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diff --git a/graphics/pgf/base/doc/pgfmanual-en-base-internalregisters.tex b/graphics/pgf/base/doc/pgfmanual-en-base-internalregisters.tex new file mode 100644 index 0000000000..fe28a3c5e8 --- /dev/null +++ b/graphics/pgf/base/doc/pgfmanual-en-base-internalregisters.tex @@ -0,0 +1,127 @@ +% Copyright 2018 by Christian Feuersaenger +% +% This file may be distributed and/or modified +% +% 1. under the LaTeX Project Public License and/or +% 2. under the GNU Free Documentation License. +% +% See the file doc/generic/pgf/licenses/LICENSE for more details. + + +\section{Adding libraries to \pgfname: temporary registers} +\label{section-internal-registers} + +This section is intended for those who like to write libraries to extend +\pgfname. Of course, this requires a good deal of knowledge about +\TeX-programming and the structure of the \pgfname\ basic layer. Besides, one +will encounter the need of temporary variables and, especially, temporary \TeX\ +registers. This section describes how to use a set of pre-allocated temporary +registers of the basic layer without needing to allocate more of them. + +A part of these internals are already mentioned in +section~\ref{section-internal-pointcmds}, but the basic layer provides more +temporaries than |\pgf@x| and |\pgf@y|. + +\begin{internallist}[dimen register]{\pgf@x,\pgf@y} + These registers are used to process point coordinates in the basic layer of + \pgfname, see section~\ref{section-internal-pointcmds}. After a + |\pgfpoint|$\dotsc$ command, they contain the final $x$ and $y$ coordinate, + respectively. + + The values of |\pgf@x| and |\pgf@y| are set \emph{globally} in contrast to + other available \pgfname\ registers. You should never assume anything about + their value unless the context defines them explicitly. + + Please prefer the |\pgf@xa|, |\pgf@xb|, $\dotsc$ registers for temporary + dimen registers unless you are writing point coordinate commands. +\end{internallist} + +\begin{internallist}[dimen register]{ + \pgf@xa, + \pgf@xb, + \pgf@xc, + \pgf@ya, + \pgf@yb, + \pgf@yc% +} + Temporary registers for \TeX\ dimensions which can be modified freely. Just + make sure changes occur only within \TeX\ groups. + + \paragraph{Attention:} + % + \pgfname\ uses these registers to perform path operations. For reasons of + efficiency, path commands do not always guard them. As a consequence, the + code + % +\begin{codeexample}[code only] +\pgfpointadd{\pgfpoint{\pgf@xa}{\pgf@ya}}{\pgfpoint{\pgf@xb}{\pgf@yb}} +\end{codeexample} + % + \noindent may fail: Inside |\pgfpointadd|, the |\pgf@xa| and friend + registers might be modified. In particular, it might happen that |\pgf@xb| + is changed before |\pgfpoint{\pgf@xb}{\pgf@yb}| is evaluated. The right + thing to do would be to first expand everything using |\edef| and process + the values afterwards, resulting in unnecessary expensive operations. Of + course, one can avoid this by simply looking into the source code of + |\pgfpointadd| to see which registers are used. +\end{internallist} + +\begin{internallist}[dimen register]{\pgfutil@tempdima,\pgfutil@tempdimb} + Further multi-purpose temporary dimen registers. For \LaTeX, these + registers are already allocated as |\@tempdima| and |\@tempdimb| and are + simply |\let| to the |\pgfutil@|$\dotsc$ names. +\end{internallist} + +\begin{internallist}[count register]{ + \c@pgf@counta, + \c@pgf@countb, + \c@pgf@countc, + \c@pgf@countd% +} + These multiple-purpose count registers are used throughout \pgfname\ to + perform integer computations. Feel free to use them as well, just make sure + changes are scoped by local \TeX\ groups. +\end{internallist} + +\begin{internallist}[openout handle]{\w@pgf@writea} + An |\openout| handle which is used to generate complete output files within + locally scoped parts of \pgfname\ (for example, to interact with + |gnuplot|). You should always use |\immediate| in front of output + operations involving |\w@pgf@writea| and you should always close the file + before returning from your code. + % +\begin{codeexample}[code only] +\immediate\openout\w@pgf@writea=myfile.dat +\immediate\write\w@pgf@writea{...}% +\immediate\write\w@pgf@writea{...}% +\immediate\closeout\w@pgf@writea% +\end{codeexample} + % +\end{internallist} + +\begin{internallist}[openin handle]{\r@pgf@reada} + An |\openin| handle which is used to read files within locally scoped parts + of \pgfname, for example to check if a file exists or to read data files. + You should always use |\immediate| in front of output operations involving + |\r@pgf@writea| and you should always close the file before returning from + your code. + % +\begin{codeexample}[code only] +\immediate\openin\r@pgf@reada=myfile.dat +% do something with \macro +\ifeof\r@pgf@reada + % end of file or it doesn't exist +\else + % loop or whatever + \immediate\read\r@pgf@reada to\macro + ... +\fi +\immediate\closein\r@pgf@reada +\end{codeexample} + % +\end{internallist} + +\begin{internallist}[box]{\pgfutil@tempboxa} + A box for temporary use inside of local \TeX\ scopes. For \LaTeX, this box + is the same as the already pre-allocated |\@tempboxa|. +\end{internallist} |