Why is substr(string,0,1) the same as substr(string,1,1)?

[Marc de Bourget]

I have had an issue with a check:
if (substr(oldstr,s-1,1) ~ CONST_ALPHANUMERIC_ONESIGN)

If s is equal to 1, the result of substr(oldstr,0,1)
is the same as substr(oldstr,1,1) but it should not (in my case).
I would have expected an empty string as result for substr(string,0,1).

I have solved it by using
if ((s-1 > 0) && (substr(oldstr,s-1,1) ~ CONST_ALPHANUMERIC_ONESIGN))

However, there must be a reason why AWK developpers decided to do so?

[Ed Morton]

It's undefined behavior per POSIX so gawk (and others?) treat any invalid start
position as 1. From the gawk manual
(https://www.gnu.org/software/gawk/manual/gawk.html#String-Functions):

"If start is less than one, substr() treats it as if it was one. (POSIX doesn’t
specify what to do in this case: BWK awk acts this way, and therefore gawk does
too.) If start is greater than the number of characters in the string, substr()
returns the null string. Similarly, if length is present but less than or equal
to zero, the null string is returned."

Ed.

[Marc de Bourget]

Thank you Ed. GAWK and TAWK behave the same while MAWK returns the null string.