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(autoconf.info)Autoconf Language

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The Autoconf Language
---------------------

   The Autoconf language is very different from many other computer
languages because it treats actual code the same as plain text.  Whereas
in C, for instance, data and instructions have very different syntactic
status, in Autoconf their status is rigorously the same.  Therefore, we
need a means to distinguish literal strings from text to be expanded:
quotation.

   When calling macros that take arguments, there must not be any blank
space between the macro name and the open parenthesis.  Arguments should
be enclosed within the M4 quote characters `[' and `]', and be
separated by commas.  Any leading spaces in arguments are ignored,
unless they are quoted.  You may safely leave out the quotes when the
argument is simple text, but _always_ quote complex arguments such as
other macro calls.  This rule applies recursively for every macro call,
including macros called from other macros.

   For instance:

     AC_CHECK_HEADER([stdio.h],
                     [AC_DEFINE([HAVE_STDIO_H])],
                     [AC_MSG_ERROR([Sorry, can't do anything for you])])

is quoted properly.  You may safely simplify its quotation to:

     AC_CHECK_HEADER(stdio.h,
                     [AC_DEFINE(HAVE_STDIO_H)],
                     [AC_MSG_ERROR([Sorry, can't do anything for you])])

Notice that the argument of `AC_MSG_ERROR' is still quoted; otherwise,
its comma would have been interpreted as an argument separator.

   The following example is wrong and dangerous, as it is underquoted:

     AC_CHECK_HEADER(stdio.h,
                     AC_DEFINE(HAVE_STDIO_H),
                     AC_MSG_ERROR([Sorry, can't do anything for you]))

   In other cases, you may have to use text that also resembles a macro
call.  You must quote that text even when it is not passed as a macro
argument:

     echo "Hard rock was here!  --[AC_DC]"

which will result in

     echo "Hard rock was here!  --AC_DC"

When you use the same text in a macro argument, you must therefore have
an extra quotation level (since one is stripped away by the macro
substitution).  In general, then, it is a good idea to _use double
quoting for all literal string arguments_:

     AC_MSG_WARN([[AC_DC stinks  --Iron Maiden]])

   You are now able to understand one of the constructs of Autoconf that
has been continually misunderstood...  The rule of thumb is that
_whenever you expect macro expansion, expect quote expansion_; i.e.,
expect one level of quotes to be lost.  For instance:

     AC_COMPILE_IFELSE([char b[10];],, [AC_MSG_ERROR([you lose])])

is incorrect: here, the first argument of `AC_COMPILE_IFELSE' is `char
b[10];' and will be expanded once, which results in `char b10;'.
(There was an idiom common in Autoconf's past to address this issue via
the M4 `changequote' primitive, but do not use it!)  Let's take a
closer look: the author meant the first argument to be understood as a
literal, and therefore it must be quoted twice:

     AC_COMPILE_IFELSE([[char b[10];]],, [AC_MSG_ERROR([you lose])])

Voila`, you actually produce `char b[10];' this time!

   The careful reader will notice that, according to these guidelines,
the "properly" quoted `AC_CHECK_HEADER' example above is actually
lacking three pairs of quotes!  Nevertheless, for the sake of
readability, double quotation of literals is used only where needed in
this manual.

   Some macros take optional arguments, which this documentation
represents as [ARG] (not to be confused with the quote characters).
You may just leave them empty, or use `[]' to make the emptiness of the
argument explicit, or you may simply omit the trailing commas.  The
three lines below are equivalent:

     AC_CHECK_HEADERS(stdio.h, [], [], [])
     AC_CHECK_HEADERS(stdio.h,,,)
     AC_CHECK_HEADERS(stdio.h)

   It is best to put each macro call on its own line in `configure.ac'.
Most of the macros don't add extra newlines; they rely on the newline
after the macro call to terminate the commands.  This approach makes
the generated `configure' script a little easier to read by not
inserting lots of blank lines.  It is generally safe to set shell
variables on the same line as a macro call, because the shell allows
assignments without intervening newlines.

   You can include comments in `configure.ac' files by starting them
with the `#'.  For example, it is helpful to begin `configure.ac' files
with a line like this:

     # Process this file with autoconf to produce a configure script.