greim@sbsvax.UUCP (Michael Greim) (08/03/89)
About 2 weeks I asked in <800@sbsvax.UUCP> whether a certain program was
incorrect and whether C compilers which only gave me a warning were
in error to do so, ie. is it permissible for a compiler only to warn
about the error?
Here is the program (just to refresh your memory):
---------- cut here ----------------------------
/*
* What does the compiler do with this?
*/
# include <stdio.h>
struct link {
struct link * next;
int count;
};
int fix0;
struct link * t;
int fix1;
main ()
{
printf ("t lies at %1d, brk is at %1d\n", (int)(&t), (int)sbrk(0));
printf ("Addresses of %1d byte objects around t : %1d and %1d\n",
sizeof(int), (int)(&fix0), (int)(&fix1));
t = NULL;
t.count = 5;
if (t.count == 5)
printf ("Assignment worked.\n");
else
printf ("Assignment did not work\n");
printf ("Value 5 has been put at address %1d\n", &(t.count));
}
---------- cut here ----------------------------
Output from VAX running 43bsd:
Script started on Wed Jul 19 11:44:22 1989
% cc test78.c
"test78.c", line 21: warning: struct/union or struct/union pointer required
"test78.c", line 22: warning: struct/union or struct/union pointer required
"test78.c", line 26: warning: struct/union or struct/union pointer required
% a.out
t lies at 6008, brk is at 6176
Addresses of 4 byte objects around t : 6004 and 6012
Assignment worked.
Value 5 has been put at address 6012
% exit
%
script done on Wed Jul 19 11:44:47 1989
Obviously the compiler took the address of t as the base for computing
the address of the member count and put the constant 5 there.
Thanks to all who answered.
Here are the results:
Of course the program is incorrect. t is defined as a pointer not
as a structure. (I knew that of course)
As to the second (and most important question) "Is it permissible for
a compiler only to warn about this?" I can only cite Chris Torek:
>The program is not legal C, but compiling it (particularly with a warning)
>is not wrong: at least, not according to the pANS. Most language standards
>do not require that a compiler detect and complain about incorrect
>programs; they say only what the compiler must do with correct programs.
>(This is necessary in general since the variety of incorrect programs
>seems to be infinite. :-) )
>
>PCC is overly trusting of structure and union field references so as
>to be backward compatible with Version 4 Unix. (The source for adb
>depended on the ability to declare a double, and then use structure
>field operators on that variable, until recently. Taking backward
>compatibility hacks out of the compiler reveals a surprising amount
>of bad software sometimes. . . .)
In my opinion any compiler should give me an error for the program, but
I can make do with the warning.
A note to all who warned me about the usage of the variables fix0 and
fix1: I know that I cannot rely on them being at any address relative
to t. It was just a quick test to see where the constant was written
to. The program could also have dumped core.
-mg
--
Michael Greim Email : greim@sbsvax.informatik.uni-saarland.dbp.de
or : ...!uunet!unido!sbsvax!greim
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