bug in roundup(3) from <sys/param.h>

[Alejandro Colomar <alx.manpages@gmail.com>]

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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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