(autoconf.info)acconfig Header

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17.2 `acconfig.h'

In order to produce `config.h.in', `autoheader' needs to build or to
find templates for each symbol.  Modern releases of Autoconf use
`AH_VERBATIM' and `AH_TEMPLATE' (Note: Autoheader Macros), but in
older releases a file, `acconfig.h', contained the list of needed
templates.  `autoheader' copied 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

   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 ancient 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

   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] --'

   Using only the `AH_' macros, `configure.ac' should be
self-contained, and should not depend upon `acconfig.h' etc.

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