# 2019q1W7上: 鄭珮彣
###### tags: `linux2019`
## 測驗 `1`

static inline size_t find_next_bit(const unsigned long *bitmap,
                                   size_t bits,
                                   size_t start)
{
    unsigned long t;
    size_t l = BITS_TO_LONGS(bits);
    size_t first_long = start / BITS_PER_LONG;
    size_t long_lower = start - (start % BITS_PER_LONG);

    if (start >= bits)
        return bits;

    t = bitmap[first_long] & BITMAP_FIRST_WORD_MASK(start);
    for (size_t i = first_long + 1; !t && i < l; i++) {
        long_lower += BITS_PER_LONG;
		t = bitmap[i];
    }
    
    if (!t)
        return bits;

    size_t pos;
    pos = long_lower + bitops_ffs(t) - 1;
	if (pos >= bits)
		return bits;
    return pos;
}

## 測驗 `2`

#include <time.h>
#include <stdlib.h>
int main(void) {
    unsigned long test[BITS_TO_LONGS(MAX_TEST_BITS)];
    int i, j;
    srand(time(NULL));
    for (i = 0; i < BITS_TO_LONGS(MAX_TEST_BITS); i++) {
        test[i] = rand() % BITS_TO_LONGS(MAX_TEST_BITS);
        for (j = i - 1; j >= 0; j--)
            if (test[i] == test[j])
                i--;
    }
    for (int i = 1; i < MAX_TEST_BITS; i++) {
        bitmap_fill(test, i);
        assert(find_next_bit(test, i, 0) == 0);

        bitmap_zero(test, i);
        assert(find_next_bit(test, i, 0) == i);
    }
    return 0;
}

## 測驗 `3`

:::info
參見 [Kernel’s Bug Reporting](https://lkw.readthedocs.io/en/latest/doc/06_kernel_bug_reporting.html) 和 [BUG 使用的討論](https://yarchive.net/comp/linux/BUG.html)
:::

#include <assert.h>
#include <stdbool.h>
#include <stddef.h>
#include <stdint.h>
#include <string.h>
#include <stdio.h>

#define BITS_PER_BYTE 8
#define BITOPS_BUILD_BUG_ON(e) ((void)sizeof(char[1 - 2 * !!(e)]))
#define BITS_PER_LONG (sizeof(unsigned long) * BITS_PER_BYTE)
/**
 * bitops_ffs() - find (least significant) first set bit plus one
 * @x: unsigned long to check
 * Return: plus-one index of first set bit; zero when x is zero
 */
static inline size_t bitops_ffs(unsigned long x)
{   size_t i = 1;

	BITOPS_BUILD_BUG_ON(BITS_PER_LONG != 32 && BITS_PER_LONG != 64);

	if (x == 0)
		return 0;

	if (BITS_PER_LONG == 64) {
		if ((0x00000000fffffffful & x) == 0) {
			i += 32;
			x >>= 32;
		}
	}

	if ((0x0000fffful & x) == 0) {
		i += 16;
		x >>= 16;
	}

	if ((0x00fful & x) == 0) {
		i += 8;
		x >>= 8;
	}

	if ((0x0ful & x) == 0) {
		i += 4;
		x >>= 4;
	}

	if ((0x3ul & x) == 0) {
		i += 2;
		x >>= 2;
	}

	if ((0x1ul & x) == 0) {
		i += 1;
		x >>= 1;
	}

	return i;

}

int main()
{
    unsigned long x ;
    x=16;//x=10000
    bitops_ffs(x);
    printf("%ld\n", bitops_ffs(x));

}

>輸出結果

5

## 測驗 `4`

#include <assert.h>
#include <stdbool.h>
#include <stddef.h>
#include <stdint.h>
#include <string.h>
#include <stdio.h>
#include <limits.h>

#define BITS_PER_BYTE 8
#define BITOPS_BUILD_BUG_ON(e) ((void)sizeof(char[1 - 2 * !!(e)]))
#define BITS_PER_LONG (sizeof(unsigned long) * BITS_PER_BYTE)
/**
 * bitops_ffz() - find (least significant) first zero bit plus one
 * @x: unsigned long to check
 *
 * Return: plus-one index of first zero bit; zero when x is ULONG_MAX
 */
static inline size_t bitops_ffz(unsigned long x)
{   x=~x;
    size_t i = 1;

	BITOPS_BUILD_BUG_ON(BITS_PER_LONG != 32 && BITS_PER_LONG != 64);

	if (~x == ULONG_MAX)
		return 0;

    if (x == 0)
		return 1;

	if (BITS_PER_LONG == 64) {
		if ((0x00000000fffffffful & x) == 0) {
			i += 32;
			x >>= 32;
		}
	}

	if ((0x0000fffful & x) == 0) {
		i += 16;
		x >>= 16;
	}

	if ((0x00fful & x) == 0) {
		i += 8;
		x >>= 8;
	}

	if ((0x0ful & x) == 0) {
		i += 4;
		x >>= 4;
	}

	if ((0x3ul & x) == 0) {
		i += 2;
		x >>= 2;
	}

	if ((0x1ul & x) == 0) {
		i += 1;
		x >>= 1;
	}

	return i;

}

int main()
{
    unsigned long x ;
    x=7;//x=0111
    bitops_ffz(x);
    printf("%ld\n", bitops_ffz(x));

}

>輸出結果

4