strtoul(3) Library Functions Manual strtoul(3)

strtoul, strtoull, strtouq - convert a string to an unsigned long integer

Standard C library (libc, -lc)

#include <stdlib.h>
unsigned long strtoul(const char *restrict nptr,
                      char **_Nullable restrict endptr, int base);
unsigned long long strtoull(const char *restrict nptr,
                      char **_Nullable restrict endptr, int base);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):


        || /* glibc <= 2.19: */ _SVID_SOURCE || _BSD_SOURCE

The strtoul() function converts the initial part of the string in nptr to an unsigned long value according to the given base, which must be between 2 and 36 inclusive, or be the special value 0.

The string may begin with an arbitrary amount of white space (as determined by isspace(3)) followed by a single optional '+' or '-' sign. If base is zero or 16, the string may then include a "0x" prefix, and the number will be read in base 16; otherwise, a zero 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 first character which is not a valid digit in the given base. (In bases above 10, the letter 'A' in either uppercase or lowercase represents 10, 'B' represents 11, and so forth, with 'Z' representing 35.)

If endptr is not NULL, and the base is supported, strtoul() stores the address of the first invalid character in *endptr. If there were no digits at all, strtoul() stores the original value of nptr in *endptr (and returns 0). In particular, if *nptr is not '\0' but **endptr is '\0' on return, the entire string is valid.

The strtoull() function works just like the strtoul() function but returns an unsigned long long value.

The strtoul() function returns either the result of the conversion or, if there was a leading minus sign, the negation of the result of the conversion represented as an unsigned value, unless the original (nonnegated) value would overflow; in the latter case, strtoul() returns ULONG_MAX and sets errno to ERANGE. Precisely the same holds for strtoull() (with ULLONG_MAX instead of ULONG_MAX).

This function does not modify errno on success.

(not in C99) The given base contains an unsupported value.
The resulting value was out of range.

The implementation may also set errno to EINVAL in case no conversion was performed (no digits seen, and 0 returned).

For an explanation of the terms used in this section, see attributes(7).

Interface Attribute Value
strtoul (), strtoull (), strtouq () Thread safety MT-Safe locale

In locales other than the "C" locale, other strings may be accepted. (For example, the thousands separator of the current locale may be supported.)

BSD also has

u_quad_t strtouq(const char *nptr, char **endptr, int base);

with completely analogous definition. Depending on the wordsize of the current architecture, this may be equivalent to strtoull() or to strtoul().

C11, POSIX.1-2008.

POSIX.1-2001, C89, SVr4.
POSIX.1-2001, C99.

Since strtoul() can legitimately return 0 or ULONG_MAX (ULLONG_MAX for strtoull()) on both success and failure, the calling program should set errno to 0 before the call, and then determine if an error occurred by checking whether errno has a nonzero value after the call.

Negative values are considered valid input and are silently converted to the equivalent unsigned long value.

See the example on the strtol(3) manual page; the use of the functions described in this manual page is similar.

a64l(3), atof(3), atoi(3), atol(3), strtod(3), strtol(3), strtoumax(3)

2024-06-16 Linux man-pages 6.9.1