大小端數(shù)據(jù)存儲(chǔ)

概念：

大端模式：是指數(shù)據(jù)的高字節(jié)保存在內(nèi)存的低地址中，而低子節(jié)數(shù)據(jù)保存在內(nèi)存的高地址中。

小端模式：是指數(shù)據(jù)的高字節(jié)保存在內(nèi)存的高地址中，而數(shù)據(jù)的低字節(jié)保存在內(nèi)存的低地址中。

新的需求：

這是移動(dòng)端日志數(shù)據(jù)傳給服務(wù)器的格式：

{“flag”:16,“item”:{“group”:“xxxxxx”,“mirror”:1,“items”:[{“flag”:1,“key”:“l(fā)og”,“datas”:[“data:application/octet-stream;base64,xxxxxxx”]}]}}

[total_len(4b)][format(4b)][type_len(1b)][type_padding_len(1b)][data_padding_len(1b)] [reserved(1b)][type][data]。

format為格式，固定長(zhǎng)度，為4個(gè)字節(jié)。type_len一種type的實(shí)際長(zhǎng)度。Type_padding_len為type補(bǔ)位個(gè)數(shù)。data_padding _len為data補(bǔ)位個(gè)數(shù)。total_len為整個(gè)數(shù)據(jù)的長(zhǎng)度，占4個(gè)字節(jié),大端字節(jié)序存儲(chǔ)。type為存儲(chǔ)數(shù)據(jù)類型，data為存儲(chǔ)數(shù)據(jù)，都建議4字節(jié)對(duì)齊，末位補(bǔ)/0。

疑惑：

之前不知道大小端數(shù)據(jù)是咋回事，一頭霧水。問(wèn)了安卓同事才勉強(qiáng)明白，我們先了解下Byte，Object-C里面其實(shí)是沿用C++的，下面是Byte的定義：

*********************************************************************************/ //Byte是UInt8的別名 typedef UInt8 Byte; typedef SInt8 SignedByte; typedef wide * WidePtr; typedef UnsignedWide * UnsignedWidePtr; typedef Float80 extended80; typedef Float96 extended96; typedef SInt8 VHSelect; /********************************************************************************* /********************************************************************************Base integer types for all target OS's and CPU'sUInt8 8-bit unsigned integer SInt8 8-bit signed integerUInt16 16-bit unsigned integer SInt16 16-bit signed integer UInt32 32-bit unsigned integer SInt32 32-bit signed integer UInt64 64-bit unsigned integer SInt64 64-bit signed integer *********************************************************************************/ //UInt8是unsigned char 別名 typedef unsigned char UInt8; typedef signed char SInt8; typedef unsigned short UInt16; typedef signed short SInt16;#if __LP64__ typedef unsigned int UInt32; typedef signed int SInt32; #else typedef unsigned long UInt32; typedef signed long SInt32; #endif

下面是iOS的實(shí)現(xiàn)：

- (NSString *)logByteFill:(NSString *)data format:(NSString *)format type:(NSString *)type {if (data == NULL) {return @"";}int type_padding_len = 0;int type_len = (int)type.length;//type的實(shí)際長(zhǎng)度,都是英文，一個(gè)英文單詞一個(gè)字節(jié)int temp = 4 - type_len % 4;if (temp != 4) {type_padding_len = temp;}//JSON字符串的字節(jié)長(zhǎng)度int data_len = (int)[data lengthOfBytesUsingEncoding:NSUTF8StringEncoding];int data_padding_len = 4 - data_len % 4;if (data_padding_len != 4) {data_len = data_len + data_padding_len;} else {data_padding_len = 0;}int total_len = 12 + type_len + data_len;if (type_padding_len != 4) {total_len = 12 + type_len + type_padding_len + data_len;}NSMutableString *mutable_cmd = [[NSMutableString alloc] initWithString:type];for (int i = 0; i < type_padding_len; i++) {[mutable_cmd appendString:@"0"];}NSMutableString *mutable_data = [[NSMutableString alloc] initWithString:data];for (int i = 0; i < data_padding_len; i++) {[mutable_data appendString:@"0"];}Byte finalArray[12];//total段Byte total_byte[4] = {0};Byte *total_ptr = total_byte;//int 轉(zhuǎn) byte數(shù)組*(int*)total_ptr = total_len;finalArray[0] = total_byte[0];finalArray[1] = total_byte[1];finalArray[2] = total_byte[2];finalArray[3] = total_byte[3];//format段,固定“json”格式Byte format_byte[4];Byte *format_ptr = format_byte;//int 轉(zhuǎn) byte數(shù)組int format_len = (int)format.length;*(int*)format_ptr = format_len;finalArray[4] = format_byte[0];finalArray[5] = format_byte[1];finalArray[6] = format_byte[2];finalArray[7] = format_byte[3];//type段,占一個(gè)字節(jié)Byte type_byte[1];Byte *type_ptr = type_byte;//int 轉(zhuǎn) byte數(shù)組*(int*)type_ptr = type_len;finalArray[8] = type_byte[0];//type_padding段,占一個(gè)字節(jié)Byte type_padding_byte[1];Byte *type_padding_ptr = type_padding_byte;//int 轉(zhuǎn) byte數(shù)組*(int*)type_padding_ptr = type_padding_len;finalArray[9] = type_padding_byte[0];//type_padding段,占一個(gè)字節(jié)Byte data_padding_byte[1];Byte *data_padding_ptr = data_padding_byte;//int 轉(zhuǎn) byte數(shù)組*(int*)data_padding_ptr = data_padding_len;finalArray[10] = data_padding_byte[0];//reased Byte段,占一個(gè)字節(jié)finalArray[11] = 0x00; //strtoul("0x00",0,16);//固定12個(gè)字節(jié)dataNSData *fiex_data = [[NSData alloc] initWithBytes:&finalArray length:12];//type數(shù)據(jù)段NSData *type_data = [mutable_cmd dataUsingEncoding:NSUTF8StringEncoding];//data數(shù)據(jù)段NSData *data_data = [mutable_data dataUsingEncoding:NSUTF8StringEncoding];NSMutableData *all_data = [[NSMutableData alloc] init];[all_data appendData:fiex_data];[all_data appendData:type_data];[all_data appendData:data_data];NSData *base64Data = [all_data base64EncodedDataWithOptions:0];NSString *baseString = [[NSString alloc] initWithData:base64Data encoding:NSUTF8StringEncoding];NSString *base64 = [NSString stringWithFormat:@"data:application/octet-stream;base64,%@",baseString];return base64;}

那個(gè)Byte數(shù)組那里的代碼有點(diǎn)重復(fù)，我想抽一下，因?yàn)镺bject-C Byte類型不能作為函數(shù)值，所以就這樣了。int轉(zhuǎn)Byte數(shù)組有種C++方式：

測(cè)試：

//json 33 + cmd + 10 +補(bǔ)位json(3)+cmd(2)+固定長(zhǎng)度12=60NSString *json = @"{\"fileName\":\"log.ios.frfr4.h264\"}";NSString *format = @"json";NSString *cmd = @"openStream";NSString *data = [self logByteFill:json format:format type:cmd];NSLog(@"最后DATA == %@",data);

總結(jié)

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