The strtoul() function converts the initial portion of the string pointed to by str to a type unsigned long int representation. First it decomposes the input string into three parts: an initial, possibly empty, sequence of white-space characters (as specified by isspace(3C)); a subject sequence interpreted as an integer represented in some radix determined by the value of base; and a final string of one or more unrecognised characters, including the terminating null byte of the input string. Then it attempts to convert the subject sequence to an unsigned integer, and returns the result.

If the value of base is 0, the expected form of the subject sequence is that of a decimal constant, octal constant or hexadecimal constant, any of which may be preceded by a + or - sign. A decimal constant begins with a non-zero digit, and consists of a sequence of decimal digits. An octal constant consists of the prefix 0 optionally followed by a sequence of the digits 0 to 7 only. A hexadecimal constant consists of the prefix 0x or 0X followed by a sequence of the decimal digits and letters a (or A) to f (or F) with values 10 to 15 respectively.

If the value of base is between 2 and 36, the expected form of the subject sequence is a sequence of letters and digits representing an integer with the radix specified by base, optionally preceded by a + or - sign. The letters from a (or A) to z (or Z) inclusive are ascribed the values 10 to 35; only letters whose ascribed values are less than that of base are permitted. If the value of base is 16, the characters 0x or 0X may optionally precede the sequence of letters and digits, following the sign if present.

The subject sequence is defined as the longest initial subsequence of the input string, starting with the first non-white-space character, that is of the expected form. The subject sequence contains no characters if the input string is empty or consists entirely of white-space characters, or if the first non-white-space character is other than a sign or a permissible letter or digit.

If the subject sequence has the expected form and the value of base is 0, the sequence of characters starting with the first digit is interpreted as an integer constant. If the subject sequence has the expected form and the value of base is between 2 and 36, it is used as the base for conversion, ascribing to each letter its value as given above. If the subject sequence begins with a minus sign, the value resulting from the conversion is negated. A pointer to the final string is stored in the object pointed to by endptr, provided that endptr is not a null pointer.

In other than the POSIX locale, additional implementation-dependent subject sequence forms may be accepted.

If the subject sequence is empty or does not have the expected form, no conversion is performed; the value of str is stored in the object pointed to by endptr, provided that endptr is not a null pointer.

The strtoull() function is identical to strtoul() except that it returns the value represented by str as an unsigned long long.

Upon successful completion strtoul() returns the converted value, if any. If no conversion could be performed, 0 is returned and errno may be set to EINVAL. If the correct value is outside the range of representable values, ULONG_MAX is returned and errno is set to ERANGE.

The strtoul() function will fail if:

EINVAL

The value of base is not supported.

ERANGE

The value to be returned is not representable.

The strtoul() function may fail if:

EINVAL

No conversion could be performed.

Because 0 and ULONG_MAX are returned on error and are also valid returns on success, an application wishing to check for error situations should set errno to 0, then call strtoul(), then check errno and if it is non-zero, assume an error has occurred.

Unlike strtod(3C) and strtol(3C), strtoul() must always return a non-negative number; so, using the return value of strtoul() for out-of-range numbers with strtoul() could cause more severe problems than just loss of precision if those numbers can ever be negative.

See attributes(5) for descriptions of the following attributes:

isalpha(3C), isspace(3C), scanf(3C), strtod(3C), strtol(3C), attributes(5)