I had read the GCC documentation and I did not know about the forward parameter declaration. I did know about the other stuff that is mentioned there (and in the first part).

Declarations in for loops is something that I had only ever used in macros (I had not found it useful in other circumstances), such as:

  #define lpt_document() for(int lpt_document_=lpt_begin();lpt_document_;lpt_document_=(lpt_end(),0))
  #define win_form(xxx) for(win_memo win_mem=win_begin_();;win_step_(&win_mem,xxx))

(The compiler will optimize out the loop and the declared variable in the use of the lpt_document macro; I had tested this.)