Why does this code have an array of 3 bytes to build a "whole byte"?

Question

I found the following code on the internet, it converts NSString representations such as
@"00F04100002712" into an actual array of bytes. The code works and does generate the correct output; I just don't understand why there is char byte_chars[3] instead of char byte_chars[2] since only the first two positions are used in the code.

void hexStringToBytes(NSString *s, NSMutableData *data)
{
    unsigned char whole_byte;
    char byte_chars[3] = {'\0','\0','\0'};
    int commandLength = (int)[s length];

    // convert hex values to bytes
    for (int i=0; i < commandLength/2; i++)
    {
        byte_chars[0] = [s characterAtIndex:i*2];
        byte_chars[1] = [s characterAtIndex:i*2+1];
        whole_byte = strtol(byte_chars, NULL, 16);
        [data appendBytes:&whole_byte length:1];
    }
}

I think it has something to do with the strtol function call but I am not sure what.

Can someone explain how and why this works?

Answer 1

C style strings have a terminating zero (aka null) character. An ASCII representation of an 8 bit byte in hexadecimal will be two characters plus that terminator.

Answer 2

Yes it does. strtol expects a "string". In C strings are null terminated. Thus the extra byte for the null.

Answer 3

C strings must be NULL (0) terminated. Since this is using a C string with a function expecting NULL terminated strings, the character array must have space for the NULL.

Answer 4

C strings must be NUL (0) terminated. The call to strtol function expects that.

Answer 5

You are messed up with C, C++ and Objective-C.

C++ and Obj-c uses full use of array. C++ creates an array of size + extra 1 space for NULL ('\0'). Obj-c has NSString class that is just a pointer, so NULL is not required.

In C you need to terminate an array of characters by NULL ('\0) and the last space is used from the allocated size. So if you write char str[10]; then you are allowed to use 9 characters and last one is automatically given to NULL.

In you code snippet you are using strtol, this is a C function which expects a C-char-array. there for last space is not used explicitly. NULL is occupying that place.