The GNU Modula-2 front end to GCC

Building a shared library

This section describes building a tiny shared library implemented entirely in Modula-2. Suppose a project consists of two definition modules and two implementation modules and a program module a.def, a.mod, b.def, b.mod and c.mod. The first step is to compile the modules using position independent code. This can be achieved by the following three commands:

gm2 -fiso -g -c -fPIC a.mod
gm2 -fiso -g -c -fPIC b.mod
gm2 -fiso -g -c -fPIC c.mod

The second step is to perform the link. In a simple project such as this you can use the single command:

gm2 -fiso -shared -fshared -fPIC -g c.mod -o c.so

At this point the shared library c.so will have been created. All procedures which are exported from the definition modules are also visible to the user of the shared library.

The BEGIN END code at module scope in a.mod, b.mod, c.mod and all dependent library modules will be initialized when the shared library is loaded. Likewise the FINALLY END code at all module scopes is executed when the library is unloaded. Both the initialization and termination is implemented transparently using the GNU/Linux mechanism:

void __attribute__ ((constructor)) initcode (void);
void __attribute__ ((destructor)) fincode (void);

The linking command above utilizes a number of subcomponents which determine the runtime initialization order, create the C++ initialization and finalization function and links all required modules together. You can inspect and modify the initialization and finalization order by using the following commands:

gm2 -c -fmakelist -fiso -fPIC -g c.mod

The command above will create a file c.lst which is a plain ascii file which contains the modules in initialization order. The # is a comment which continues to the next newline. It is worth noting that the first five or so modules which are listed before the commented lines are the critical runtime modules which have a default order. You can override these using the command line option -fruntime-modules=. However you should only do this if you are building an application for an embedded system or your own set of Modula-2 low level libraries.

You can modify the order of the modules in this file, although it is wise to leave the initial five modules alone. It is much more useful to order your own modules which will be near the end of the file. Once you are happy with the order you can conclude the link with the command:

gm2 -fonlylink -fuselist -fiso -fPIC -g c.mod -o c.so

which creates c.so and c_m2.cpp. The initialization and finalization code is contained within c_m2.cpp which is compiled and linked with all the modules required to satisfy all the program module c.mod dependencies.