17.1 Dynamic Modules

In order to dynamically load some code into your executable, that code must be compiled in some special but architecture dependent fashion. Depending on the compiler you use and the platform you are compiling for, there are different conventions you must observe in the code for the module, and for the particular combination of compiler options you need to select if the resulting objects are to be suitable for use in a dynamic module. For the rest of this chapter I will concentrate on the conventions used when compiling dynamic modules with GCC on GNU/Linux, which although peculiar to this particular combination of compiler and host architecture, are typical of the sorts of conventions you would need to observe on other architectures or with a different compiler.

With GCC on GNU/Linux, you must compile each of the source files with `-fPIC'(38), the resulting objects must be linked into a loadable module with gcc's `-shared' option:

$ gcc -fPIC -c foo.c $ gcc -fPIC -c bar.c $ gcc -shared -o baz.so foo.o bar.o

This is pretty similar to how you might go about linking a shared library, except that the `baz.so' module will never be linked with a `-lbaz' option, so the `lib' prefix isn't necessary. In fact, it would probably be confusing if you used the prefix. Similarly, there is no constraint to use any particular filename suffix, but it is sensible to use the target's native shared library suffix (GNU/Linux uses `.so') to make it obvious that the compiled file is some sort of shared object, and not a normal executable.

Apart from that, the only difference between a shared library built for linking at compile-time and a dynamic module built for loading at run-time is that the module must provide known entry points for the main executable to call. That is, when writing code destined for a dynamic module, you must provide functions or variables with known names and semantics that the main executable can use to access the functionality of the module. This is different to the function and variable names in a regular library, which are already known when you write the client code, since the libraries are always written before the code that uses them; a runtime module loading system must, by definition, be able to cope with modules that are written after the code that uses those modules.