The GNU FDL is incompatible in both directions with the GPL: that is GNU FDL material cannot be put into GPL code and GPL code cannot be put into a GNU FDL manual.  Because of this, code samples are often dual-licensed so that they may appear in documentation and can be incorporated into a free software program. 
The GNU FDL is incompatible in both directions with the GPL: that is GNU FDL material cannot be put into GPL code and GPL code cannot be put into a GNU FDL manual.  Because of this, code samples are often dual-licensed so that they may appear in documentation and can be incorporated into a free software program. 