Usage: /usr/bin/ifnames [OPTION]... [FILE]...

Scan all of the C source FILES (or the standard input, if none are
given) and write to the standard output a sorted list of all the
identifiers that appear in those files in '#if', '#ifdef', '#ifndef',
'#elif', '#elifdef', or '#elifndef' directives.  Print each identifier
on a line, followed by a space-separated list of the files in which
that identifier occurs.

  -h, --help      print this help, then exit
  -V, --version   print version number, then exit

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or by email to <bug-autoconf@gnu.org>.

The full documentation for Autoconf can be read via 'info autoconf',
or on the Web at <https://www.gnu.org/software/autoconf/manual/>.