autoconf.info: ifnames Invocation

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Using `ifnames' to List Conditionals

`ifnames' can help you write `configure.ac' for a software package.  It
prints the identifiers that the package already uses in C preprocessor
conditionals.  If a package has already been set up to have some
portability, `ifnames' can thus help you figure out what its
`configure' needs to check for.  It may help fill in some gaps in a
`configure.ac' generated by `autoscan' (*note autoscan Invocation::).
   `ifnames' scans all of the C source files named on the command line
(or the standard input, if none are given) and writes to the standard
output a sorted list of all the identifiers that appear in those files
in `#if', `#elif', `#ifdef', or `#ifndef' directives.  It prints each
identifier on a line, followed by a space-separated list of the files
in which that identifier occurs.
`ifnames' accepts the following options:
`--help'
`-h'
     Print a summary of the command line options and exit.
`--version'
`-V'
     Print the version number of Autoconf and exit.