[pdftex] Experiment with a new TeX primitive

[Hans Hagen]

At 12:03 31/01/2004, Nagy Bence wrote:
>Dear developers,
>
>I made a small experiment to understand better how web2c works
>(calling pascal<->C codes), so i defined a new TeX primitive
>(\experiment). It calls an external C-function to print out its
>parameter, and this function gives back a new string (like a search and
>replace engine).
>
>The code from the C-file:
>
>strnumber experiment(integer t)
>{
>     tex_printf("<%s>",makecstring(tokenstostring(t)));
>     return maketexstring("apple");
>}
>
>The main inserted code from pdftex.ch:
>
>@ @<Implement \.{\\experiment}@>=
>begin
>     call_func(scan_toks(false, true));
>     back_list(link(experiment(def_ref)));
>     delete_token_ref(def_ref);
>end
>
>All works fine except that I can not put back the new string ("apple")
>to the token list. The function back_list causes a segmentation
>fault, but it should work like the \lowercase or \uppercase primitive
>(the experiment C-function gives back an integer value well). What is
>wrong?

You need to simulate something:

\ifx\scratchread \undefined \newread \scratchread  \fi
\ifx\scratchwrite\undefined \newwrite\scratchwrite \fi

\let\endofresultline\relax
\let\resultofrun    \empty

\def\resultrunfile {\jobname.src.tmp}
\def\resultrunerror{no result}

\def\runsystemcommand#1%
   {\bgroup
    % reset results
    \global\let\endofresultline\relax
    \global\let\resultofrun\empty
    % erase resultfile
    \immediate\openout\scratchwrite\resultrunfile
    \immediate\write\scratchwrite{\resultrunerror}%
    \immediate\closeout\scratchwrite
    % run command
    \immediate\write18{#1 > \resultrunfile}%
    % define collector
    \def\runsystemcommand
      {\ifeof\scratchread \else
         \endlinechar -1
         \readline\scratchread to \next
         \xdef\resultofrun{\resultofrun\next\endofresultline}%
         \expandafter\runsystemcommand
       \fi}%
    % fetch result
    \openin\scratchread\resultrunfile\relax
    \runsystemcommand
    \closein\scratchread
    % done
    \egroup}

\starttext

\runsystemcommand{dir /s} \let\endofresultline\par \tt \resultofrun

\stoptext

so, the best place to look is in the etex code and see how the \readline 
results end up in tex

Hans