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From: ccc_ldo@waikato.ac.nz
Newsgroups: comp.sys.mac.programmer
Subject: More on MPW C 3.0 question
Message-ID: <162.25dfe4b3@waikato.ac.nz>
Date: 19 Feb 90 00:25:04 GMT
Lines: 91

The story so far:

While porting a large C program between versions of MPW C, I came
across a strange quirk in the 3.0 compiler, which is summed up in
the following program:

	typedef unsigned char
	    Str255[256];

	main()
	  {
	    Str255
		temps, *tempp;

	    tempp = &temps;
	  }

When I try to compile this, the compiler throws up the following
error message:

	#
	#    tempp = &temps;
	#                  ?
	### Error 225 Incompatible types for assignment
	#-----------------------------------------------------------------------
		File "test.c"; Line 9
	#-----------------------------------------------------------------------

A number of people have made the same suggestion to me, viz, replace
the line

	    tempp = &temps;

with

	    tempp = temps;

Would you believe, this doesn't work? I get the exact same error, on the
same line.

Only one bright soul suggested the obvious, "brute force" solution--a type
cast. This does work. Here is the modified program, with an extra line added
to keep the compiler from optimising away the body of the routine completely:

	typedef unsigned char
	    Str255[256];

	main()
	  {
	    Str255
		temps, *tempp;

	    tempp = (Str255 *) &temps;
	    do_something_with(tempp);
	  }

It even generates the right code. *And* taking out the ampersand, as per
those other suggestions, makes no difference--it still compiles, and
still generates the same code.

To all those people who tried to explain to me that, in C, arrays are
actually just constant pointers--thanks, but I knew that. Also, all the
C compilers I've met, except one, have allowed me to use "&array" to mean
the same thing as "array", where "array" was an array variable. The MPW
3.0 compiler is no exception, as the following version of the troublesome
program demonstrates:

	typedef unsigned char
	    Str255[256];

	main()
	  {
	    Str255
		temps;
	    unsigned char
		*tempp;

	    tempp = &temps;
	    do_something_with(tempp);
	  }

This version also compiles without error, and generates the right code.

Thanks everybody for your help. But, the question remains: is this behaviour
a requirement of the ANSI C standard, or is it a bug in Apple's compiler?

Lawrence D'Oliveiro
Mac-hacker-in-residence
Computer Services Department, University of Waikato, Hamilton, New Zealand
All opinions expressed in this message are sacred to Epimetheus, the
    Greek god of hindsight.