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Re: z/OS enum size pitfall


From: Ben Pfaff
Subject: Re: z/OS enum size pitfall
Date: Tue, 22 Aug 2017 13:25:55 -0700
User-agent: Mutt/1.5.23 (2014-03-12)

On Tue, Aug 22, 2017 at 04:13:54PM -0400, Daniel Richard G. wrote:
> Hello list,
> 
> I'm writing in to report a bizarre issue with the IBM z/OS XLC compiler
> that is currently causing one gnulib test to fail (test-timespec), and
> may present an issue for application code simply because no other
> compiler does things this way. My hope is to have gnulib integrate a
> workaround so that this won't bite anyone else.
> 
> I have been in contact with IBM about this, originally reporting the
> issue as a compiler bug. However, they responded that the compiler
> behavior is conformant to the C standard and that they are less
> concerned with matching the behavior of other systems than keeping
> things as-is for the benefit of existing customer application code.
> 
> The problem has to do with the implicit integer type that is used for
> enum symbols. Here is a sample program that illustrates the issue:
> 
> --------8<--------
> #include <stdio.h>
> 
> enum { BILLION = 1000000000 };
> 
> static const unsigned int BILLION2 = 1000000000;
> 
> int main(void) {
>     int x = -999999999;
>     printf("BILLION = %d\n", (int)BILLION);
>     printf("x = %d\n", x);
>     printf("x / BILLION = %d\n", (int)(x / BILLION));
>     return 0;
> }
> -------->8--------
> 
> On GNU/Linux and AIX, with a minimal compiler invocation, this
> program prints
> 
>     BILLION = 1000000000
>     x = -999999999
>     x / BILLION = 0
> 
> However, on z/OS, it prints
> 
>     BILLION = 1000000000
>     x = -999999999
>     x / BILLION = 3
> 
> What happens is that BILLION is implicitly typed as an unsigned int,
> rather than an int. If you edit the code above to use BILLION2 instead
> of BILLION, you'll see the same result on GNU/Linux.

It's odd that they claim that this conforms to the C standard.  C11
says, in section 6.4.4.3 "Enumeration constants":

    An identifier declared as an enumeration constant has type int.

It also says in section 6.7.2.2 "Enumeration specifiers":

    The identifiers in an enumerator list are declared as constants that
    have type int and may appear wherever such are permitted.

This seems pretty clear to me, so I wonder how this interpretation
arises.



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