21.4.5 Diversions

Diversions are a facility in M4 for diverting text from the input stream into a holding buffer. There is a large number of diversion buffers in GNU M4, limited only by available memory. Text can be diverted into any one of these buffers and then `undiverted' back to the output (diversion number 0) at a later stage.

Text is diverted and undiverted using the divert and undivert macros. They expand to the empty string, with the side effect of setting the diversion. Here is an illustrative example:

divert(1)dnl This goes at the end. divert(0)dnl This goes at the beginning. undivert(1)dnl =>This goes at the beginning. =>This goes at the end.

It is unlikely that you will want to use diversions in your own macros, and it is difficult to do reliably without understanding the internals of Autoconf. However, it is interesting to note that this is how autoconf generates fragments of shell code on-the-fly that must precede shell code at the current point in the `configure' script.