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This methodology, also known as "shared components" or "archive libraries", groups together multiple compiled object code files into a single file known as a library. Typically C functions/C++ classes and methods which can be shared by more than one application are broken out of the application's source code, compiled and bundled into a library. The C standard libraries and C++ STL are examples of shared components which can be linked with your code. The benefit is that each and every object file need not be stated when linking because the developer can reference the individual library. This simplifies the multiple use and sharing of software components between applications. It also allows application vendors a way to simply release an API to interface with an application. Components which are large can be created for dynamic use, thus the library remain separate from the executable reducing it's size and thus disk space used. The library components are then called by various applications for use when needed.

There are two Linux C/C++ library types which can be created:

1. Static libraries (.a): Library of object code which is linked with, and becomes part of the application.
2. Dynamically linked shared object libraries (.so): There is only one form of this library but it can be used in two ways.
   1. Dynamically linked at run time but statically aware. The libraries must be available during compile/link phase. The shared objects are not included into the executable component but are tied to the execution.
   2. Dynamically loaded/unloaded and linked during execution (i.e. browser plug-in) using the dynamic linking loader system functions.

Library naming conventions:

Libraries are typically names with the prefix "lib". This is true for all the C standard libraries. When linking, the command line reference to the library will not contain the library prefix or suffix.

Thus the following link command: gcc src-file.c -lm -lpthread
The libraries referenced in this example for inclusion during linking are the math library and the thread library. They are found in /usr/lib/libm.a and /usr/lib/libpthread.a.

Note: The GNU compiler now has the command line option "-pthread" while older versions of the compiler specify the pthread library explicitly with "-lpthread". Thus now you are more likely to see gcc src-file.c -lm -pthread

How to generate a library:

• Compile: cc -Wall -c ctest1.c ctest2.c
  Compiler options:
  • -Wall: include warnings. See man page for warnings specified.
• Create library "libctest.a": ar -cvq libctest.a ctest1.o ctest2.o
• List files in library: ar -t libctest.a
• Linking with the library:
  • cc -o executable-name prog.c libctest.a
  • cc -o executable-name prog.c -L/path/to/library-directory -lctest
• Example files:
  • ctest1.c

    void ctest1(int *i) { *i=5; }

  • ctest2.c

    void ctest2(int *i) { *i=100; }

  • prog.c

    #include <stdio.h> void ctest1(int *); void ctest2(int *); int main() { int x; ctest1(&x); printf("Valx=%d\n",x); return 0; }

Historical note: After creating the library it was once necessary to run the command: ranlib ctest.a. This created a symbol table within the archive. Ranlib is now embedded into the "ar" command.

Note for MS/Windows developers: The Linux/Unix ".a" library is conceptually the same as the Visual C++ static ".lib" libraries.

How to generate a shared object: (Dynamically linked object library file.) Note that this is a two step process.

1. Create object code
2. Create library
3. Optional: create default version using a symbolic link.

Library creation example:

    gcc -Wall -fPIC -c *.c
    gcc -shared -Wl,-soname,libctest.so.1 -o libctest.so.1.0   *.o
    mv libctest.so.1.0 /opt/lib
    ln -sf /opt/lib/libctest.so.1.0 /opt/lib/libctest.so.1
    ln -sf /opt/lib/libctest.so.1.0 /opt/lib/libctest.so
    

This creates the library libctest.so.1.0 and symbolic links to it.

It is also valid to cascade the linkage:

    ln -sf /opt/lib/libctest.so.1.0 /opt/lib/libctest.so.1
    ln -sf /opt/lib/libctest.so.1   /opt/lib/libctest.so
    

If you look at the libraries in /lib/ and /usr/lib/ you will find both methodologies present. Linux developers are not consistent. What is important is that the symbolic links eventually point to an actual library.

Compiler options:

• -Wall: include warnings. See man page for warnings specified.
• -fPIC: Compiler directive to output position independent code, a characteristic required by shared libraries. Also see "-fpic".
• -shared: Produce a shared object which can then be linked with other objects to form an executable.
• -Wl,options: Pass options to linker.
  In this example the options to be passed on to the linker are: "-soname libctest.so.1". The name passed with the "-o" option is passed to gcc.
• Option -o: Output of operation. In this case the name of the shared object to be output will be "libctest.so.1.0"

Library Links:

• The link to /opt/lib/libctest.so allows the naming convention for the compile flag -lctest to work.
• The link to /opt/lib/libctest.so.1 allows the run time binding to work. See dependency below.

Compile main program and link with shared object library:

Compiling for runtime linking with a dynamically linked libctest.so.1.0:

    gcc -Wall -I/path/to/include-files -L/path/to/libraries prog.c -lctest -o prog
Use:
    gcc -Wall -L/opt/lib prog.c -lctest -o prog
      

Where the name of the library is libctest.so. (This is why you must create the symbolic links or you will get the error "/usr/bin/ld: cannot find -lctest".)
The libraries will NOT be included in the executable but will be dynamically linked during runtime execution.

List Dependencies:

The shared library dependencies of the executable can be listed with the command: ldd name-of-executable

Example: ldd prog

        libctest.so.1 => /opt/lib/libctest.so.1 (0x00002aaaaaaac000)
        libc.so.6 => /lib64/tls/libc.so.6 (0x0000003aa4e00000)
        /lib64/ld-linux-x86-64.so.2 (0x0000003aa4c00000)
    

Run Program:

• Set path: export LD_LIBRARY_PATH=/opt/lib:$LD_LIBRARY_PATH
• Run: prog

Man Pages:

• gcc - GNU C compiler
• ld - The GNU Linker
• ldd - List dependencies

Links:

• LDP: Shared libraries

In order for an executable to find the required libraries to link with during run time, one must configure the system so that the libraries can be found. Methods available: (Do at least one of the following)

1. Add library directories to be included during dynamic linking to the file /etc/ld.so.conf

   Sample: /etc/ld.so.conf

   /usr/X11R6/lib /usr/lib ... .. /usr/lib/sane /usr/lib/mysql /opt/lib

   Add the library path to this file and then execute the command (as root) ldconfig to configure the linker run-time bindings.
   You can use the "-f file-name" flag to reference another configuration file if you are developing for different environments.
   See man page for command ldconfig.

   OR

2. Add specified directory to library cache: (as root)
   ldconfig -n /opt/lib
   Where /opt/lib is the directory containing your library libctest.so
   (When developing and just adding your current directory: ldconfig -n . Link with -L.)

   This will NOT permanently configure the system to include this directory. The information will be lost upon system reboot.

   OR

3. Specify the environment variable LD_LIBRARY_PATH to point to the directory paths containing the shared object library. This will specify to the run time loader that the library paths will be used during execution to resolve dependencies.
   (Linux/Solaris: LD_LIBRARY_PATH, SGI: LD_LIBRARYN32_PATH, AIX: LIBPATH, Mac OS X: DYLD_LIBRARY_PATH, HP-UX: SHLIB_PATH)

   Example (bash shell): export LD_LIBRARY_PATH=/opt/lib:$LD_LIBRARY_PATHor add to your ~/.bashrc file:

   ... if [ -d /opt/lib ]; then LD_LIBRARY_PATH=/opt/lib:$LD_LIBRARY_PATH fi ... export LD_LIBRARY_PATH

   
   This instructs the run time loader to look in the path described by the environment variable LD_LIBRARY_PATH, to resolve shared libraries. This will include the path /opt/lib.

Library paths used should conform to the "Linux Standard Base" directory structure.

The command "nm" lists symbols contained in the object file or shared library.

Use the command nm -D libctest.so.1.0
(or nm --dynamic libctest.so.1.0)

0000000000100988 A __bss_start
000000000000068c T ctest1
00000000000006a0 T ctest2
                 w __cxa_finalize
00000000001007b0 A _DYNAMIC
0000000000100988 A _edata
0000000000100990 A _end
00000000000006f8 T _fini
0000000000100958 A _GLOBAL_OFFSET_TABLE_
                 w __gmon_start__
00000000000005b0 T _init
                 w _Jv_RegisterClasses
      

Man page for nm

Symbol Type	Description
A	The symbol's value is absolute, and will not be changed by further linking.
B	Un-initialized data section
D	Initialized data section
T	Normal code section
U	Undefined symbol used but not defined. Dependency on another library.
W	Doubly defined symbol. If found, allow definition in another library to resolve dependency.

Also see: objdump man page

Library versions should be specified for shared objects if the function interfaces are expected to change (C++ public/protected class definitions), more or fewer functions are included in the library, the function prototype changes (return data type (int, const int, ...) or argument list changes) or data type changes (object definitions: class data members, inheritance, virtual functions, ...).

The library version can be specified when the shared object library is created. If the library is expected to be updated, then a library version should be specified. This is especially important for shared object libraries which are dynamically linked. This also avoids the Microsoft "DLL hell" problem of conflicting libraries where a system upgrade which changes a standard library breaks an older application expecting an older version of the the shared object function.

Versioning occurs with the GNU C/C++ libraries as well. This often make binaries compiled with one version of the GNU tools incompatible with binaries compiled with other versions unless those versions also reside on the system. Multiple versions of the same library can reside on the same system due to versioning. The version of the library is included in the symbol name so the linker knows which version to link with.

One can look at the symbol version used: nm csub1.o

00000000 T ctest1

No version is specified in object code by default.

ld and object file layout

There is one GNU C/C++ compiler flag that explicitly deals with symbol versioning. Specify the version script to use at compile time with the flag: --version-script=your-version-script-file
Note: This is only useful when creating shared libraries. It is assumed that the programmer knows which libraries to link with when static linking. Runtime linking allows opportunity for library incompatibility.

GNU/Linux, see examples of version scripts here: sysdeps/unix/sysv/linux/Versions

Some symbols may also get version strings from assembler code which appears in glibc headers files. Look at include/libc-symbols.h.

Example: nm /lib/libc.so.6 | more

00000000 A GCC_3.0
00000000 A GLIBC_2.0
00000000 A GLIBC_2.1
00000000 A GLIBC_2.1.1
00000000 A GLIBC_2.1.2
00000000 A GLIBC_2.1.3
00000000 A GLIBC_2.2
00000000 A GLIBC_2.2.1
00000000 A GLIBC_2.2.2
00000000 A GLIBC_2.2.3
00000000 A GLIBC_2.2.4
...
..
      

Note the use of a version script.

Library referencing a versioned library: nm /lib/libutil-2.2.5.so

..
...
         U strcpy@@GLIBC_2.0
         U strncmp@@GLIBC_2.0
         U strncpy@@GLIBC_2.0
...
..
      

Links:

• Symbol versioning
• GNU.org: ld

These libraries are dynamically loaded / unloaded and linked during execution. Usefull for creating a "plug-in" architecture.

Prototype include file for the library: ctest.h

#ifndef CTEST_H #define CTEST_H #ifdef __cplusplus extern "C" { #endif void ctest1(int *); void ctest2(int *); #ifdef __cplusplus } #endif #endif

Use the notation extern "C" so the libraries can be used with C and C++. This statement prevents the C++ from name mangling and thus creating "unresolved symbols" when linking.

Load and unload the library libctest.so (created above), dynamically:

#include <stdio.h> #include <dlfcn.h> #include "ctest.h" int main(int argc, char **argv) { void *lib_handle; double (*fn)(int *); int x; char *error; lib_handle = dlopen("/opt/lib/libctest.so", RTLD_LAZY); if (!lib_handle) { fprintf(stderr, "%s\n", dlerror()); exit(1); } fn = dlsym(lib_handle, "ctest1"); if ((error = dlerror()) != NULL) { fprintf(stderr, "%s\n", error); exit(1); } (*fn)(&x); printf("Valx=%d\n",x); dlclose(lib_handle); return 0; }

gcc -rdynamic -o progdl progdl.c -ldl

Explanation:

• dlopen("/opt/lib/libctest.so", RTLD_LAZY);
  Open shared library named "libctest.so".
  The second argument indicates the binding. See include file dlfcn.h.
  Returns NULL if it fails.
  Options:
  • RTLD_LAZY: If specified, Linux is not concerned about unresolved symbols until they are referenced.
  • RTLD_NOW: All unresolved symbols resolved when dlopen() is called.
  • RTLD_GLOBAL: Make symbol libraries visible.
• dlsym(lib_handle, "ctest1");
  Returns address to the function which has been loaded with the shared library..
  Returns NULL if it fails.
  Note: When using C++ functions, first use nm to find the "mangled" symbol name or use the extern "C" construct to avoid name mangling.
  i.e. extern "C" void function-name();

Object code location: Object code archive libraries can be located with either the executable or the loadable library. Object code routines used by both should not be duplicated in each. This is especially true for code which use static variables such as singleton classes. A static variable is global and thus can only be represented once. Including it twice will provide unexpected results. The programmer can specify that specific object code be linked with the executable by using linker commands which are passed on by the compiler.

Use the "-Wl" gcc/g++ compiler flag to pass command line arguments on to the GNU "ld" linker.

Example makefile statement: g++ -rdynamic -o appexe $(OBJ) $(LINKFLAGS) -Wl,--whole-archive -L{AA_libs} -laa -Wl,--no-whole-archive $(LIBS)

• --whole-archive: This linker directive specifies that the libraries listed following this directive (in this case AA_libs) shall be included in the resulting output even though there may not be any calls requiring its presence. This option is used to specify libraries which the loadable libraries will require at run time.
• -no-whole-archive: This needs to be specified whether you list additional object files or not. The gcc/g++ compiler will add its own list of archive libraries and you would not want all the object code in the archive library linked in if not needed. It toggles the behavior back to normal for the rest of the archive libraries.

Man pages:

• dlopen() - gain access to an executable object file
• dclose() - close a dlopen object
• dlsym() - obtain the address of a symbol from a dlopen object
• dlvsym() - Programming interface to dynamic linking loader.
• dlerror() - get diagnostic information

Links:

• GNOME Glib dynamic loading of modules - cross platform API for dynamically loading "plug-ins".

C++ and name mangling:

When running the above "C" examples with the "C++" compiler one will quickly find that "C++" function names get mangled and thus will not work unless the function definitions are protected with extern "C"{}.

Note that the following are not equivalent:

extern "C" { int functionx(); }

extern "C" int functionx();

The following are equivalent:

extern "C" { extern int functionx(); }

extern "C" int functionx();

Dynamic loading of C++ classes:

The dynamic library loading routines enable the programmer to load "C" functions. In C++ we would like to load class member functions. In fact the entire class may be in the library and we may want to load and have access to the entire object and all of its member functions. Do this by passing a "C" class factory function which instantiates the class.

The class ".h" file:

class Abc { ... ... }; // Class factory "C" functions typedef Abc* create_t; typedef void destroy_t(Abc*);

The class ".cpp" file:

Abc::Abc() { ... } extern "C" { // These two "C" functions manage the creation and destruction of the class Abc Abc* create() { return new Abc; } void destroy(Abc* p) { delete p; // Can use a base class or derived class pointer here } }

This file is the source to the library. The "C" functions to instantiate (create) and destroy a class defined in the dynamically loaded library where "Abc" is the C++ class.

Main executable which calls the loadable libraries:

// load the symbols create_t* create_abc = (create_t*) dlsym(lib_handle, "create"); ... ... destroy_t* destroy_abc = (destroy_t*) dlsym(lib_handle, "destroy"); ... ...

Pitfalls:

• The new/delete of the C++ class should both be provided by the executable or the library but not split. This is so that there is no surprise if one overloads new/delete in one or the other.

Links:

• LinuxJournal.com: Dynamic Class Loading for C++ on Linux
• dlopen howto

The Microsoft Windows equivalent to the Linux / Unix shared object (".so") is the ".dll". The Microsoft Windows DLL file usually has the extension ".dll", but may also use the extension ".ocx". On the old 16 bit windows, the dynamically linked libraries were also named with the ".exe" suffix. "Executing" the DLL will load it into memory.

The Visual C++ .NET IDE wizard will create a DLL framework through the GUI, and generates a ".def" file. This "module definition file" lists the functions to be exported. When exporting C++ functions, the C++ mangled names are used. Using the Visual C++ compiler to generate a ".map" file will allow you to discover the C++ mangled name to use in the ".def" file. The "SECTIONS" label in the ".def" file will define the portions which are "shared". Unfortunately the generation of DLLs are tightly coupled to the Microsoft IDE, so much so that I would not recomend trying to create one without it.

The Microsoft Windows C++ equivalent functions to libdl are the following functions:

• ::LoadLibrary() - dlopen()
• ::GetProcAddress() - dlsym()
• ::FreeLibrary() - dlclose()

[Potential Pitfall]: Microsoft Visual C++ .NET compilers do not allow the linking controll that the GNU linker "ld" allows (i.e. --whole-archive, -no-whole-archive). All symbols need to be resolved by the VC++ compiler for both the loadable library and the application executable individually and thus it can cause duplication of libraries when the library is loaded. This is especially bad when using static variables (i.e. used in singleton patterns) as you will get two memory locations for the static variable, one used by the loadable library and the other used by the program executable. This breaks the whole static variable concept and the singleton pattern. Thus you can not use a static variable which is referenced by by both the loadable library and the application executable as they will be unique and different. To use a unique static variable, you must pass a pointer to that static variable to the other module so that each module (main executable and DLL library) can use the same instatiation. On MS/Windows you can use shared memory or a memory mapped file so that the main executable and DLL library can share a pointer to an address they both will use.

Cross platform (Linux and MS/Windows) C++ code snippet: Include file declaration: (.h or .hpp)

class Abc{ public: static Abc* Instance(); // Function declaration. Could also be used as a public class member function. private: static Abc *mInstance; // Singleton. Use this declaration in C++ class member variable declaration. ... }

C/C++ Function source: (.cpp)

/// Singleton instantiation Abc* Abc::mInstance = 0; // Use this declaration for C++ class member variable // (Defined outside of class definition in ".cpp" file) // Return unique pointer to instance of Abc or create it if it does not exist. // (Unique to both exe and dll) static Abc* Abc::Instance() // Singleton { #ifdef WIN32 // If pointer to instance of Abc exists (true) then return instance pointer else look for // instance pointer in memory mapped pointer. If the instance pointer does not exist in // memory mapped pointer, return a newly created pointer to an instance of Abc. return mInstance ? mInstance : (mInstance = (Abc*) MemoryMappedPointers::getPointer("Abc")) ? mInstance : (mInstance = (Abc*) MemoryMappedPointers::createEntry("Abc",(void*)new Abc)); #else // If pointer to instance of Abc exists (true) then return instance pointer // else return a newly created pointer to an instance of Abc. return mInstance ? mInstance : (mInstance = new Abc); #endif }

Windows linker will pull two instances of object, one in exe and one in loadable module. Specify one for both to use by using memory mapped pointer so both exe and loadable library point to same variable or object.
Note that the GNU linker does not have this problem. For more on singletons see the YoLinux.com C++ singleton software design pattern tutorial.

#ifndef USE_PRECOMPILED_HEADERS #ifdef WIN32 #include <direct.h> #include <windows.h> #else #include <sys/types.h> #include <dlfcn.h> #endif #include <iostream> #endif using namespace std; #ifdef WIN32 HINSTANCE lib_handle; #else void *lib_handle; #endif // Where retType is the pointer to a return type of the function // This return type can be int, float, double, etc or a struct or class. typedef retType* func_t; // load the library ------------------------------------------------- #ifdef WIN32 string nameOfLibToLoad("C:\opt\lib\libctest.dll"); lib_handle = LoadLibrary(TEXT(nameOfLibToLoad.c_str())); if (!lib_handle) { cerr << "Cannot load library: " << TEXT(nameOfDllToLoad.c_str()) << endl; } #else string nameOfLibToLoad("/opt/lib/libctest.so"); lib_handle = dlopen(nameOfLibToLoad.c_str(), RTLD_LAZY); if (!lib_handle) { cerr << "Cannot load library: " << dlerror() << endl; } #endif ... ... ... // load the symbols ------------------------------------------------- #ifdef WIN32 func_t* fn_handle = (func_t*) GetProcAddress(lib_handle, "superfunctionx"); if (!fn_handle) { cerr << "Cannot load symbol superfunctionx: " << GetLastError() << endl; } #else // reset errors dlerror(); // load the symbols (handle to function "superfunctionx") func_t* fn_handle= (func_t*) dlsym(lib_handle, "superfunctionx"); const char* dlsym_error = dlerror(); if (dlsym_error) { cerr << "Cannot load symbol superfunctionx: " << dlsym_error << endl; } #endif ... ... ... // unload the library ----------------------------------------------- #ifdef WIN32 FreeLibrary(lib_handle); #else dlclose(lib_handle); #endif