NSString 转 NSData

NSString * str = @"str"; 
NSData *data = [str dataUsingEncoding:NSUTF8StringEncoding];

NSData 转 NSString

NSString * str  =[[NSString alloc] initWithData:data encoding:NSUTF8StringEncoding];

data 转 char

NSData * data; 
char * haha = [data bytes]; 

char 转 data

byte * byteData = malloc(sizeof(byte)*16); 
NSData *content=[NSData dataWithBytes:byteData length:16];