[Qt-interest] LGPL and static linking

[andrew.m.goth at l-3com.com]

Daniel Grubbs wrote:
> The fact that one's program is dynamically linked to the Qt 
> library doesn't preclude the possibility that you may also 
> have modified parts of the library statically included in 
> your program as well.

This is true of all third-party programs and libraries, not just Qt.  I sell you a binary; how do you know it doesn't secretly incorporate some GPL code?  I'm not sure I can prove that it doesn't.  Even if I disclose my full sources in a court of law, how can I prove to your lawyers that not even the slightest bit of it was derived from any GPL codebase?  Also I would have to show that the sources I disclosed match the binary I sold.  I suspect I wouldn't be asked to defend myself in this manner, unless my code behaves suspiciously (e.g. uses mplayer's internal subtitle format as if it was its own), or I am being harrassed (SCO).

> The distinction between dynamic and static linking is arbitrary and 
> could only make a difference to a lawyer.

It's not entirely arbitrary, because dynamic linking keeps the application and the Qt library in separate files.  Separate files make it very easy to show that the Qt library has not been unmodified.  But if the Qt library has been folded into the application by means of static linking, it's much more difficult to show that no modification has taken place.  That's why I suggested delivering separate .a files to be linked by the customer.  From the perspective of a lawyer (so I imagine), this mimics the way dynamic linking works.  From the perspective of the computer, it's still static linking, no matter who does it.

-- 
Andy Goth
<amgoth at link.com>