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

<

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.