19.3.3 Creating a Stand-alone Library to be used in a non-Ada context

It is easy to adapt the SAL build procedure discussed above for use of a SAL in a non-Ada context.

The only extra step required is to ensure that library interface subprograms are compatible with the main program, by means of pragma Export or pragma Convention.

Here is an example of simple library interface for use with C main program:

     package Interface is
     
        procedure Do_Something;
        pragma Export (C, Do_Something, "do_something");
     
        procedure Do_Something_Else;
        pragma Export (C, Do_Something_Else, "do_something_else");
     
     end Interface;

On the foreign language side, you must provide a “foreign” view of the library interface; remember that it should contain elaboration routines in addition to interface subprograms.

The example below shows the content of mylib_interface.h (note that there is no rule for the naming of this file, any name can be used)

     /* the library elaboration procedure */
     extern void mylibinit (void);
     
     /* the library finalization procedure */
     extern void mylibfinal (void);
     
     /* the interface exported by the library */
     extern void do_something (void);
     extern void do_something_else (void);

Libraries built as explained above can be used from any program, provided that the elaboration procedures (named mylibinit in the previous example) are called before the library services are used. Any number of libraries can be used simultaneously, as long as the elaboration procedure of each library is called.

Below is an example of a C program that uses the mylib library.

     #include "mylib_interface.h"
     
     int
     main (void)
     {
        /* First, elaborate the library before using it */
        mylibinit ();
     
        /* Main program, using the library exported entities */
        do_something ();
        do_something_else ();
     
        /* Library finalization at the end of the program */
        mylibfinal ();
        return 0;
     }

Note that invoking any library finalization procedure generated by gnatbind shuts down the Ada run-time environment. Consequently, the finalization of all Ada libraries must be performed at the end of the program. No call to these libraries or to the Ada run-time library should be made after the finalization phase.