruction   Here is a complete working example   Interface declaration  Interface hpp   class Base   public      virtual  Base          virtual void foo   const   0      using Base creator t   Base           Shared library content    include  Interface hpp   class Derived  public Base   public      void foo   const override        extern  C    Base   create         return new Derived        Dynamic shared library handler  Derived factory hpp    include  Interface hpp   include  lt dlfcn h gt   class Derived factory   public      Derived factory             handler   dlopen  libderived so   RTLD NOW           if    handler                throw std  runtime error dlerror                       Reset dlerror            creator   reinterpret cast lt Base creator t gt  dlsym handler   create             Check dlerror               std  unique ptr lt Base gt  create   const           return std  unique ptr lt Base gt  creator                 Derived factory             if  handler                dlclose handler                    private      void   handler   nullptr      Base creator t creator   nullptr       static void Reset dlerror             dlerror               static void Check dlerror             const char   dlsym error   dlerror            if  dlsym error                throw std  runtime error dlsym error                        Client code    include  Derived factory hpp         Derived factory factory      std  unique ptr lt Base gt  base   factory create        base- gt foo        Note    I put everything in header files for conciseness  In real life you should of course split your code between  hpp and  cpp files  To simplify  I ignored the case where you want to handle a new delete overload    Two clear articles to get more details    C   dlopen mini how-to C   Dynamic Loading of Shared Objects at Runtime

User · Answer

Basically  you should include the class  header file in the code where you want to use the class in the shared library   Then  when you link  use the  -l  flag to link your code with the shared library   Of course  this requires the  so to be where the OS can find it   See 3 5  Installing and Using a Shared Library  Using dlsym is for when you don t know at compile time which library you want to use   That doesn t sound like it s the case here   Maybe the confusion is that Windows calls the dynamically loaded libraries whether you do the linking at compile or run-time  with analogous methods    If so  then you can think of dlsym as the equivalent of LoadLibrary   If you really do need to dynamically load the libraries  i e   they re plug-ins   then this FAQ should help

User · Answer

myclass h   ifndef   MYCLASS H    define   MYCLASS H    class MyClass   public    MyClass          use virtual otherwise linker will try to perform static linkage      virtual void DoSomething     private    int x       endif   myclass cc   include  myclass h   include  lt iostream gt   using namespace std   extern  C  MyClass  create object       return new MyClass     extern  C  void destroy object  MyClass  object       delete object     MyClass  MyClass       x   20     void MyClass  DoSomething       cout lt  lt x lt  lt endl      class user cc   include  lt dlfcn h gt   include  lt iostream gt   include  myclass h   using namespace std   int main int argc  char   argv         on Linux  use    myclass so       void  handle   dlopen  myclass so   RTLD LAZY      MyClass    create       void   destroy  MyClass       create    MyClass        dlsym handle   create object      destroy    void     MyClass   dlsym handle   destroy object       MyClass  myClass    MyClass  create      myClass- gt DoSomething      destroy  myClass        On Mac OS X  compile with   g   -dynamiclib -flat namespace myclass cc -o myclass so g   class user cc -o class user   On Linux  compile with   g   -fPIC -shared myclass cc -o myclass so g   class user cc -ldl -o class user   If this were for a plugin system  you would use MyClass as a base class and define all the required functions virtual  The plugin author would then derive from MyClass  override the virtuals and implement create object and destroy object  Your main application would not need to be changed in any way

User · Answer

The following shows an example of a shared class library shared  h cpp  and a main cpp module using the library  It s a very simple example and the makefile could be made much better  But it works and may help you   shared h defines the class   class myclass      int myx     public       myclass     myx 0        void setx int newx       int  getx         shared cpp defines the getx setx functions    include  shared h   void myclass  setx int newx    myx   newx    int  myclass  getx     return myx      main cpp uses the class    include  lt iostream gt   include  shared h   using namespace std   int main int argc  char  argv        myclass m     cout  lt  lt  m getx    lt  lt  endl    m setx 10     cout  lt  lt  m getx    lt  lt  endl      and the makefile that generates libshared so and links main with the shared library   main  libshared so main o       CXX  -o main  main o -L  -lshared  libshared so  shared cpp       CXX  -fPIC -c shared cpp -o shared o       CXX  -shared  -Wl -soname libshared so -o libshared so shared o  clean       rm   o   so   To actual run  main  and link with libshared so you will probably need to specify the load path  or put it in  usr local lib or similar    The following specifies the current directory as the search path for libraries and runs main  bash syntax    export LD LIBRARY PATH     main   To see that the program is linked with libshared so you can try ldd   LD LIBRARY PATH   ldd main   Prints on my machine       prj test shared  LD LIBRARY PATH   ldd main     linux-gate so 1   gt    0xb7f88000      libshared so   gt    libshared so  0xb7f85000      libstdc   so 6   gt   usr lib libstdc   so 6  0xb7e74000      libm so 6   gt   lib libm so 6  0xb7e4e000      libgcc s so 1   gt   usr lib libgcc s so 1  0xb7e41000      libc so 6   gt   lib libc so 6  0xb7cfa000       lib ld-linux so 2  0xb7f89000

User · Answer

The following shows an example of a shared class library shared  h cpp  and a main cpp module using the library  It s a very simple example and the makefile could be made much better  But it works and may help you   shared h defines the class   class myclass      int myx     public       myclass     myx 0        void setx int newx       int  getx         shared cpp defines the getx setx functions    include  shared h   void myclass  setx int newx    myx   newx    int  myclass  getx     return myx      main cpp uses the class    include  lt iostream gt   include  shared h   using namespace std   int main int argc  char  argv        myclass m     cout  lt  lt  m getx    lt  lt  endl    m setx 10     cout  lt  lt  m getx    lt  lt  endl      and the makefile that generates libshared so and links main with the shared library   main  libshared so main o       CXX  -o main  main o -L  -lshared  libshared so  shared cpp       CXX  -fPIC -c shared cpp -o shared o       CXX  -shared  -Wl -soname libshared so -o libshared so shared o  clean       rm   o   so   To actual run  main  and link with libshared so you will probably need to specify the load path  or put it in  usr local lib or similar    The following specifies the current directory as the search path for libraries and runs main  bash syntax    export LD LIBRARY PATH     main   To see that the program is linked with libshared so you can try ldd   LD LIBRARY PATH   ldd main   Prints on my machine       prj test shared  LD LIBRARY PATH   ldd main     linux-gate so 1   gt    0xb7f88000      libshared so   gt    libshared so  0xb7f85000      libstdc   so 6   gt   usr lib libstdc   so 6  0xb7e74000      libm so 6   gt   lib libm so 6  0xb7e4e000      libgcc s so 1   gt   usr lib libgcc s so 1  0xb7e41000      libc so 6   gt   lib libc so 6  0xb7cfa000       lib ld-linux so 2  0xb7f89000

User · Answer

The following shows an example of a shared class library shared  h cpp  and a main cpp module using the library  It s a very simple example and the makefile could be made much better  But it works and may help you   shared h defines the class   class myclass      int myx     public       myclass     myx 0        void setx int newx       int  getx         shared cpp defines the getx setx functions    include  shared h   void myclass  setx int newx    myx   newx    int  myclass  getx     return myx      main cpp uses the class    include  lt iostream gt   include  shared h   using namespace std   int main int argc  char  argv        myclass m     cout  lt  lt  m getx    lt  lt  endl    m setx 10     cout  lt  lt  m getx    lt  lt  endl      and the makefile that generates libshared so and links main with the shared library   main  libshared so main o       CXX  -o main  main o -L  -lshared  libshared so  shared cpp       CXX  -fPIC -c shared cpp -o shared o       CXX  -shared  -Wl -soname libshared so -o libshared so shared o  clean       rm   o   so   To actual run  main  and link with libshared so you will probably need to specify the load path  or put it in  usr local lib or similar    The following specifies the current directory as the search path for libraries and runs main  bash syntax    export LD LIBRARY PATH     main   To see that the program is linked with libshared so you can try ldd   LD LIBRARY PATH   ldd main   Prints on my machine       prj test shared  LD LIBRARY PATH   ldd main     linux-gate so 1   gt    0xb7f88000      libshared so   gt    libshared so  0xb7f85000      libstdc   so 6   gt   usr lib libstdc   so 6  0xb7e74000      libm so 6   gt   lib libm so 6  0xb7e4e000      libgcc s so 1   gt   usr lib libgcc s so 1  0xb7e41000      libc so 6   gt   lib libc so 6  0xb7cfa000       lib ld-linux so 2  0xb7f89000

User · Answer

On top of previous answers  I d like to raise awareness about the fact that you should use the RAII  Resource Acquisition Is Initialisation  idiom to be safe about handler destruction   Here is a complete working example   Interface declaration  Interface hpp   class Base   public      virtual  Base          virtual void foo   const   0      using Base creator t   Base           Shared library content    include  Interface hpp   class Derived  public Base   public      void foo   const override        extern  C    Base   create         return new Derived        Dynamic shared library handler  Derived factory hpp    include  Interface hpp   include  lt dlfcn h gt   class Derived factory   public      Derived factory             handler   dlopen  libderived so   RTLD NOW           if    handler                throw std  runtime error dlerror                       Reset dlerror            creator   reinterpret cast lt Base creator t gt  dlsym handler   create             Check dlerror               std  unique ptr lt Base gt  create   const           return std  unique ptr lt Base gt  creator                 Derived factory             if  handler                dlclose handler                    private      void   handler   nullptr      Base creator t creator   nullptr       static void Reset dlerror             dlerror               static void Check dlerror             const char   dlsym error   dlerror            if  dlsym error                throw std  runtime error dlsym error                        Client code    include  Derived factory hpp         Derived factory factory      std  unique ptr lt Base gt  base   factory create        base- gt foo        Note    I put everything in header files for conciseness  In real life you should of course split your code between  hpp and  cpp files  To simplify  I ignored the case where you want to handle a new delete overload    Two clear articles to get more details    C   dlopen mini how-to C   Dynamic Loading of Shared Objects at Runtime

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