Re: z/OS enum size pitfall
On Tue, Aug 22, 2017 at 10:09:22PM -0400, Daniel Richard G. wrote: > On Tue, 2017 Aug 22 15:01-0700, Ben Pfaff wrote: > > > > I don't know what XLC conforms to. > > > > C11 has the same text in 6.7.2.2p4. The specification for enums has > > not changed significantly since C89. > > > > Paul Eggert already explained the distinction between enumeration > > constants and enumeration types, so I won't repeat it. > > All the commentary here is greatly appreciated; please excuse my delay > in replying. > > Here is what IBM said relating to standards: > > The compiler behavior is correct. The C standard says that the > enumerated type can be char, or any signed or unsigned integer type > (6.7.2.2#4). > > The test case uses a default enumsize which is small and for the > range of values in the test case it reserves an unsigned int to hold > the enumeration values (this is documented in the User's Guide). > > The conversion rules make the signed into unsigned int (6.3.1.8#1) > meaning that expression becomes (unsigned int)x/BILLION because > BILLION is an unsigned int; > > The conversion of -9 produces 3294967297 (UINT_MAX +1 > -9) which when divided by 10 produces 3 as the > result. > > There was no further qualification of "C standard" in the first > paragraph. However, the test program still returns 3 when XLC is > in C99 mode: > > $ xlc -qlanglvl=stdc99 -o enumsize enumsize.c > $ ./enumsize > BILLION = 10 > x = -9 > x / BILLION = 3 > > Now, from what I'm hearing here, it sounds like IBM may be in the wrong--- > and this would be quite convenient, because while they may not be too > bothered when their interpretation of POSIX et al. is different from the > rest of the world's, they _will_ care about a red-letter violation of a > spec they claim to support. > > I can't standards-lawyer my way out of a paper bag, but if someone here > could provide a line of argument that IBM's enum shenanigans are > categorically unsupported by the standard, I'll gladly send it on in the > hope that it'll get the beast to move. The C99 rationale is even clearer than the text, in section 6.4.4.3 "Enumeration constants": Whereas an enumeration variable may have any integer type that correctly represents all its values when widened to int, an enumeration constant is only usable as the value of an expression. Hence its type is simply int.
Re: z/OS enum size pitfall
On Tue, 2017 Aug 22 15:01-0700, Ben Pfaff wrote: > > I don't know what XLC conforms to. > > C11 has the same text in 6.7.2.2p4. The specification for enums has > not changed significantly since C89. > > Paul Eggert already explained the distinction between enumeration > constants and enumeration types, so I won't repeat it. All the commentary here is greatly appreciated; please excuse my delay in replying. Here is what IBM said relating to standards: The compiler behavior is correct. The C standard says that the enumerated type can be char, or any signed or unsigned integer type (6.7.2.2#4). The test case uses a default enumsize which is small and for the range of values in the test case it reserves an unsigned int to hold the enumeration values (this is documented in the User's Guide). The conversion rules make the signed into unsigned int (6.3.1.8#1) meaning that expression becomes (unsigned int)x/BILLION because BILLION is an unsigned int; The conversion of -9 produces 3294967297 (UINT_MAX +1 -9) which when divided by 10 produces 3 as the result. There was no further qualification of "C standard" in the first paragraph. However, the test program still returns 3 when XLC is in C99 mode: $ xlc -qlanglvl=stdc99 -o enumsize enumsize.c $ ./enumsize BILLION = 10 x = -9 x / BILLION = 3 Now, from what I'm hearing here, it sounds like IBM may be in the wrong--- and this would be quite convenient, because while they may not be too bothered when their interpretation of POSIX et al. is different from the rest of the world's, they _will_ care about a red-letter violation of a spec they claim to support. I can't standards-lawyer my way out of a paper bag, but if someone here could provide a line of argument that IBM's enum shenanigans are categorically unsupported by the standard, I'll gladly send it on in the hope that it'll get the beast to move. One minor addendum: In reviewing the support ticket, I noticed a suggestion to use #pragma enum(int) as a workaround, which would probably be more convenient than a compiler flag (this could go right into config.h). But even though it is supposed to be equivalent to the ENUMSIZE option, it does not affect the output of the test program. Seems like another PMR is in order... --Daniel -- Daniel Richard G. || sk...@iskunk.org My ASCII-art .sig got a bad case of Times New Roman.
Re: z/OS enum size pitfall
On Tue, Aug 22, 2017 at 10:43:07PM +0200, Tim Rühsen wrote: > On Dienstag, 22. August 2017 13:25:55 CEST Ben Pfaff wrote: > > On Tue, Aug 22, 2017 at 04:13:54PM -0400, Daniel Richard G. wrote: > > > 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. > > Do you know to which C standard the XLC compiler complies to ? > > C99, 6.7.2.2p4 says > > Each enumerated type shall be compatible with char, a signed integer > type, > or an unsigned integer type. The choice of type is > implementation-defined,108) > but shall be capable of representing the values of all the members of the > enumeration. I don't know what XLC conforms to. C11 has the same text in 6.7.2.2p4. The specification for enums has not changed significantly since C89. Paul Eggert already explained the distinction between enumeration constants and enumeration types, so I won't repeat it.
Re: z/OS enum size pitfall
On 08/22/2017 01:13 PM, Daniel Richard G. wrote:
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.
There seems to be some miscommunication here. The enum type might be
either signed or unsigned, and I expect this is what IBM is talking
about. However, the enum constants that are declared all must be of type
'int'. This requirement has been in the standard for ages. For example,
given:
enum { a, b, c } v = a;
The expression "a < -1" must return 0, because a is zero and is of type
int. However, the expression "v < -1" might return 0 (if v is signed)
and it might return 1 (if v is unsigned). This is the case even though v
is zero, just as a is. Since the code in question is using the enum
constants, not the enum type, it must treat the values as signed
integers in any conforming compiler.
Re: z/OS enum size pitfall
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
>
> enum { BILLION = 10 };
>
> static const unsigned int BILLION2 = 10;
>
> int main(void) {
> int x = -9;
> 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 = 10
> x = -9
> x / BILLION = 0
>
> However, on z/OS, it prints
>
> BILLION = 10
> x = -9
> 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.
z/OS enum size pitfall
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
enum { BILLION = 10 };
static const unsigned int BILLION2 = 10;
int main(void) {
int x = -9;
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 = 10
x = -9
x / BILLION = 0
However, on z/OS, it prints
BILLION = 10
x = -9
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.
test-timespec fails not because of the time function being tested, but
because of how TIMESPEC_RESOLUTION is defined in timespec.h.
IBM, in their response, suggested specifying the flag -qenumsize=4 .
With this flag, the output on z/OS matches that of other systems,
test-timespec passes, and the rest of the gnulib test suite is
unaffected. I think it may be worth considering having gnulib add this
flag by default on z/OS, to get the expected behavior.
--Daniel
--
Daniel Richard G. || sk...@iskunk.org
My ASCII-art .sig got a bad case of Times New Roman.
