本文主要是介绍RT-Thread内核-__rt_ffs函数，希望对大家解决编程问题提供一定的参考价值，需要的开发者们随着小编来一起学习吧！
1、原型
int __rt_ffs(int value)
2、作用
用于找到参数 value 用二进制表示时，第一个被置 1 的位的索引
3、示例程序
#include <stdio.h>const unsigned char __lowest_bit_bitmap[] =
{/* 00 */ 0, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0,/* 10 */ 4, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0,/* 20 */ 5, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0,/* 30 */ 4, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0,/* 40 */ 6, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0,/* 50 */ 4, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0,/* 60 */ 5, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0,/* 70 */ 4, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0,/* 80 */ 7, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0,/* 90 */ 4, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0,/* A0 */ 5, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0,/* B0 */ 4, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0,/* C0 */ 6, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0,/* D0 */ 4, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0,/* E0 */ 5, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0,/* F0 */ 4, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0
};/*** This function finds the first bit set (beginning with the least significant bit)* in value and return the index of that bit.** Bits are numbered starting at 1 (the least significant bit). A return value of* zero from any of these functions means that the argument was zero.** @return return the index of the first bit set. If value is 0, then this function* shall return 0.*/
int __rt_ffs(int value)
{if (value == 0) return 0;if (value & 0xff)return __lowest_bit_bitmap[value & 0xff] + 1;if (value & 0xff00)return __lowest_bit_bitmap[(value & 0xff00) >> 8] + 9;if (value & 0xff0000)return __lowest_bit_bitmap[(value & 0xff0000) >> 16] + 17;return __lowest_bit_bitmap[(value & 0xff000000) >> 24] + 25;
}int main(int argc, char *argv[])
{printf("hello tyustli\r\n");printf("%d, set bit is %d\r\n", __rt_ffs(0b10000), __rt_ffs(0b10000) - 1);printf("%d, set bit is %d\r\n", __rt_ffs(0b100000000), __rt_ffs(0b100000000) - 1);printf("%d, set bit is %d\r\n", __rt_ffs(0b10000000000000000), __rt_ffs(0b10000000000000000) - 1);printf("%d, set bit is %d\r\n", __rt_ffs(0b1000000000000000000000000), __rt_ffs(0b1000000000000000000000000) - 1);while (1);
}/*
hello tyustli
5, set bit is 4
9, set bit is 8
17, set bit is 16
25, set bit is 24
*//********** end of file ****************/
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