From mboxrd@z Thu Jan  1 00:00:00 1970
To: 9fans@cse.psu.edu
Date: Wed,  2 Aug 2000 09:39:05 +0000
From: "Douglas A. Gwyn" <dagwyn@home.com>
Message-ID: <3987E4B0.FEA6B593@home.com>
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References: <8m6dhf$74d$1@inputplus.demon.co.uk>, <200008011603.MAA05857@egyptian-gods.MIT.EDU>
Subject: Re: [9fans] Installing the updates
Topicbox-Message-UUID: f2700dfa-eac8-11e9-9e20-41e7f4b1d025

Greg Hudson wrote:
> a low cost (and one which can be optimized away for ifdef-protected
> headers; I'm told gcc does so).

The general trick is for the preprocessor to maintain
a table of (lock_symbol,header_name) entries, and when
a new header file is read, to make a note whether it
consists of <white>#ifndef symbol...#endif<white>
(after comment processing).  If so, the symbol and
file name go into the table.  #include processing
always checks the table and if there is an entry for
the file, if the associated lock_symbol is defined,
the file is not even opened.  This trick does no harm.