How do I source/link external functions in C or C++?

Question

EDIT: I suppose I should clarify, in case it matters. I am on a AIX Unix box, so I am using VAC compilers - no gnu compilers. End edit

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I am pretty rusty in C/C++, so forgive me if this is a simple question.

I would like to take common functions out of a few of my C programs and put them in shared libraries or shared objects. If I was doing this in perl I would put my subs in a perl module and use that module when needed.

For the sake of an example, let's say I have this function:

int giveInteger()
{
    return 1034;
}

Obviously this is not a real world example, but if I wanted to share that function, how would I proceed?

I'm pretty sure I have 2 options:

1. Put my shared function in a file, and have it compile with my main program at compile time. If I ever make changes to my shared function, I would have to recompile my main program.
2. Put my shared function in a file, and compile it as a shared library (if I have my terms correct), and have my main program link to that shared library. Any changes I make to my shared library (after compiling it) would be integrated into my main program at runtime without re-compiling my main program.

Am I correct on that thinking?

If so, how can I complish either/both of those methods? I've searched a lot and I seem to find information how how I could have my own program link to someone else's shared library, but not how to create my own shared functions and compile them in a way I can use them in my own program.

Thanks so much!

Brian

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EDIT:

Conclusion

Thanks everyone for your help! I thought I would add to this post what is working for me (for dynamic shared libraries on AIX) so that others can benefit:

I compile my shared functions:

xlc -c sharedFunctions.c -o sharedFunctions.o

Then make it a shared object:

xlc -qmkshrobj -qexpfile=exportlist sharedFunctions.o
xlc -G -o libsharedFunctions.so sharedFunctions.o  -bE:exportlist

Then link it another program:

xlc -brtl -o mainProgram mainProgram.c  -L. -lsharedFunctions

And another comment helped me find this link, which also helped: http://publib.boulder.ibm.com/infocenter/comphelp/v7v91/topic/com.ibm.vacpp7a.doc/proguide/ref/compile_library.htm

Thanks again to all who helped me out!

Answer 1

Yeah you are correct. The first is called a static library, while the second is called a shared library, because the code is not bound to the executable at compile time, but everytime again when your program is loaded.

Static library

Compile your library's code as follows:

gcc -c *.c

The -c tells the program not to link the object file, but just leaves you with object files for each .c file that was compiled. Now, archive them into one static library:

ar rcs libmystuff.a *.o 

man ar will tell you what the rcs options mean. Now, libmystuff.a is a archive file (you can open it with some zip-file viewers) which contain those object files, together with an index of symbols for each object file. You can link it to your program:

gcc *.c libmystuff.a -o myprogram

Now, your program is ready. Note that the order of where the static libraries appear in the command matter. See my Link order answer.

Shared library

For a shared library, you will create your library with

gcc -shared -o libmystuff.so *.c

That's all it takes, libmystuff.so is now a shared object file. If you want to link a program to it, you have to put it into a directory that is listed in the /etc/ld.so.conf file, or that is given by the -L switch to GCC, or listed in the LD_LIBRARY_PATH variable. When linking, you cut the lib prefix and .so suffix from the library name you tell gcc.

gcc -L. -lmystuff *.c -o myprogram

Internally, gcc will just pass your arguments to the GNU linker. You can see what arguments it pass using the -### option: Gcc will print the exact arguments given to each sub process.

For details about the linking process (how some stuff is done internally), view my Linux GCC linker answer.

Answer 2

You've got a third option. In general, your C++ compiler should be able to link C routines. The necessary options may vary from compiler to compiler, so R your fine M, but basically, you should be able to compile with g++ as here:

$ g++ -o myapp myapp.cpp myfunc.c giveint.c

... or compile separately

$ gcc -c myfunc.c
$ gcc -c giveint.c
$ g++ -c myapp.cpp
$ g++ -o myapp myapp.o myfunc.o

You also need to include your declaration of the functions; you do that in C++ as

extern "C" {
    int myfunc(int,int);
    int giveInterger(void);
}