一 概念：
循环冗余码校验英文名称为Cyclical Redundancy Check，简称CRC。它是利用除法及余数的原理来作错误侦测（Error Detecting）的。实际应用时，发送装置计算出CRC值并随数据一同发送给接收装置，接收装置对收到的数据重新计算CRC并与收到的CRC相比较，若两个CRC值不同，则说明数据通讯出现错误。
二 CRC16源码解析：
1 函数实现
#include <stdbool.h> #include <stdlib.h> #include <stdint.h> #define CRC_POLY_16 0xA001 #define CRC_START_16 0x0000 static bool crc_tab16_init = false; static uint16_t crc_tab16[256]; /* * uint16_t crc_16( const unsigned char *input_str, size_t num_bytes ); * * The function crc_16() calculates the 16 bits CRC16 in one pass for a byte * string of which the beginning has been passed to the function. The number of * bytes to check is also a parameter. The number of the bytes in the string is * limited by the constant SIZE_MAX. */ uint16_t crc_16( const unsigned char *input_str, size_t num_bytes ) { uint16_t crc; const unsigned char *ptr; size_t a; if ( ! crc_tab16_init ) init_crc16_tab(); crc = CRC_START_16; ptr = input_str; if ( ptr != NULL ) for (a=0; a<num_bytes; a++) { crc = (crc >> 8) ^ crc_tab16[ (crc ^ (uint16_t) *ptr++) & 0x00FF ]; } return crc; } /* crc_16 */ /* * static void init_crc16_tab( void ); * * For optimal performance uses the CRC16 routine a lookup table with values * that can be used directly in the XOR arithmetic in the algorithm. This * lookup table is calculated by the init_crc16_tab() routine, the first time * the CRC function is called. */ void init_crc16_tab( void ) { uint16_t i; uint16_t j; uint16_t crc; uint16_t c; for (i=0; i<256; i++) { crc = 0; c = i; for (j=0; j<8; j++) { if ( (crc ^ c) & 0x0001 ) crc = ( crc >> 1 ) ^ CRC_POLY_16; else crc = crc >> 1; c = c >> 1; } crc_tab16[i] = crc; } crc_tab16_init = true; } /* init_crc16_tab */
2 计算结果：
#include <stdio.h> #include <string.h> typedef unsigned char uint8_t; uint8_t test_data[20]; int main() { uint16_t crc_value = 0x00; memset(test_data,0x11,20); crc_value = crc_16(test_data,10); printf("crc_value is:0x%x \n\r",crc_value); return 0; }