Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!ames!pasteur!helios.ee.lbl.gov!ace.ee.lbl.gov!jef
From: jef@ace.ee.lbl.gov (Jef Poskanzer)
Newsgroups: comp.software-eng
Subject: Re: Recursive #includes
Message-ID: <1991@helios.ee.lbl.gov>
Date: 28 Feb 89 01:07:59 GMT
References: <18136@glacier.STANFORD.EDU>
Sender: usenet@helios.ee.lbl.gov
Reply-To: Jef Poskanzer <jef@helios.ee.lbl.gov>
Organization: Paratheo-Anametamystikhood Of Eris Esoteric, Ada Lovelace Cabal
Lines: 37

In the referenced message, jbn@glacier.UUCP (John B. Nagle) wrote:
}>> #ifndef H_FOO
}>> #define H_FOO
}>>...stuff...
}>> #endif
}
}      It does add to your compile times, since the entire file you're not
}including has to be read and parsed.  

Read, yes; parsed, no.  You would be surprised at how few cycles are
used in reading an include file and just handling the #ifs.  On a
Sun 3/260:

stdio.h's	user		system		elapsed
1		0.6		0.4		5
  delta		   6.3		   1.5		  7
501		6.9		1.9		12
  delta		   6.5		   1.3		  7
1001		13.4		3.2		19
  delta		   6.7		   1.0		  10
1501		20.1		4.2		29

Looks to me like handling a stdio-sized include file for a second time
costs about 0.013 user, 0.003 system, and 0.015 elapsed.  I think I can
live with that, especially considering the minutes to hours of my time
that the current idiotic #include scheme wastes each time I need to
figure out what files to include.

I argued this issue a few years ago when I was working at UniSoft.
I provided data similar to the above, and assumed that the benefits
of including only the files you explicitly reference would be obvious
to all.  Sadly, I was ignored.
---
Jef

            Jef Poskanzer   jef@helios.ee.lbl.gov   ...well!pokey
                         In Stereo (where available)