Man page 9 : kstrtoll

kstrtoll - convert a string to a long long

int kstrtoll(const char *  s , unsigned int  base , long long *  res );

s
    The start of the string. The string must be null-terminated, and may also include a single newline before its terminating null. The first character may also be a plus sign or a minus sign.

base
    The number base to use. The maximum supported base is 16. If base is given as 0, then the base of the string is automatically detected with the conventional semantics - If it begins with 0x the number will be parsed as a hexadecimal (case insensitive), if it otherwise begins with 0, it will be parsed as an octal number. Otherwise it will be parsed as a decimal.

res
    Where to write the result of the conversion on success.

Returns 0 on success, -ERANGE on overflow and -EINVAL on parsing error. Used as a replacement for the obsolete simple_strtoull. Return code must be checked.