CRC32这种轻量级的校验算法在嵌入式开发中使用非常广泛,比如User data的存储,非关键数据的 保存(对于重要数据,一般不建议只用CRC32去校验其完整性,还会考虑加密处理).在PC端常用脚本 语言(例如python)去生成CRC32校验值值,而在嵌入式设备端,由于设备端能力有限,会直接使用C编写的程序去 做CRC32校验;
python实现crc32(同时有md5和sha1校验算法demo)算法如下:
#!/usr/bin/python import sys import os from zlib import crc32 import hashlib import binascii def getcrc32(filepath): file = open(filepath,'rb') return crc32(file.read()) def getbinasciiCRC(filepath): file = open(filepath,'rb') return (binascii.crc32(file.read())&0xffffffff) def getmd5(filepath): file = open(filepath,'rb') md5func = hashlib.md5() md5func.update(file.read()) return md5func.hexdigest() def getsha1(filepath): file = open(filepath,'rb') sha1func = hashlib.sha1() sha1func.update(file.read()) return sha1func.hexdigest() path = sys.argv[1] print sys.argv[0], " ",path print "crc32 value:%X"%(getcrc32(path)&0xffffffff) print "binascii crc32 value:%X"%(getbinasciiCRC(path)) print "md5 value:%s"%(getmd5(path)) print "sha1 value:%s"%(getsha1(path))该例测试用python版本为2.7.4,需要安装zlib/hashlib/binascii这三个module,在实际应用中zlib和binascii可只装其一,这两者都含有计算crc32的方法;
因为在嵌入式设备端的计算能力有限,所以通常的做法是使用crc32查表法去计算crc32,利用提前计算好的256个elements的table,去计算crc32(使用多项式:x^{{32}}+x^{{26}}+x^{{23}}+x^{{22}}+x^{{16}}+x^{{12}}+x^{{11}}+x^{{10}}+x^{8}+x^{7}+x^{5}+x^{4}+x^{2}+x+1); 用C实现的table生成方法如下:
static unsigned int CRC32_Table[256]; /** * 产生CRC32查表法所用的表, * 0xEDB88320:CRC32的生成项是1 0000 0100 1100 0001 0001 1101 1011 0111，去掉最高位的1，保留32bit，然后再颠倒后 * 转换成16进制就是0xEDB88320 */ static void _createCRCTable(void) { unsigned int crc32; unsigned int idx = 0; unsigned int bit = 0; for(idx = 0; idx < 256; idx++) { crc32 = idx; for(bit = 0; bit < 8; bit++) { if(crc32&1) { crc32 = (crc32 >> 1)^(0xEDB88320); } else { crc32 = crc32 >> 1; } } CRC32_Table[idx] = crc32; printf("0xx, ", CRC32_Table[idx]); if((idx+1)%8 == 0) printf("\n"); } }后续计算CRC32就直接使用已经生成的table:CRC32_Table[256];在正式使用时,可直接将table固定在代码中, 如下:
static const unsigned int CRC32_Table[256] = { 0x00000000, 0x77073096, 0xEE0E612C, 0x990951BA, 0x076DC419, 0x706AF48F, 0xE963A535, 0x9E6495A3, 0x0EDB8832, 0x79DCB8A4, 0xE0D5E91E, 0x97D2D988, 0x09B64C2B, 0x7EB17CBD, 0xE7B82D07, 0x90BF1D91, 0x1DB71064, 0x6AB020F2, 0xF3B97148, 0x84BE41DE, 0x1ADAD47D, 0x6DDDE4EB, 0xF4D4B551, 0x83D385C7, 0x136C9856, 0x646BA8C0, 0xFD62F97A, 0x8A65C9EC, 0x14015C4F, 0x63066CD9, 0xFA0F3D63, 0x8D080DF5, 0x3B6E20C8, 0x4C69105E, 0xD56041E4, 0xA2677172, 0x3C03E4D1, 0x4B04D447, 0xD20D85FD, 0xA50AB56B, 0x35B5A8FA, 0x42B2986C, 0xDBBBC9D6, 0xACBCF940, 0x32D86CE3, 0x45DF5C75, 0xDCD60DCF, 0xABD13D59, 0x26D930AC, 0x51DE003A, 0xC8D75180, 0xBFD06116, 0x21B4F4B5, 0x56B3C423, 0xCFBA9599, 0xB8BDA50F, 0x2802B89E, 0x5F058808, 0xC60CD9B2, 0xB10BE924, 0x2F6F7C87, 0x58684C11, 0xC1611DAB, 0xB6662D3D, 0x76DC4190, 0x01DB7106, 0x98D220BC, 0xEFD5102A, 0x71B18589, 0x06B6B51F, 0x9FBFE4A5, 0xE8B8D433, 0x7807C9A2, 0x0F00F934, 0x9609A88E, 0xE10E9818, 0x7F6A0DBB, 0x086D3D2D, 0x91646C97, 0xE6635C01, 0x6B6B51F4, 0x1C6C6162, 0x856530D8, 0xF262004E, 0x6C0695ED, 0x1B01A57B, 0x8208F4C1, 0xF50FC457, 0x65B0D9C6, 0x12B7E950, 0x8BBEB8EA, 0xFCB9887C, 0x62DD1DDF, 0x15DA2D49, 0x8CD37CF3, 0xFBD44C65, 0x4DB26158, 0x3AB551CE, 0xA3BC0074, 0xD4BB30E2, 0x4ADFA541, 0x3DD895D7, 0xA4D1C46D, 0xD3D6F4FB, 0x4369E96A, 0x346ED9FC, 0xAD678846, 0xDA60B8D0, 0x44042D73, 0x33031DE5, 0xAA0A4C5F, 0xDD0D7CC9, 0x5005713C, 0x270241AA, 0xBE0B1010, 0xC90C2086, 0x5768B525, 0x206F85B3, 0xB966D409, 0xCE61E49F, 0x5EDEF90E, 0x29D9C998, 0xB0D09822, 0xC7D7A8B4, 0x59B33D17, 0x2EB40D81, 0xB7BD5C3B, 0xC0BA6CAD, 0xEDB88320, 0x9ABFB3B6, 0x03B6E20C, 0x74B1D29A, 0xEAD54739, 0x9DD277AF, 0x04DB2615, 0x73DC1683, 0xE3630B12, 0x94643B84, 0x0D6D6A3E, 0x7A6A5AA8, 0xE40ECF0B, 0x9309FF9D, 0x0A00AE27, 0x7D079EB1, 0xF00F9344, 0x8708A3D2, 0x1E01F268, 0x6906C2FE, 0xF762575D, 0x806567CB, 0x196C3671, 0x6E6B06E7, 0xFED41B76, 0x89D32BE0, 0x10DA7A5A, 0x67DD4ACC, 0xF9B9DF6F, 0x8EBEEFF9, 0x17B7BE43, 0x60B08ED5, 0xD6D6A3E8, 0xA1D1937E, 0x38D8C2C4, 0x4FDFF252, 0xD1BB67F1, 0xA6BC5767, 0x3FB506DD, 0x48B2364B, 0xD80D2BDA, 0xAF0A1B4C, 0x36034AF6, 0x41047A60, 0xDF60EFC3, 0xA867DF55, 0x316E8EEF, 0x4669BE79, 0xCB61B38C, 0xBC66831A, 0x256FD2A0, 0x5268E236, 0xCC0C7795, 0xBB0B4703, 0x220216B9, 0x5505262F, 0xC5BA3BBE, 0xB2BD0B28, 0x2BB45A92, 0x5CB36A04, 0xC2D7FFA7, 0xB5D0CF31, 0x2CD99E8B, 0x5BDEAE1D, 0x9B64C2B0, 0xEC63F226, 0x756AA39C, 0x026D930A, 0x9C0906A9, 0xEB0E363F, 0x72076785, 0x05005713, 0x95BF4A82, 0xE2B87A14, 0x7BB12BAE, 0x0CB61B38, 0x92D28E9B, 0xE5D5BE0D, 0x7CDCEFB7, 0x0BDBDF21, 0x86D3D2D4, 0xF1D4E242, 0x68DDB3F8, 0x1FDA836E, 0x81BE16CD, 0xF6B9265B, 0x6FB077E1, 0x18B74777, 0x88085AE6, 0xFF0F6A70, 0x66063BCA, 0x11010B5C, 0x8F659EFF, 0xF862AE69, 0x616BFFD3, 0x166CCF45, 0xA00AE278, 0xD70DD2EE, 0x4E048354, 0x3903B3C2, 0xA7672661, 0xD06016F7, 0x4969474D, 0x3E6E77DB, 0xAED16A4A, 0xD9D65ADC, 0x40DF0B66, 0x37D83BF0, 0xA9BCAE53, 0xDEBB9EC5, 0x47B2CF7F, 0x30B5FFE9, 0xBDBDF21C, 0xCABAC28A, 0x53B39330, 0x24B4A3A6, 0xBAD03605, 0xCDD70693, 0x54DE5729, 0x23D967BF, 0xB3667A2E, 0xC4614AB8, 0x5D681B02, 0x2A6F2B94, 0xB40BBE37, 0xC30C8EA1, 0x5A05DF1B, 0x2D02EF8D };计算CRC32:
/** * [_getCRC32 使用查表法计算CRC32] * @param buffer [缓存地址] * @param bufferLen [缓存长度] * @return [计算出的CRC32值] */ static unsigned int _getCRC32(void *buffer, unsigned int bufferLen) { unsigned int crc32Value = 0xffffffff; unsigned char *pTmpBuffer = buffer; while(bufferLen--) { crc32Value = CRC32_Table[(crc32Value^ *pTmpBuffer++) & 0xff] ^ (crc32Value >> 8); } return (crc32Value^0xffffffff); }测试函数如下:
int main(int argc, char *argv[]) { FILE* pFile = NULL; void* tmpBuffer = NULL; unsigned int crc32Value = 0; char *FilePath[1024]; struct stat fileInfo; if(argc != 2) { printf("The program run format:./crc32bin [filepath] \n"); return -1; } printf("open FilePath:%s\n", argv[1]); strncpy((char*)FilePath, (char*)argv[1], 1023); if(stat((const char*)FilePath, &fileInfo)<0) { printf("Stat file failed!\n"); return -1; } printf("file size:%u\n", fileInfo.st_size); tmpBuffer = malloc((unsigned int)(fileInfo.st_size)+0x10); if(!tmpBuffer) { printf("alloc buffer failed!\n"); return -1; } _createCRCTable(); pFile = fopen((const char*)FilePath, "rb+"); fread(tmpBuffer, fileInfo.st_size, 1, pFile); crc32Value = _getCRC32(tmpBuffer, fileInfo.st_size); printf("crc32Value is:0x%8x\n", crc32Value); fclose(pFile); free(tmpBuffer); return 0; }测试同一个文件,分别使用python和C,算出来的crc32值一致,完工!
