strtouq converts the string nptr to a uquadt value. The conversion is done according to the given base, which must be between 2 and 36 inclusive, or be the special value 0.
nptr may begin with an arbitrary amount of white space (as determined by isspace(3)), followed by a single optional + or - sign character. If base is 0 or 16, the string may then include a 0x prefix, and the number will be read in base 16; otherwise, a 0 base is taken as 10 (decimal), unless the next character is 0, in which case it is taken as 8 (octal).
The remainder of the string is converted to an unsigned long value in the obvious manner, stopping at the end of the string or at the first character that does not produce a valid digit in the given base. (In bases above 10, the letter A in either upper or lower case represents 10, B represents 11, and so forth, with Z representing 35.)
On success, strtouq returns either the result of the conversion or, if there was a leading minus sign, the negation of the result of the conversion, unless the original (non-negated) value would overflow. In the case of an overflow the function returns UQUAD_MAX and the global variable errno is set to ERANGE.