ptrdiff_t maximum

J

John Kelly

# include <stddef.h>
# include <stdio.h>

int
main (void)
{
ptrdiff_t const dx =
((ptrdiff_t) ~ ((ptrdiff_t) - 1 < 1 ? (-1 - (((ptrdiff_t) 1
<< (sizeof (ptrdiff_t) * 8 - 2)) - 1 + ((ptrdiff_t) 1
<< (sizeof (ptrdiff_t) * 8 - 2)))) : (ptrdiff_t) 0));

printf ("ptrdiff_t maximum %d\n", dx);

return 0;
}
 
I

Ian Collins

On 08/23/10 07:43 AM, John Kelly wrote:

If an implementation has ptrdiff_t, it'll have PTRDIFF_MIN and
PTRDIFF_MAX defined.
 
J

John Kelly

If an implementation has ptrdiff_t, it'll have PTRDIFF_MIN and
PTRDIFF_MAX defined.
Click to expand...

Interix has ptrdiff_t but no PTRDIFF_MAX.

I grepped /usr/include and /opt/gcc.3.3. It's not there.

On Interix 3.5 we have the bare essentials:

/usr/include/limits.h:60:#define INT_MAX 2147483647
/usr/include/limits.h:61:#define UINT_MAX 0xffffffff

Blame it on Microsoft's lip service to UNIX interoperability.

(just kidding Geoff!)

It's not a bad tool once you learn your way around.
 
B

Ben Pfaff

Ian Collins said:
If an implementation has ptrdiff_t, it'll have PTRDIFF_MIN and
PTRDIFF_MAX defined.
Click to expand...

All implementations, both C89 and C99, have ptrdiff_t, so the
"if" part doesn't make sense. But only C99 has PTRDIFF_MIN and
PTRDIFF_MAX.
 
I

Ian Collins

All implementations, both C89 and C99, have ptrdiff_t, so the
"if" part doesn't make sense. But only C99 has PTRDIFF_MIN and
PTRDIFF_MAX.
Click to expand...

Ah, sorry I've become too used to using the one true C!
 
B

Ben Bacarisse

John Kelly said:
# include <stddef.h>
# include <stdio.h>

int
main (void)
{
ptrdiff_t const dx =
((ptrdiff_t) ~ ((ptrdiff_t) - 1 < 1 ? (-1 - (((ptrdiff_t) 1
<< (sizeof (ptrdiff_t) * 8 - 2)) - 1 + ((ptrdiff_t) 1
<< (sizeof (ptrdiff_t) * 8 - 2)))) : (ptrdiff_t) 0));

printf ("ptrdiff_t maximum %d\n", dx);

return 0;
}
Click to expand...

This prints -1 on my machine.

ptrdiff_t is a signed type so there is no need for the conditional
expression.

Why subtract from -1 and then bit-wise negate? Surely the expression
you subtract is just as good? I.e.

((ptrdiff_t)1 << sizeof dx * CHAR_BIT - 2) - 1 +
((ptrdiff_t)1 << sizeof dx * CHAR_BIT - 2)

is just as good and avoids a problem on machines that have symmetric
positive and negative ranges.

This must be for C90 since C99 has PTRDIFF_MAX. In C90 the safest way
to print types for which there is no conversion specifier is with a
cast to the longest type of the correct "signedness":

printf("ptrdiff_t maximum %ld\n", (long)dx);

CHAR_BIT would be better than 8 and even so you can't be sure that all
sizeof dx * CHAR_BIT bit are value bits.
 
P

Peter Nilsson

John Kelly said:
# include <stddef.h>
# include <stdio.h>

int
main (void)
{
    ptrdiff_t const dx =
        ((ptrdiff_t) ~ ((ptrdiff_t) - 1 < 1 ? (-1 - (((ptrdiff_t) 1
                        << (sizeof (ptrdiff_t) * 8 - 2)) - 1 + ((ptrdiff_t) 1
                        << (sizeof (ptrdiff_t) * 8 - 2)))) : (ptrdiff_t) 0));

    printf ("ptrdiff_t maximum %d\n", dx);

    return 0;}
Click to expand...

Under C90...

#include <stdio.h>
#include <stddef.h>

int main(void)
{
ptrdiff_t max, new;
for (max = 32767; (new = 2 * max + 1) > max; )
max = new;
printf("%ld\n", (long) max);
return 0;
}
 
J

John Kelly

Under C90...

#include <stdio.h>
#include <stddef.h>

int main(void)
{
ptrdiff_t max, new;
for (max = 32767; (new = 2 * max + 1) > max; )
max = new;
printf("%ld\n", (long) max);
return 0;
}
Click to expand...

I like that.

The signed integer multiplication overflow will get Seebs dancing. ;-)
 
B

Ben Pfaff

Peter Nilsson said:
Under C90...

#include <stdio.h>
#include <stddef.h>

int main(void)
{
ptrdiff_t max, new;
for (max = 32767; (new = 2 * max + 1) > max; )
max = new;
printf("%ld\n", (long) max);
return 0;
}
Click to expand...

Doesn't this make assumptions about the behavior of signed
integer overflow, which yields undefined behavior?
 
J

John Kelly

Doesn't this make assumptions about the behavior of signed
integer overflow, which yields undefined behavior?
Click to expand...

Yes but ISTM you can (double) cast the intermediate result

for (max = 32767; (new = 2 * (double)max + 1) > max; )

to fix it up.
 
I

Ian Collins

# include<stddef.h>
# include<stdio.h>

int
main (void)
{
ptrdiff_t const dx =
((ptrdiff_t) ~ ((ptrdiff_t) - 1< 1 ? (-1 - (((ptrdiff_t) 1
<< (sizeof (ptrdiff_t) * 8 - 2)) - 1 + ((ptrdiff_t) 1
<< (sizeof (ptrdiff_t) * 8 - 2)))) : (ptrdiff_t) 0));

printf ("ptrdiff_t maximum %d\n", dx);

return 0;
}
Click to expand...

Or:

#include <stdio.h>
#include <stddef.h>
#include <limits.h>

int main()
{
printf("ptrdiff_t maximum %ld\n",
(long)(~(1UL<<(sizeof(ptrdiff_t)*CHAR_BIT-1))));
}
 
J

John Kelly

Doesn't this make assumptions about the behavior of signed
integer overflow, which yields undefined behavior?
Click to expand...

Yes, but here's a fix.

If somebody with a 64-bit machine would run this, and post the output I
would appreciate it.



# include <stdio.h>
# include <stddef.h>

int main(void) {

ptrdiff_t last, next;

for (last = 32767; (next = 2 * (double)last + 1) > last; ) {
printf ("last = %ld double = %f next = %ld\n", (long)last, 2 *
(double)last, (long)next);
last = next;

}
printf("%ld\n", (long) last);
return 0;
}
 
G

Geoff

Yes, but here's a fix.

If somebody with a 64-bit machine would run this, and post the output I
would appreciate it.



# include <stdio.h>
# include <stddef.h>

int main(void) {

ptrdiff_t last, next;

for (last = 32767; (next = 2 * (double)last + 1) > last; ) {
printf ("last = %ld double = %f next = %ld\n", (long)last, 2 *
(double)last, (long)next);
last = next;

}
printf("%ld\n", (long) last);
return 0;
}
Click to expand...

last = 32767 double = 65534.000000 next = 65535
last = 65535 double = 131070.000000 next = 131071
last = 131071 double = 262142.000000 next = 262143
last = 262143 double = 524286.000000 next = 524287
last = 524287 double = 1048574.000000 next = 1048575
last = 1048575 double = 2097150.000000 next = 2097151
last = 2097151 double = 4194302.000000 next = 4194303
last = 4194303 double = 8388606.000000 next = 8388607
last = 8388607 double = 16777214.000000 next = 16777215
last = 16777215 double = 33554430.000000 next = 33554431
last = 33554431 double = 67108862.000000 next = 67108863
last = 67108863 double = 134217726.000000 next = 134217727
last = 134217727 double = 268435454.000000 next = 268435455
last = 268435455 double = 536870910.000000 next = 536870911
last = 536870911 double = 1073741822.000000 next = 1073741823
last = 1073741823 double = 2147483646.000000 next = 2147483647
last = 2147483647 double = 4294967294.000000 next = -1
last = -1 double = 8589934590.000000 next = -1
last = -1 double = 17179869182.000000 next = -1
last = -1 double = 34359738366.000000 next = -1
last = -1 double = 68719476734.000000 next = -1
last = -1 double = 137438953470.000000 next = -1
last = -1 double = 274877906942.000000 next = -1
last = -1 double = 549755813886.000000 next = -1
last = -1 double = 1099511627774.000000 next = -1
last = -1 double = 2199023255550.000000 next = -1
last = -1 double = 4398046511102.000000 next = -1
last = -1 double = 8796093022206.000000 next = -1
last = -1 double = 17592186044414.000000 next = -1
last = -1 double = 35184372088830.000000 next = -1
last = -1 double = 70368744177662.000000 next = -1
last = -1 double = 140737488355326.000000 next = -1
last = -1 double = 281474976710654.000000 next = -1
last = -1 double = 562949953421310.000000 next = -1
last = -1 double = 1125899906842622.000000 next = -1
last = -1 double = 2251799813685246.000000 next = -1
last = -1 double = 4503599627370494.000000 next = -1
last = -1 double = 9007199254740990.000000 next = -1
last = -1 double = 18014398509481982.000000 next = 0
last = 0 double = 36028797018963968.000000 next = 0
last = 0 double = 72057594037927936.000000 next = 0
last = 0 double = 144115188075855870.000000 next = 0
last = 0 double = 288230376151711740.000000 next = 0
last = 0 double = 576460752303423490.000000 next = 0
last = 0 double = 1152921504606847000.000000 next = 0
last = 0 double = 2305843009213694000.000000 next = 0
last = 0 double = 4611686018427387900.000000 next = 0
0
 
J

John Kelly

last = 0 double = 36028797018963968.000000 next = 0
last = 0 double = 72057594037927936.000000 next = 0
last = 0 double = 144115188075855870.000000 next = 0
last = 0 double = 288230376151711740.000000 next = 0
last = 0 double = 576460752303423490.000000 next = 0
last = 0 double = 1152921504606847000.000000 next = 0
last = 0 double = 2305843009213694000.000000 next = 0
last = 0 double = 4611686018427387900.000000 next = 0
Click to expand...

looks like the printf needs long long, can you fix that and try again?
 
G

Geoff

I pasted and posted too quick. I think you wanted something like this:

# include <stdio.h>
# include <stddef.h>

int main(void) {

ptrdiff_t last, next;

for (last = 32767; (next = 2 * (double)last + 1) > last; ) {
printf ("last = %lld double = %f next = %lld\n",
(long long)last, 2 * (double)last, (long long)next);
last = next;

}
printf("%lld\n", (long long) last);
return 0;
}

With output like this:

last = 32767 double = 65534.000000 next = 65535
last = 65535 double = 131070.000000 next = 131071
last = 131071 double = 262142.000000 next = 262143
last = 262143 double = 524286.000000 next = 524287
last = 524287 double = 1048574.000000 next = 1048575
last = 1048575 double = 2097150.000000 next = 2097151
last = 2097151 double = 4194302.000000 next = 4194303
last = 4194303 double = 8388606.000000 next = 8388607
last = 8388607 double = 16777214.000000 next = 16777215
last = 16777215 double = 33554430.000000 next = 33554431
last = 33554431 double = 67108862.000000 next = 67108863
last = 67108863 double = 134217726.000000 next = 134217727
last = 134217727 double = 268435454.000000 next = 268435455
last = 268435455 double = 536870910.000000 next = 536870911
last = 536870911 double = 1073741822.000000 next = 1073741823
last = 1073741823 double = 2147483646.000000 next = 2147483647
last = 2147483647 double = 4294967294.000000 next = 4294967295
last = 4294967295 double = 8589934590.000000 next = 8589934591
last = 8589934591 double = 17179869182.000000 next = 17179869183
last = 17179869183 double = 34359738366.000000 next = 34359738367
last = 34359738367 double = 68719476734.000000 next = 68719476735
last = 68719476735 double = 137438953470.000000 next = 137438953471
last = 137438953471 double = 274877906942.000000 next = 274877906943
last = 274877906943 double = 549755813886.000000 next = 549755813887
last = 549755813887 double = 1099511627774.000000 next = 1099511627775
last = 1099511627775 double = 2199023255550.000000 next =
2199023255551
last = 2199023255551 double = 4398046511102.000000 next =
4398046511103
last = 4398046511103 double = 8796093022206.000000 next =
8796093022207
last = 8796093022207 double = 17592186044414.000000 next =
17592186044415
last = 17592186044415 double = 35184372088830.000000 next =
35184372088831
last = 35184372088831 double = 70368744177662.000000 next =
70368744177663
last = 70368744177663 double = 140737488355326.000000 next =
140737488355327
last = 140737488355327 double = 281474976710654.000000 next =
281474976710655
last = 281474976710655 double = 562949953421310.000000 next =
562949953421311
last = 562949953421311 double = 1125899906842622.000000 next =
1125899906842623
last = 1125899906842623 double = 2251799813685246.000000 next =
2251799813685247
last = 2251799813685247 double = 4503599627370494.000000 next =
4503599627370495
last = 4503599627370495 double = 9007199254740990.000000 next =
9007199254740991
last = 9007199254740991 double = 18014398509481982.000000 next =
18014398509481984
last = 18014398509481984 double = 36028797018963968.000000 next =
36028797018963968
last = 36028797018963968 double = 72057594037927936.000000 next =
72057594037927936
last = 72057594037927936 double = 144115188075855870.000000 next =
144115188075855872
last = 144115188075855872 double = 288230376151711740.000000 next =
288230376151711744
last = 288230376151711744 double = 576460752303423490.000000 next =
576460752303423488
last = 576460752303423488 double = 1152921504606847000.000000 next =
1152921504606846976
last = 1152921504606846976 double = 2305843009213694000.000000 next =
2305843009213693952
last = 2305843009213693952 double = 4611686018427387900.000000 next =
4611686018427387904
4611686018427387904
 
F

Francesco S. Carta

Yes, but here's a fix.

If somebody with a 64-bit machine would run this, and post the output I
would appreciate it.



# include<stdio.h>
# include<stddef.h>

int main(void) {

ptrdiff_t last, next;

for (last = 32767; (next = 2 * (double)last + 1)> last; ) {
printf ("last = %ld double = %f next = %ld\n", (long)last, 2 *
(double)last, (long)next);
last = next;

}
printf("%ld\n", (long) last);
return 0;
}
Click to expand...

Caveat: I have not even the slightest idea whether these results are
relevant or not, I only know that I have a 64-bit machine.

MinGW 4.4.0, Windows7, AMD Athlon(tm) II Dual-Core M300 2.00 Ghz

last = 32767 double = 65534.000000 next = 65535
last = 65535 double = 131070.000000 next = 131071
last = 131071 double = 262142.000000 next = 262143
last = 262143 double = 524286.000000 next = 524287
last = 524287 double = 1048574.000000 next = 1048575
last = 1048575 double = 2097150.000000 next = 2097151
last = 2097151 double = 4194302.000000 next = 4194303
last = 4194303 double = 8388606.000000 next = 8388607
last = 8388607 double = 16777214.000000 next = 16777215
last = 16777215 double = 33554430.000000 next = 33554431
last = 33554431 double = 67108862.000000 next = 67108863
last = 67108863 double = 134217726.000000 next = 134217727
last = 134217727 double = 268435454.000000 next = 268435455
last = 268435455 double = 536870910.000000 next = 536870911
last = 536870911 double = 1073741822.000000 next = 1073741823
last = 1073741823 double = 2147483646.000000 next = 2147483647
2147483647
 
G

Geoff

This prints -1 on my machine.

ptrdiff_t is a signed type so there is no need for the conditional
expression.

Why subtract from -1 and then bit-wise negate? Surely the expression
you subtract is just as good? I.e.

((ptrdiff_t)1 << sizeof dx * CHAR_BIT - 2) - 1 +
((ptrdiff_t)1 << sizeof dx * CHAR_BIT - 2)

is just as good and avoids a problem on machines that have symmetric
positive and negative ranges.

This must be for C90 since C99 has PTRDIFF_MAX. In C90 the safest way
to print types for which there is no conversion specifier is with a
cast to the longest type of the correct "signedness":

printf("ptrdiff_t maximum %ld\n", (long)dx);

CHAR_BIT would be better than 8 and even so you can't be sure that all
sizeof dx * CHAR_BIT bit are value bits.
Click to expand...

On a Win7 box running an i7 and VS 2010 Pro:

# include <stddef.h>
# include <stdio.h>
# include <limits.h>

int
main (void)
{
ptrdiff_t const dx =
((ptrdiff_t)1 << (sizeof dx * CHAR_BIT - 2)) - 1 +
((ptrdiff_t)1 << (sizeof dx * CHAR_BIT - 2));

printf ("ptrdiff_t maximum %llx = %lld\n", dx, dx);
printf ("ptrdiff_t minimum %llx = %lld\n", -dx - 1, -dx - 1);

return 0;
}

----------------------------

Output:

ptrdiff_t maximum 7fffffffffffffff = 9223372036854775807
ptrdiff_t minimum 8000000000000000 = -9223372036854775808
 
F

Francesco S. Carta

On a Win7 box running an i7 and VS 2010 Pro:

# include<stddef.h>
# include<stdio.h>
# include<limits.h>

int
main (void)
{
ptrdiff_t const dx =
((ptrdiff_t)1<< (sizeof dx * CHAR_BIT - 2)) - 1 +
((ptrdiff_t)1<< (sizeof dx * CHAR_BIT - 2));

printf ("ptrdiff_t maximum %llx = %lld\n", dx, dx);
printf ("ptrdiff_t minimum %llx = %lld\n", -dx - 1, -dx - 1);

return 0;
}

----------------------------

Output:

ptrdiff_t maximum 7fffffffffffffff = 9223372036854775807
ptrdiff_t minimum 8000000000000000 = -9223372036854775808
Click to expand...


The last code above on:
MinGW 4.4.0, Windows7, AMD Athlon(tm) II Dual-Core M300 2.00 Ghz

Gives:
ptrdiff_t maximum 7fffffff7fffffff = 8541177558308426082
ptrdiff_t minimum 8000000080000000 = 8541177558308426082
 
F

Francesco S. Carta

The last code above on:
MinGW 4.4.0, Windows7, AMD Athlon(tm) II Dual-Core M300 2.00 Ghz

Gives:
ptrdiff_t maximum 7fffffff7fffffff = 8541177558308426082
ptrdiff_t minimum 8000000080000000 = 8541177558308426082
Click to expand...

Are these (and the others from me, elsethread) results meaningful?

If not I'll just avoid posting them - I did report them because they
seem weird at best.
 
J

John Kelly

Are these (and the others from me, elsethread) results meaningful?

If not I'll just avoid posting them - I did report them because they
seem weird at best.
Click to expand...

I don't know much about MinGW, but it's good to have a variety of data.
 

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