Hi!
I was trying to understand what roundup() is (defined in <sys/param,h>).
It seems to be kind of:
SYNOPSIS
#include <sys/param.h>
roundup(x, step);
DESCRIPTION
This macro rounds x to the nearest multiple of step that is not less
than x.
I found that it doesn't work for negative numbers; but that's expected, and it
could be documented as such. However, it doesn't work nicely with unsigned
integers either: for values close to zero, where wrap around happens, the result
is also bogus. See my experiments below.
$ sed -n 92,98p /usr/include/x86_64-linux-gnu/sys/param.h
#ifdef __GNUC__
# define roundup(x, y) (__builtin_constant_p (y) && powerof2 (y) \
? (((x) + (y) - 1) & ~((y) - 1)) \
: ((((x) + ((y) - 1)) / (y)) * (y)))
#else
# define roundup(x, y) ((((x) + ((y) - 1)) / (y)) * (y))
#endif
$ cat roundup.c
#include <stdint.h>
#include <stdio.h>
#include <sys/param.h>
int
main(void)
{
/* signed */
{
int32_t n, m;
m = 3;
n = 10;
puts("signed:");
for (int32_t x = -n; x < 0; x++)
printf("roundup(%d, %d) == %d\n", x, m, roundup(x, m));
puts("");
for (int32_t x = 0; x < n; x++)
printf("roundup(%d, %d) == %d\n", x, m, roundup(x, m));
puts("");
for (int32_t x = INT32_MIN; x < INT_MIN + n; x++)
printf("roundup(%d, %d) == %d\n", x, m, roundup(x, m));
puts("");
for (int32_t x = INT32_MAX; x > INT32_MAX - n; x--)
printf("roundup(%d, %d) == %d\n", x, m, roundup(x, m));
}
/* unsigned */
{
uint32_t n, m;
m = 3;
n = 10;
puts("\nunsigned:");
for (uint32_t x = 1; x < n; x++)
printf("roundup(%u, %u) == %u\n", -x, m, roundup(-x, m));
puts("");
for (uint32_t x = 0; x < n; x++)
printf("roundup(%u, %u) == %u\n", x, m, roundup(x, m));
}
}
$ cc -Wall -Wextra -Werror roundup.c
$ ./a.out
signed:
roundup(-10, 3) == -6
roundup(-9, 3) == -6
roundup(-8, 3) == -6
roundup(-7, 3) == -3
roundup(-6, 3) == -3
roundup(-5, 3) == -3
roundup(-4, 3) == 0
roundup(-3, 3) == 0
roundup(-2, 3) == 0
roundup(-1, 3) == 0
/* These values are nonsense, but OK, let's ignore the negative */
roundup(0, 3) == 0
roundup(1, 3) == 3
roundup(2, 3) == 3
roundup(3, 3) == 3
roundup(4, 3) == 6
roundup(5, 3) == 6
roundup(6, 3) == 6
roundup(7, 3) == 9
roundup(8, 3) == 9
roundup(9, 3) == 9
/* These make sense */
roundup(-2147483648, 3) == -2147483646
roundup(-2147483647, 3) == -2147483643
roundup(-2147483646, 3) == -2147483643
roundup(-2147483645, 3) == -2147483643
roundup(-2147483644, 3) == -2147483640
roundup(-2147483643, 3) == -2147483640
roundup(-2147483642, 3) == -2147483640
roundup(-2147483641, 3) == -2147483637
roundup(-2147483640, 3) == -2147483637
roundup(-2147483639, 3) == -2147483637
/* Nonsense; ignore the negative */
roundup(2147483647, 3) == -2147483646 // UB; ignore
roundup(2147483646, 3) == -2147483646 // UB; ignore
roundup(2147483645, 3) == 2147483646
roundup(2147483644, 3) == 2147483646
roundup(2147483643, 3) == 2147483643
roundup(2147483642, 3) == 2147483643
roundup(2147483641, 3) == 2147483643
roundup(2147483640, 3) == 2147483640
roundup(2147483639, 3) == 2147483640
roundup(2147483638, 3) == 2147483640
/* These make sense */
unsigned:
roundup(4294967295, 3) == 0 // Wrong; should be: 4294967295
roundup(4294967294, 3) == 0 // Wrong; should be: 4294967295
roundup(4294967293, 3) == 4294967295
roundup(4294967292, 3) == 4294967292
roundup(4294967291, 3) == 4294967292
roundup(4294967290, 3) == 4294967292
roundup(4294967289, 3) == 4294967289
roundup(4294967288, 3) == 4294967289
roundup(4294967287, 3) == 4294967289
roundup(0, 3) == 0
roundup(1, 3) == 3
roundup(2, 3) == 3
roundup(3, 3) == 3
roundup(4, 3) == 6
roundup(5, 3) == 6
roundup(6, 3) == 6
roundup(7, 3) == 9
roundup(8, 3) == 9
roundup(9, 3) == 9
Do you think this is something to be fixed without important performance
penalties, or should we just document the bug and live with it?
Cheers, Alex -- <http://www.alejandro-colomar.es/>
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