Scripted wrappers for legacy applications

Let's start with an easy example, the kind you'll want to do on your own: create a C source file, test1.c:

        #include  
        #include  
        main() 
        { 
            struct timeval timevalue; 
            struct timezone timezone; 
 
            gettimeofday(&timevalue, &timezone); 
            printf("The current time is %s", 
                     ctime((time_t *) &timevalue.tv_sec)); 
        } 

and make an executable program from it. Under Unix, you'd generally do that with a command-line invocation:

        cc -o test1 test1.c 

Now create the Perl/Tk source file, test1.pl:

       use Tk; 
 
       $mw = MainWindow->new; 
       $label = $mw->Label(); 
       $label->pack; 
       $mw->Button(-text => "Show the time", 
            -command => sub{ 
                $label->configure(-text => `./test1`) 
             })->pack; 
       MainLoop; 

There! If you execute:

        perl test1.pl 

you'll see a small GUI panel with a button. Pushing the button launches the C-coded program, retrieves output, and displays it. The problem's solved, for both Unix and Windows.

There's more to the story

For many programmers, though, that is only the beginning. Let's look at the example in more detail.

To run the example, we launch a Perl program, which itself controls the legacy test1 as a separate process. With all the examples in this article, it doesn't matter that test1 is compiled from C source; it could be a Fortran, C++, Java, or even another Perl program. All that matters in that architecture is that it can be launched as a command-line application, which puts its results to standard output.

Most modern languages available for Unix and Win* have at least one way of invoking an external process and retrieving its result in that way. The gluing languages we discuss in this column are particularly rich in those facilities. Most accessible to beginners are the backtick operator or function used in test1.pl above, which is also available in much the same form in Ruby, Tcl, Bourne shell, and several other languages.

A simple control panel of that sort is a great way to wrap a legacy application with complex command-line arguments or inputs.

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