strtoul man page

Prolog

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strtoul, strtoull — convert a string to an unsigned long

Synopsis

#include <stdlib.h>

unsigned long strtoul(const char *restrict str,
    char **restrict endptr, int base);
unsigned long long strtoull(const char *restrict str,
    char **restrict endptr, int base);

Description

The functionality described on this reference page is aligned with the ISO C standard. Any conflict between the requirements described here and the ISO C standard is unintentional. This volume of POSIX.1‐2008 defers to the ISO C standard.

These functions shall convert the initial portion of the string pointed to by str to a type unsigned long and unsigned long long representation, respectively. First, they decompose the input string into three parts:

1.
An initial, possibly empty, sequence of white-space characters (as specified by isspace())
2.
A subject sequence interpreted as an integer represented in some radix determined by the value of base
3.
A final string of one or more unrecognized characters, including the terminating NUL character of the input string

Then they shall attempt to convert the subject sequence to an unsigned integer, and return 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 shall contain 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 shall be interpreted as an integer constant. If the subject sequence has the expected form and the value of base is between 2 and 36, it shall be 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 shall be negated. A pointer to the final string shall be stored in the object pointed to by endptr, provided that endptr is not a null pointer.

In other than the C or POSIX locales, other implementation-defined subject sequences may be accepted.

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

These functions shall not change the setting of errno if successful.

Since 0, {ULONG_MAX}, and {ULLONG_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() or strtoull(), then check errno.

Return Value

Upon successful completion, these functions shall return the converted value, if any. If no conversion could be performed, 0 shall be returned and errno may be set to [EINVAL].

If the value of base is not supported, 0 shall be returned and errno shall be set to [EINVAL].

If the correct value is outside the range of representable values, {ULONG_MAX} or {ULLONG_MAX} shall be returned and errno set to [ERANGE].

Errors

These functions shall fail if:

EINVAL
The value of base is not supported.
ERANGE
The value to be returned is not representable.

These functions may fail if:

EINVAL
No conversion could be performed.

The following sections are informative.

Application Usage

Since the value of *endptr is unspecified if the value of base is not supported, applications should either ensure that base has a supported value (0 or between 2 and 36) before the call, or check for an [EINVAL] error before examining *endptr.

See Also

fscanf(), isalpha(), strtod(), strtol()

The Base Definitions volume of POSIX.1‐2008, <stdlib.h>

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2013 IEEE/The Open Group POSIX Programmer's Manual