[Info-vax] Significance of 2020 / 2021 date

[Stephen Hoffman]

On 2021-01-09 21:38:07 +0000, 1tim.lovern at gmail.com said:

> On Thursday, January 7, 2021 at 2:00:26 PM UTC-7, issinoho wrote:
>> ...
>> if (atoi (date_string + 5) > 2020).
> 
> you are adding 5 to the ascii value of the first byte of that date string.

The cited C code above is not adding 5 to the ASCII value of the first 
byte. The date_string + 5 syntax shown is performing pointer math, and 
moves the begining of the string by five of whatever the pointer is 
pointing to. That'll be 5 characters, if date_string is a character 
pointer as would be typical.

A word or longword pointer would be somewhat unusual here, though 
pointer math works with that, too. Adding 5 would relocate by 10 or by 
20 bytes for a word or longword pointer, respectively. But I digress.

If this is a request to add 5 to the ASCII value of the first byte, 
then the code would typically be date_string[0] + 5, or maybe 
*date_string + 5, assuming a character pointer. date_string[0] and 
*date_string both return the byte value for the specified pointer, 
again assuming a character pointer.

> code should read
> if ((atoi(date_string) + 5)  > 2020)'

That C code will convert the contents of the bytes at the specified 
address to an integer, then add 5 to the result.

(Valid input for atoi can include an optional leading + or -, then some 
number of decimal digits up to the first non-decimal-digit or the first 
null byte. Invalid string input gets a 0 as a return from atoi.)

But without more context to the code, it's difficult to determine the 
intent of the programmer with absolute certainty.

Hoff


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