Linux library types

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.a Static library, the contents become part of the compiled executable.

.so Dynamically linked shared object library. Do not become part of the executable but must be present at run-time.


How to generate a static library (.a)
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


How to generate a dynamically linked shared object library: .so

Create object code

gcc -Wall -fPIC -c *.c


Create library

gcc -shared -Wl,-soname,libctest.so.1 -o libctest.so.1.0 *.o


Optional: create default version using symbolic link

mv libctest.so.1.0 /usr/lib
ln -sf /usr/lib/libctest.so.1.0 /usr/lib/libctest.so
ln -sf /usr/lib/libctest.so.1.0 /usr/lib/libctest.so.1


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


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.


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