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Making Fatal Hidden Assumptions

We often find hidden, and totally unnecessary, assumptions being
made in code. The following leans heavily on one particular
example, which happens to be in C. However similar things can (and
do) occur in any language.

These assumptions are generally made because of familiarity with
the language. As a non-code example, consider the idea that the
faulty code is written by blackguards bent on foulling the
language. The term blackguards is not in favor these days, and for
good reason. However, the older you are, the more likely you are
to have used it since childhood, and to use it again, barring
specific thought on the subject. The same type of thing applies to
writing code.

I hope, with this little monograph, to encourage people to examine
some hidden assumptions they are making in their code. As ever, in
dealing with C, the reference standard is the ISO C standard.
Versions can be found in text and pdf format, by searching for N869
and N1124. [1] The latter does not have a text version, but is
more up-to-date.

We will always have innocent appearing code with these kinds of
assumptions built-in. However it would be wise to annotate such
code to make the assumptions explicit, which can avoid a great deal
of agony when the code is reused under other systems.

In the following example, the code is as downloaded from the
referenced URL, and the comments are entirely mine, including the
'every 5' linenumber references.

/* Making fatal hidden assumptions */
/* Paul Hsiehs version of strlen.
http://www.azillionmonkeys.com/qed/asmexample.html

Some sneaky hidden assumptions here:
1. p = s - 1 is valid. Not guaranteed. Careless coding.
2. cast (int) p is meaningful. Not guaranteed.
3. Use of 2's complement arithmetic.
4. ints have no trap representations or hidden bits.
5. 4 == sizeof(int) && 8 == CHAR_BIT.
6. size_t is actually int.
7. sizeof(int) is a power of 2.
8. int alignment depends on a zeroed bit field.

Since strlen is normally supplied by the system, the system
designer can guarantee all but item 1. Otherwise this is
not portable. Item 1 can probably be beaten by suitable
code reorganization to avoid the initial p = s - 1. This
is a serious bug which, for example, can cause segfaults
on many systems. It is most likely to foul when (int)s
has the value 0, and is meaningful.

He fails to make the valid assumption: 1 == sizeof(char).
*/

#define hasNulByte(x) ((x - 0x01010101) & ~x & 0x80808080)
#define SW (sizeof (int) / sizeof (char))

int xstrlen (const char *s) {
const char *p; /* 5 */
int d;

p = s - 1;
do {
p++; /* 10 */
if ((((int) p) & (SW - 1)) == 0) {
do {
d = *((int *) p);
p += SW;
} while (!hasNulByte (d)); /* 15 */
p -= SW;
}
} while (*p != 0);
return p - s;
} /* 20 */

Let us start with line 1! The constants appear to require that
sizeof(int) be 4, and that CHAR_BIT be precisely 8. I haven't
really looked too closely, and it is possible that the ~x term
allows for larger sizeof(int), but nothing allows for larger
CHAR_BIT. A further hidden assumption is that there are no trap
values in the representation of an int. Its functioning is
doubtful when sizeof(int) is less that 4. At the least it will
force promotion to long, which will seriously affect the speed.

This is an ingenious and speedy way of detecting a zero byte within
an int, provided the preconditions are met. There is nothing wrong
with it, PROVIDED we know when it is valid.

In line 2 we have the confusing use of sizeof(char), which is 1 by
definition. This just serves to obscure the fact that SW is
actually sizeof(int) later. No hidden assumptions have been made
here, but the usage helps to conceal later assumptions.

Line 4. Since this is intended to replace the systems strlen()
function, it would seem advantageous to use the appropriate
signature for the function. In particular strlen returns a size_t,
not an int. size_t is always unsigned.

In line 8 we come to a biggie. The standard specifically does not
guarantee the action of a pointer below an object. The only real
purpose of this statement is to compensate for the initial
increment in line 10. This can be avoided by rearrangement of the
code, which will then let the routine function where the
assumptions are valid. This is the only real error in the code
that I see.

In line 11 we have several hidden assumptions. The first is that
the cast of a pointer to an int is valid. This is never
guaranteed. A pointer can be much larger than an int, and may have
all sorts of non-integer like information embedded, such as segment
id. If sizeof(int) is less than 4 the validity of this is even
less likely.

Then we come to the purpose of the statement, which is to discover
if the pointer is suitably aligned for an int. It does this by
bit-anding with SW-1, which is the concealed sizeof(int)-1. This
won't be very useful if sizeof(int) is, say, 3 or any other
non-poweroftwo. In addition, it assumes that an aligned pointer
will have those bits zero. While this last is very likely in
todays systems, it is still an assumption. The system designer is
entitled to assume this, but user code is not.

Line 13 again uses the unwarranted cast of a pointer to an int.
This enables the use of the already suspicious macro hasNulByte in
line 15.

If all these assumptions are correct, line 19 finally calculates a
pointer difference (which is valid, and of type size_t or ssize_t,
but will always fit into a size_t). It then does a concealed cast
of this into an int, which could cause undefined or implementation
defined behaviour if the value exceeds what will fit into an int.
This one is also unnecessary, since it is trivial to define the
return type as size_t and guarantee success.

I haven't even mentioned the assumption of 2's complement
arithmetic, which I believe to be embedded in the hasNulByte
macro. I haven't bothered to think this out.

Would you believe that so many hidden assumptions can be embedded
in such innocent looking code? The sneaky thing is that the code
appears trivially correct at first glance. This is the stuff that
Heisenbugs are made of. Yet use of such code is fairly safe if we
are aware of those hidden assumptions.

I have cross-posted this without setting follow-ups, because I
believe that discussion will be valid in all the newsgroups posted.

[1] The draft C standards can be found at:
<http://www.open-std.org/jtc1/sc22/wg14/www/docs/>

--
"If you want to post a followup via groups.google.c om, don't use
the broken "Reply" link at the bottom of the article. Click on
"show options" at the top of the article, then click on the
"Reply" at the bottom of the article headers." - Keith Thompson
More details at: <http://cfaj.freeshell. org/google/>
Also see <http://www.safalra.com/special/googlegroupsrep ly/>

Mar 6 '06
351 13213
msg

On the other hand, when Seymour Cray started his own company, those
machines where 2's complement.
The Cray blood-line starting at least with "Little Character" (prototype
for the 160) was 1's complement, implemented with subtraction as the
basis of arithmetic (the so-called 'adder pyramid'). Even the CDC
3000 series which were mostly others' designs retained 1's complement
arithmetic. The 6000 and 7000 series PPUs were essentially 160s also.
I should think it safe to say one could find 1's complement in Cray
designs from at least 1957 through the early 1980s.
And he shifted from 60 to 64 bit
words, but still retained octal notation (he did not like hexadecimal
at all).


Nor did he have truck with integrated circuits until absolutely necessary.

Michael Grigoni
Cybertheque Museum

Mar 9 '06 #101
On Thu, 09 Mar 2006 00:15:17 +0000, Christian Bau wrote:
I just tried the following program (CodeWarrior 10 on MacOS X):


Same for gcc4 on MacOS X. However this slight permutation of your
program (only the comparison line has changed):

#include <stdio.h>

#define SIZE (50*1000000L)
typedef struct {
char a [SIZE];
} bigstruct;

static bigstruct bigarray [8];

int main(void)
{
printf("%lx\n", (unsigned long) &bigarray [0]);
printf("%lx\n", (unsigned long) &bigarray [9]);
printf("%lx\n", (unsigned long) &bigarray [-1]);

if (&bigarray [-1] - & bigarray [0] < 0)
printf ("Everything is fine\n");
else
printf ("The C Standard is right: &bigarray [-1] is broken\n");

return 0;
}

produces:
3080
1ad2a500
fd054000
Everything is fine

So what we see is that (a) pointer comparisons use direct unsigned integer
comparison, instead of checking the sign of the pointer difference---since
pointer comparisons only make sense in the context of an indivdual object,
I'd argue that the compiler is doing the wrong thing here, and the
comparison should instead have been done in the context of a pointer
difference; and (b) your printf string about "&bigarray[-1] is broken" is
wrong, since that's not what the code showed at all. What it showed is
that &bigarray[-1] could be formed, that &bigarray[0] was one element to
the right of it, and that hell did not freeze over (nor was any trap
taken), since you did not attempt to access any memory there.

Cheers,

--
Andrew

Mar 9 '06 #102
On Wed, 08 Mar 2006 18:07:45 -0700, Al Balmer wrote:
On Thu, 09 Mar 2006 09:13:24 +1100, Andrew Reilly
<an************ *@areilly.bpc-users.org> wrote:
C is not a higher-level language. It's a universal assembler. Pick
another one.
Nice parrot. I think the original author of that phrase meant it as a
joke.


Most jokes contain at least a kernel of truth.
I spent 25 years writing assembler. C is a higher-level language.


Yeah, me to. Still do, regularly, on processors that will never have a C
compiler. C is as close to a universal assembler as we've got at the
moment. It doesn't stick it's neck out too far, although a more
deliberately designed universal assembler would be a really good thing.
(It's on my list of things to do...)

If you actually *want* a higher level language, there are better ones
to chose from than C.

--
Andrew

Mar 9 '06 #103
Andrew Reilly <an************ *@areilly.bpc-users.org> writes:
On Thu, 09 Mar 2006 00:00:34 +0000, Christian Bau wrote:
Question: If the C Standard guarantees that for any array a, &a [-1]
should be valid, should it also guarantee that &a [-1] != NULL


Probably, since NULL has been given the guarantee that it's unique in some
sense. In an embedded environment, or assembly language, the construct
could of course produce NULL (for whatever value you pick for NULL), and
NULL would not be special. I don't know that insisting on the existence of
a unique and special NULL pointer value is one of the standard's crowning
achievements, either. It's convenient for lots of things, but it's just
not the way simple hardware works, particularly at the limits.


How exactly do you get from NULL (more precisely, a null pointer
value) being "unique in some sense" to a guarantee that &a[-1], which
doesn't point to any object, is unequal to NULL?

The standard guarantees that a null pointer "is guaranteed to compare
unequal to a pointer to any object or function". &a[-1] is not a
pointer to any object or function, so the standard doesn't guarantee
that &a[-1] != NULL.

Plausibly, if a null pointer is represented as all-bits-zero, and
pointer arithmetic works like integer arithmetic, an object of size N
could easily happen to be allocated at address N; then pointer
arithmetic could yield a null pointer value. (In standard C, this is
one of the infinitely many possible results of undefined behavior.)

What restrictions would you be willing to impose, and/or what code
would you be willing to break, in order to make such a guarantee?

--
Keith Thompson (The_Other_Keit h) ks***@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <*> <http://users.sdsc.edu/~kst>
We must do something. This is something. Therefore, we must do this.
Mar 9 '06 #104
In article <pa************ *************** *@areilly.bpc-users.org>
Andrew Reilly <an************ *@areilly.bpc-users.org> wrote:
So what we see is that (a) pointer comparisons use direct unsigned integer
comparison, instead of checking the sign of the pointer difference ...
While the data are consistent with this conclusion, there are other
ways to arrive at the same output. But this is certainly allowed.

It is perhaps worth pointing out that in Ancient C (as in "whatever
Dennis' compiler did"), before the "unsigned" keyword even existed,
the way you got unsigned arithmetic and comparisons was to use
"char *". That is:

int a, b;
char *c, *d;
...
if (a < b) /* signed compare */
...
c = a; /* no cast needed because this was Ancient C */
d = b; /* (we could even do things like 077440->rkcsr!) */
if (c < d) /* unsigned compare */
...
I'd argue that the compiler is doing the wrong thing here ...


It sounded to me as though you liked what Dennis' original compilers
did, and wished that era still existed. In this respect, it does:
and now you argue that this is somehow "wrong".
--
In-Real-Life: Chris Torek, Wind River Systems
Salt Lake City, UT, USA (4039.22'N, 11150.29'W) +1 801 277 2603
email: forget about it http://web.torek.net/torek/index.html
Reading email is like searching for food in the garbage, thanks to spammers.
Mar 9 '06 #105
Chris Torek wrote:
Keith Thompson said:
Um, I always thought that "within" and "outside" were two different
things.

In article <du**********@n wrdmz03.dmz.ncs .ea.ibs-infra.bt.com>
Richard Heathfield <in*****@invali d.invalid> wrote:
Ask Jack to lend you his bottle. You'll soon change your mind.

To clarify a bit ...

A mathematician named Klein
Thought the Moebius band was divine
Said he, "If you glue
The edges of two
You'll get a weird bottle like mine!"

:-)

(A Moebius band has only one side. It is a two-dimensional object
that exists only in a 3-dimensional [or higher] space. A Klein
bottle can only be made in a 4-dimensional [or higher] space, and
is a 3-D object with only one side. The concept can be carried on
indefinitely, but a Klein bottle is hard enough to contemplate
already.)


See

http://www.kleinbottle.com/

--
Eric Sosman
es*****@acm-dot-org.invalid
Mar 9 '06 #106
Let me get this correct.

If I went something like

#include <stdio.h>
int main(void) {

int *p;
int arr[2];
p = arr + 4;

return 0;
}

This would be undefine behavior because I'm writing two past the array
instead of one. Right?

Chad

Mar 9 '06 #107

On Wed, 8 Mar 2006, Chad wrote:

Let me get this correct.
If I went something like

#include <stdio.h>
int main(void) {
int *p;
int arr[2];
p = arr + 4;
return 0;
}

This would be undefined behavior because I'm writing two past the array
instead of one. Right?


Wrong. It would be undefined behavior because you're constructing
a pointer that points /three/ elements past the end of the array.
("Writing" has nothing to do with it.) But yes, it's undefined
behavior in C (and C++).

-Arthur
Mar 9 '06 #108
"Chad" <cd*****@gmail. com> writes:
Let me get this correct.

If I went something like

#include <stdio.h>
int main(void) {

int *p;
int arr[2];
p = arr + 4;

return 0;
}

This would be undefine behavior because I'm writing two past the array
instead of one. Right?


You're not writing past the array, but yes, it's undefined behavior.

Given the above declarations, and adding "int i;":

p = arr + 1; /* ok */
i = *p; /* ok, accesses 2nd element of 2-element array */

p = arr + 2; /* ok, points just past end of array */
i = *p; /* undefined behavior */

p = arr + 3; /* undefined behavior, points too far past end of array */

--
Keith Thompson (The_Other_Keit h) ks***@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <*> <http://users.sdsc.edu/~kst>
We must do something. This is something. Therefore, we must do this.
Mar 9 '06 #109
Eric Sosman said:
http://www.kleinbottle.com/


"These elegant bottles make great gifts, fantastic classroom displays, and
inferior mouse-traps."

Now /that/ is good advertising.

--
Richard Heathfield
"Usenet is a strange place" - dmr 29/7/1999
http://www.cpax.org.uk
email: rjh at above domain (but drop the www, obviously)
Mar 9 '06 #110

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