ex:

```
NSData *data = [NSData dataWithContentsOfFile:filePath]; 
int len = [data length]; 

```

if len = 10000,
i hope i can convert 1000 to a NSData look like

```
char hoperesult[] = {0x10, 0x27, 0x00, 0x00}

```

and hoperesult[] must always 4 Bytes