Question

In terminal on OS X, if you type “say” and hit return the command doesn’t exit and any subsequent things typed in (followed by return) are said by the system. How can this kind of effect be achieved?

Answer 1

If you type cat and hit return, the command doesn’t exit and any subsequent things typed in, followed by a return, are printed on the screen (instead of being converted to speech through the speakers, as with say). So, the easy bit is reading lines until EOF; you can use fgets() for that, easily enough, or getline().

int main(void)
{
    char buffer[4096];
    while (fgets(buffer, sizeof(buffer), stdin) != 0)
    {
        fputs(buffer, stdout);
        // Or invoke appropriate text-to-speech function(s)
    }
    return(0);
}

The text-to-speech conversion will be done through calls provided by Apple. You may need to use Objective C to get at the interfaces sensibly, in which case you may also need to use some other input function or convert the string into an appropriate Objective C construct.

The /usr/bin/say command uses the libraries:

/System/Library/Frameworks/CoreFoundation.framework/Versions/A/CoreFoundation (compatibility version 150.0.0, current version 633.0.0)
/System/Library/Frameworks/ApplicationServices.framework/Versions/A/ApplicationServices (compatibility version 1.0.0, current version 41.0.0)
/System/Library/Frameworks/AudioToolbox.framework/Versions/A/AudioToolbox (compatibility version 1.0.0, current version 1.0.0)
/System/Library/Frameworks/AudioUnit.framework/Versions/A/AudioUnit (compatibility version 1.0.0, current version 1.0.0)
/System/Library/Frameworks/CoreAudio.framework/Versions/A/CoreAudio (compatibility version 1.0.0, current version 1.0.0)
/usr/lib/libstdc++.6.dylib (compatibility version 7.0.0, current version 52.0.0)
/usr/lib/libSystem.B.dylib (compatibility version 1.0.0, current version 159.0.0)

The combination of AudioToolBox, AudioUnit and CoreAudio will provide the text-to-speech services.