[TUHS] origin of C header files

Steve Johnson scj at yaccman.com
Sat Dec 30 11:06:34 AEST 2017

I was told once that McIlroy and Morris invented macro instructions
for assembly language.   And certainly they wrote the definitive
paper on macros, with, as I recall, something like 10 decisions you
needed to make about a macro processor and you could generate 1024
different macro systems that way.  I wonder if that ever got
published -- it blew my mind when I encountered it at the beginning of
my career...

There were two features of the C preprocessor that were, I think,
unique to the labs for some time.  One was a problem that came up
with program generators.  In RATFOR, for example, an IF statement on
line 50 might get translated to become line 125 of the output FORTRAN
program.   And if when there was an error, it could be very painful
to discover where it came from.  I ran into the same problem in
spades with Yacc.  When someone wrote a Yacc program (especially when
using a makefile), an error on line 50 in foo.y could easily be
reported as being on line 125 in foo.c.   The natural thing was to
fix the error in foo.c.   And lo, it compiled and ran.  Until the
next time you changed foo.y, and  then it reappeared in all its
glory.   I begged Dennis for a solution, and he came up with #line,
which allowed you to say to the C compiler "treat the next line as if
it were line nnn in file fff, and following lines as successor lines
in file fff.  It instantly solved the problem, and was used multiple
times for various applications (probably most notably cfront).  So
far as I know, many languages including FORTRAN, Pascal and Python do
not have such a mechanism, making it awkward to use them as
preprocessor targets.

The other thing that I think was pretty unique was the search path for
include files (-I on the compilers).   When writing the portable C
compiler, I had a directory of source and header files that were
machine independent, and other directories that were specific to
different target architectures.   Just by changing the command line
you could compile for any desired architecture.  This would have been
a maintenance nightmare if I'd had to make different makefiles for
each desired language and keep them all in sync.


----- Original Message -----
 "Clem Cole" <clemc at ccc.com>

"Paul Winalski" <paul.winalski at gmail.com>
"TUHS main list" <tuhs at minnie.tuhs.org>
Fri, 29 Dec 2017 15:31:54 -0500
Re: [TUHS] origin of C header files

On Fri, Dec 29, 2017 at 2:28 PM, Paul Winalski
<paul.winalski at gmail.com [1]>

 When higher-level languages came along, programmers moving from
 assembly code to a HLL would want the same sort of preprocessor
 functionality.  I know that IBM PL/I had %include, and I suspect
 other HLLs of the day had similar features.

 What's very clear is that C did not invent include files or
 conditional compilation, it merely carried on existing tradition.

​I'll +1 Paul's comment and add a couple of observations. 
 Languages such a PL/1 and FORTRAN would could support a preprocessor
and conditional compilation, were more easy to use to build 'products'
- as opposed to Pascal.    Folks did splice an backwards
conditi​onal compiling scheme with include files into some Pascal
flavors but it was non-standard.

Fortran folks used tools like RATFOR or m4, but the key was the there
was some why to preprocess code for different targets.   In a
production shop, particularly where your 'target' customer was
different, this ability becomes more and more of an requirement.

I've always said as contemporary production systems programming
languages, while BLISS had a better Macro system then C, the include
file and conditional scheme worked much better/was much cleaner - to
the point that ifdef is abused and the cause of much pain in actual
code.   But the truth is that is a success problem.   When used
properly, the C header scheme, while not invented by the BTL crew, was
pretty much what people needed.   No too fancy, but all the features
you really needed and has been lasting.




[1] mailto:paul.winalski at gmail.com

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