so description

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.

gcc -Wall -L/home/hanyh/workspace/cplusplus/linux prog.c -lctest -o prog
export LD_LIBRARY_PATH=/home/hanyh/workspace/cplusplus/linux/so:$LD_LIBRARY_PATH


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

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

    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.
        * -W1: 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
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.

       nm - list symbols from object files