autoconf: acconfig Header

1 
1 18.2 `acconfig.h'
1 =================
1 
1 In order to produce `config.h.in', `autoheader' needs to build or to
1 find templates for each symbol.  Modern releases of Autoconf use
1 `AH_VERBATIM' and `AH_TEMPLATE' (⇒Autoheader Macros), but in
1 older releases a file, `acconfig.h', contained the list of needed
1 templates.  `autoheader' copied comments and `#define' and `#undef'
1 statements from `acconfig.h' in the current directory, if present.
1 This file used to be mandatory if you `AC_DEFINE' any additional
1 symbols.
1 
1    Modern releases of Autoconf also provide `AH_TOP' and `AH_BOTTOM' if
1 you need to prepend/append some information to `config.h.in'.  Ancient
1 versions of Autoconf had a similar feature: if `./acconfig.h' contains
1 the string `@TOP@', `autoheader' copies the lines before the line
1 containing `@TOP@' into the top of the file that it generates.
1 Similarly, if `./acconfig.h' contains the string `@BOTTOM@',
1 `autoheader' copies the lines after that line to the end of the file it
1 generates.  Either or both of those strings may be omitted.  An even
1 older alternate way to produce the same effect in ancient versions of
1 Autoconf is to create the files `FILE.top' (typically `config.h.top')
1 and/or `FILE.bot' in the current directory.  If they exist,
1 `autoheader' copies them to the beginning and end, respectively, of its
1 output.
1 
1    In former versions of Autoconf, the files used in preparing a
1 software package for distribution were:
1      configure.ac --.   .------> autoconf* -----> configure
1                     +---+
1      [aclocal.m4] --+   `---.
1      [acsite.m4] ---'       |
1                             +--> [autoheader*] -> [config.h.in]
1      [acconfig.h] ----.     |
1                       +-----'
1      [config.h.top] --+
1      [config.h.bot] --'
1 
1    Using only the `AH_' macros, `configure.ac' should be
1 self-contained, and should not depend upon `acconfig.h' etc.
1