Date: Sun, 16 Oct 94 17:07:29 JST
From: Stephen Turnbull <turnbull AT shako DOT sk DOT tsukuba DOT ac DOT jp>
To: walters AT smd4d DOT wes DOT army DOT mil
Cc: djgpp AT sun DOT soe DOT clarkson DOT edu
Subject: Order of "Operations" bug? <not on newslist>

This doesn't belong on the DJGPP list.  Get a good C book (K&R 2d ed,
Harbison and Steele are both good), maybe Stroustrup's book on C++,
too, and read them all the way through.

   Which FINALLY leads to my questions: Have I done something which
   resulted in an invalid installation, is this a bug in the djgpp
   code, does my having gpp installed without libraries loaded affect
   the distribution, or is it not a c standard for all arguments of a
   function (printf in this case) to be evaluated from left to right?

In C, for efficiency reasons, you are NOT allowed to rely on the order
of evaluation of the arguments of a function call (or across
assignment operators or commutative or associative operators, etc).
In fact, most C compilers push arguments from right to left in order
to simplify the programming of variadic functions like printf (note
that in this case the return address will be top of stack, and
immediately below it you will find the pointer to the format string).
However, many C++ compilers push arguments from left to right for
fixed argument functions, but switch to the right to left approach for
variadic functions.
    With a good optimizer, it is possible that

   int temp = match(a,b);
   printf("Match results: Match at %d marker at %d\n",temp,marker);

would effectively result in *exactly* the same order of instructions
as the left to right evaluation of arguments.  The difference is that
(oops, this is from Stroustrup's book, but I believe ANSI C also
mandates it) the assignment to the explicit temporary variable
guarantees the order of evaluation (although there is no guarantee
that the temporary variable will not be optimized into oblivion).

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|                           Stephen Turnbull                            |
|     University of Tsukuba, Institute of Socio-Economic Planning       |
|          Tennodai 1-chome 1--1, Tsukuba, Ibaraki 305 JAPAN            |
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|                                                                       |
|                Founder and CEO, Skinny Boy Associates                 |
|               Mechanism Design and Social Engineering                 |
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