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In order to produce `config.h.in', @command{autoheader} needs to
build or to find templates for each symbol. Modern releases of Autoconf
use AH_VERBATIM and AH_TEMPLATE (see section Autoheader Macros), but in older releases a file, `acconfig.h', contained the
list of needed templates. autoheader copies comments and
#define and #undef statements from `acconfig.h' in
the current directory, if present. This file used to be mandatory if
you AC_DEFINE any additional symbols.
Modern releases of Autoconf also provide AH_TOP and
AH_BOTTOM if you need to prepend/append some information to
`config.h.in'. Ancient versions of Autoconf had a similar feature:
if `./acconfig.h' contains the string `@TOP@',
autoheader copies the lines before the line containing
`@TOP@' into the top of the file that it generates. Similarly,
if `./acconfig.h' contains the string `@BOTTOM@',
autoheader copies the lines after that line to the end of the
file it generates. Either or both of those strings may be omitted. An
even older alternate way to produce the same effect in jurasik versions
of Autoconf is to create the files `file.top' (typically
`config.h.top') and/or `file.bot' in the current
directory. If they exist, autoheader copies them to the
beginning and end, respectively, of its output.
In former versions of Autoconf, the files used in preparing a software package for distribution were:
configure.ac --. .------> autoconf* -----> configure
+---+
[aclocal.m4] --+ `---.
[acsite.m4] ---' |
+--> [autoheader*] -> [config.h.in]
[acconfig.h] ----. |
+-----'
[config.h.top] --+
[config.h.bot] --'
Use only the AH_ macros, `configure.ac' should be
self-contained, and should not depend upon `acconfig.h' etc.
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