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docs: document how to use comment characters in rules.

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* doc/autoconf.texi (Comments in Make Rules): Explain how to
produce a `#' in a rule.
(Comments in Make Macros): Add cross reference.
Suggestion from Eric Blake.

Signed-off-by: Ralf Wildenhues <Ralf.Wildenhues@gmx.de>
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Ralf Wildenhues 2011-01-22 19:39:29 +01:00
parent f02c72e46b
commit 80ca89f803
2 changed files with 23 additions and 1 deletions
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@ -1,5 +1,11 @@
2011-01-22 Ralf Wildenhues <Ralf.Wildenhues@gmx.de>
docs: document how to use comment characters in rules.
* doc/autoconf.texi (Comments in Make Rules): Explain how to
produce a `#' in a rule.
(Comments in Make Macros): Add cross reference.
Suggestion from Eric Blake.
docs: new sections about comments and whitespace in make macros.
* doc/autoconf.texi (Top, Portable Make): Adjust menus.
(Comments in Make Macros, Trailing whitespace in Make Macros):

18
doc/autoconf.texi
View File

@ -19553,6 +19553,21 @@ all:
: "foo"
@end example
Conversely, if you want to use the @samp{#} character in some command,
you can only do so by expanding it inside a rule (@pxref{Comments in
Make Macros}). So for example, if @samp{COMMENT_CHAR} is substituted by
@command{config.status} as @samp{#}, then the following substitutes
@samp{@@COMMENT_CHAR@@} in a generated header:
@example
foo.h: foo.h.in
sed -e 's|@@''COMMENT_CHAR''@@|@@COMMENT_CHAR@@|g' \
$(srcdir)/foo.h.in > $@@
@end example
The funny shell quoting avoids a substitution at @command{config.status}
run time of the left-hand side of the @command{sed} @samp{s} command.
@node Newlines in Make Rules
@section Newlines in Make Rules
@cindex Newlines in @file{Makefile} rules
@ -19599,7 +19614,8 @@ CPPFLAGS = "-DCOMMENT_CHAR='#'"
@noindent
as @samp{CPPFLAGS} may be expanded to @samp{"-DCOMMENT_CHAR='}.
Luckily, most @command{make} implementations disregard this and treat
single and double quotes specially here.
single and double quotes specially here; @pxref{Comments in Make Rules},
for a more portable alternative.
Even without quoting involved, comments can have surprising effects,
because the whitespace before them is part of the variable value: