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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])])

Voilà, 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.