Linking Object Files

You should always use g++ to link a program that contains C++ code, even if it also contains C code. If your program contains only C code, you should use gcc instead. Because this program contains both C and C++, you should use g++, like this:

% g++ -o reciprocal main.o reciprocal.o
% ./reciprocal 7
The reciprocal of 7 is 0.142857

As you can see, g++ has automatically linked in the standard C runtime library containing the implementation of printf. If you had needed to link in another library (such as a graphical user interface toolkit), you would have specified the library with the -l option. In Linux, library names almost always start with lib. For example, the Pluggable Authentication Module (PAM) library is called libpam.a.

% g++ -o reciprocal main.o reciprocal.o -lpam

The compiler automatically adds the lib prefix and the .a suffix.

As with header files, the linker looks for libraries in some standard places, including the /lib and /usr/lib directories that contain the standard system libraries. If you want the linker to search other directories as well, you should use the -L option. You can use this line to instruct the linker to look for libraries in the /usr/local/lib/pam directory before looking in the usual places:

% g++ -o reciprocal main.o reciprocal.o -L/usr/local/lib/pam -lpam

Although you don't have to use the -I option to get the preprocessor to search the current directory, you do have to use the -L option to get the linker to search the current directory. In particular, you could use the following to instruct the linker to find the test library in the current directory:

% gcc -o app app.o -L. -ltest

By default, gcc uses shared libraries, so if you must link against static libraries, you have to use the -static option. This means that only static libraries will be used. The following example creates an executable linked against the static ncurses.

% gcc cursesapp.c -lncurses -static

• When you link against static libraries, the resulting binary is much larger than using shared libraries.
• Why use a static library, then? One common reason is to guarantee that users can run your program in the case of shared libraries, the code your program needs to run is linked dynamically at runtime, rather than statically at compile time.
• If the shared library your program requires is not installed on the user's system, she will get errors and not be able to run your program.