Why libraries are used:

Libraries employ a software design also known as "shared components" or "archive libraries", which 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 library collective. 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 can remain separate from the executable reducing it's size and thus less disk space is used for the application. The library components are then called by various applications for use when needed.

Component reuse: update one library, shared resource takes up less disk space.

Version management: Linux libraries can cohabitate old and new versions on a single system.

Component Specialization: niche and specialized developers can focus on their core competency on a single library. Simplifies testing and verification.

Benefits include:

Linux Library Types:

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

Static libraries (.a): Library of object code which is linked with, and becomes part of the application. Dynamically linked shared object libraries (.so): There is only one form of this library but it can be used in two ways. Dynamically linked at run time. 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. 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 named 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.

Consider the following compile and link command: gcc src-file.c -lm -lpthread

The libraries referenced in this example for inclusion during linking are the math library ("m") and the thread library ("pthread"). 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

Static Libraries: (.a)

How to generate a static library (object code archive file):

Compile: cc -Wall -c ctest1.c ctest2.c

Compiler options: -Wall: include warnings. See man page for warnings specified.

Compiler options: 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

",x); return 0; }



ranlib ctest.a

Historical note: After creating the library it was once necessary to run the command:. 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.

Dynamically Linked "Shared Object" Libraries: (.so)

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

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

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

libctest.so.1.0

This creates the libraryand 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

/lib/

/usr/lib/

If you look at the libraries inandyou 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.

: 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".

: 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.

: 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 .

: Pass options to linker. In this example the options to be passed on to the linker are: " ". The name passed with the "-o" option is passed to . 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.

allows the naming convention for the compile flag 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:

libctest.so.1.0

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

gcc -Wall -L/opt/lib prog.c -lctest -o prog

libctest.so

Compiling for run-time linking with a dynamically linkedUse:Where the name of the library is. (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 run-time execution.

List Dependencies:

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

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)

[Potential Pitfall]

Example:: Unresolved errors within a shared library may cause an error when the library is loaded. Example:

ERROR: unable to load libname-of-lib.so ERROR: unable to get function address

Error message at run-time:

Investigate error:

[prompt]$ ldd libname-of-lib.so libglut.so.3 => /usr/lib64/libglut.so.3 (0x00007fb582b74000) libGL.so.1 => /usr/lib64/libGL.so.1 (0x00007fb582857000) libX11.so.6 => /usr/lib64/libX11.so.6 (0x00007fb582518000) libIL.so.1 (0x00007fa0f2c0f000) libcudart.so.4 => not found

The first three libraries show that there is a path resolution. The last two are problematic.

libname-of-lib.so

Add the unresolved library path in /etc/ld.so.conf.d/ name-of-lib -x86_64.conf and/or /etc/ld.so.conf.d/ name-of-lib -i686.conf

Reload the library cache ( /etc/ld.so.cache ) with the command: sudo ldconfig

or

and/or Reload the library cache ( ) with the command: Add library and path explicitly to the compiler/linker command: -l name-of-lib -L/path/to/lib

or

Add the library path to the environment variable to fix run-time dependency:

export LD_LIBRARY_PATH=$LD_LIBRARY_PATH:/path/to/lib

The fix is to resolve dependencies of the last two libraries when linking the library

Run Program:

Set path: export LD_LIBRARY_PATH=/opt/lib:$LD_LIBRARY_PATH

Run: prog

Example with code:

Compile the library functions: gcc -Wall -fPIC -c ctest1.c ctest2.c Generate the shared library: gcc -shared -Wl,-soname,libctest.so.1 -o libctest.so.1.0 ctest1.o ctest2.o

This generates the library libctest.so.1.0 Move to lib/ directory: sudo mv libctest.so.1.0 /opt/lib

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

sudo ln -sf /opt/lib/libctest.so.1 /opt/lib/libctest.so Compile program for use with a shared library: gcc -Wall -L/opt/lib prog.c -lctest -o prog

[Potential Pitfall] : If the symbolic links are not created (above), you will get the following error: /usr/bin/ld: cannot find -lctest collect2: error: ld returned 1 exit status The reference to the library name -lctest refers to /opt/lib/libctest.so Configure the library path (see below and choose one of three mechanisms).

In this example we set the environment variable: export LD_LIBRARY_PATH=$LD_LIBRARY_PATH:/opt/lib Run the program: ./prog Valx=5

[Potential Pitfall] : You get the following error if the library path is not set: ./prog: error while loading shared libraries: libctest.so.1: cannot open shared object file: No such file or directory

Using the example code above for ctest1.c, ctest2.c and prog.c

Man Pages:

gcc - GNU C compiler

ld - The GNU Linker

ldd - List library dependencies

ldconfig - configure dynamic linker run time bindings (update cache /etc/ld.so.cache )

Links:

Library Path:

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)

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 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 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:$LD_LIBRARY_PATH or add to your ~/.bashrc file: ... if [ -d /opt/lib ]; then LD_LIBRARY_PATH=/opt/lib:$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.

Library Info:

ar: list object files in archive library

ar tf /usr/lib/x86_64-linux-gnu/libjpeg.a

jlibinit.o jcapimin.o jcapistd.o jccoefct.o jccolor.o jcdctmgr.o jchuff.o jcinit.o ... ...

This will list all of the object files held in the archive library:Also see: Man page for ar

nm: list symbols: object files, archive library and shared library

The command "nm" lists symbols contained in object files:

nm file.o

The command "nm" lists symbols contained in the archive library:

nm /usr/lib/x86_64-linux-gnu/libjpeg.a

Object symbols in static archive libraries are categorized using the source and object file hierarchy of the library:

jlibinit.o: 0000000000000000 B auxv U fclose U fopen U fread U getpagesize 0000000000000000 T libjpeg_general_init U malloc U perror jcapimin.o: U jinit_marker_writer U jinit_memory_mgr 0000000000000000 T jpeg_CreateCompress U jpeg_abort 0000000000000240 T jpeg_abort_compress U jpeg_destroy 0000000000000230 T jpeg_destroy_compress 00000000000002a0 T jpeg_finish_compress U jpeg_natural_order ... ...

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

Note that other platforms (Cygwin) may not respond to "-D". Try nm -gC libctest.so.1.0

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: Man page for nm

Also see: objdump man page

readelf: list symbols in shared library

The command "readelf" command to list symbols contained in a shared library:

readelf -s /usr/lib64/libjpeg.so

Symbol table '.dynsym' contains 144 entries: Num: Value Size Type Bind Vis Ndx Name 0: 0000000000000000 0 NOTYPE LOCAL DEFAULT UND 1: 0000000000003b30 0 SECTION LOCAL DEFAULT 10 2: 0000000000000000 0 FUNC GLOBAL DEFAULT UND getenv@GLIBC_2.2.5 (4) 3: 0000000000000000 0 FUNC GLOBAL DEFAULT UND free@GLIBC_2.2.5 (4) 4: 0000000000000000 0 FUNC GLOBAL DEFAULT UND ferror@GLIBC_2.2.5 (4) 5: 0000000000000000 0 FUNC GLOBAL DEFAULT UND fread@GLIBC_2.2.5 (4) 6: 0000000000000000 0 FUNC GLOBAL DEFAULT UND fclose@GLIBC_2.2.5 (4) 7: 0000000000000000 0 FUNC GLOBAL DEFAULT UND __stack_chk_fail@GLIBC_2.4 (5) 8: 0000000000000000 0 FUNC GLOBAL DEFAULT UND memset@GLIBC_2.2.5 (4) 9: 0000000000000000 0 NOTYPE WEAK DEFAULT UND __gmon_start__ ... ...

Use the command

Also see: readelf man page

Library Versions:

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.

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. Run-time 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:

Dynamic loading and un-loading of shared libraries using libdl:

These libraries are dynamically loaded / unloaded and linked during execution. Useful 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

extern "C"

Use the notationso 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

", dlerror()); exit(1); } fn = dlsym(lib_handle, "ctest1"); if ((error = dlerror()) != NULL) { fprintf(stderr, "%s

", error); exit(1); } (*fn)(&x); printf("Valx=%d

",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.

Open shared library named " ". The second argument indicates the binding. See include file . Returns NULL if it fails. Options: 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.

) 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++ class objects and dynamic loading:

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"{} .

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

Note that the following are not equivalent:

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

The following are equivalent:

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.

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

The class ".h" 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 } }

The class ".cpp" file: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.

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

Main executable which calls the loadable libraries:

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.

Comparison to the Microsoft DLL:

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 recommend 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 control 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 instantiation. 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:

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. ... }

Include file declaration: (.h or .hpp)

/// 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 }

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

Cross platform programming of loadable libraries:

#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

Tools:

Man pages:

ar - create, modify, and extract from archives

ranlib - generate index to archive

nm - list symbols from object files

ld - Linker

ldconfig - configure dynamic linker run-time bindings

ldconfig -p : Print the lists of directories and candidate libraries stored in the current cache.

i.e. /sbin/ldconfig -p |grep libGL

: Print the lists of directories and candidate libraries stored in the current cache. i.e. ldd - print shared library dependencies

gcc/g++ - GNU project C and C++ compiler

man page to: ld.so - a.out dynamic linker/loader

Notes:

Direct loader to pre-load a specific shared library before all others: export LD_PRELOAD=/usr/lib/libXXX.so.x; exec program . This is specified in the file /etc/ld.so.preload and extended with the environment variable LD_PRELOAD .

Also see: man page to: ld.so - a.out dynamic linker/loader LD_PRELOAD and Linux function interception.

. This is specified in the file and extended with the environment variable . Also see: Running Red Hat 7.1 (glibc 2.2.2) but compiling for Red Hat 6.2 compatibility.

See RELEASE-NOTES

export LD_ASSUME_KERNEL=2.2.5

. /usr/i386-glibc21-linux/bin/i386-glibc21-linux-env.sh



See RELEASE-NOTES Environment variable to highlight warnings, errors, etc: export CC="colorgcc"

YoLinux C/C++ Software Development Tutorials:

Links:

GNU: Libtool - script for portable shared library creation.

LDP Library Howto

Books: Books:

"Advanced Linux Programming"

by Mark Mitchell, Jeffrey Oldham, Alex Samuel, Jeffery Oldham

ISBN # 0735710430, New Riders Good book for programmers who already know how to program and just need to know the Linux specifics. Covers a variety of Linux tools, libraries, API's and techniques. If you don't know how to program, start with a book on C.

"Linux Programming Bible"

by John Goerzen

ISBN # 0764546570, Hungry Minds, Inc This covers the next step after "C" programming 101.

