digit_value

Determines the numeric value of a character as a decimal digit.

If the character is not a decimal digit according to [func@GLib.ascii_isdigit], `-1` is returned.

Differs from [func@GLib.unichar_digit_value] because it takes a char, so there's no worry about sign extension if characters are signed.

Parameters:

c	an ASCII character

Returns:

the numerical value of c if it is a decimal digit, `-1` otherwise