Source: http://www.gnu.org/software/m4/manual/m4.html

GNU M4 1.4.16 macro processor

2 Invoking m4

The format of the m4 command is:

     m4 [option...] [file...]

All options begin with ‘–’, or if long option names are used, with ‘—’.

The argument—is a marker to denote the end of options.

In other words, m4 -QPDfoo -d a -df is equivalent to m4 -Q -P -D foo -d -df -- ./a, although the latter form is considered canonical.继续阅读