nsstring nsdata char得转换
NSString 转换成NSData 对象
NSData* xmlData = [@"testdata" dataUsingEncoding:NSUTF8StringEncoding];

NSData 转换成NSString对象
NSData * data;
NSString *result = [[NSString alloc] initWithData:data encoding:NSUTF8StringEncoding];


NSData 转换成char*
NSData *data;
char *test=[data bytes];


char* 转换成NSData对象
byte* tempData = malloc(sizeof(byte)*16);
NSData *content=[NSData dataWithBytes:tempData length:16];