Section (3) strtoul

Linux manual pages Section 3  


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


#include <stdlib.h>
unsigned long int strtoul( const char *nptr,
  char **endptr,
  int base);
unsigned long long int strtoull( const char *nptr,
  char **endptr,
  int base);
[Note] Note
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
_ISOC99_SOURCE || || /* Glibc versions <= 2.19:


The strtoul() function converts the initial part of the string in nptr to an unsigned long int 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 _zsingle_quotesz_+_zsingle_quotesz_ or _zsingle_quotesz_−_zsingle_quotesz_ 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 _zsingle_quotesz_0_zsingle_quotesz_, in which case it is taken as 8 (octal).

The remainder of the string is converted to an unsigned long int 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 _zsingle_quotesz_A_zsingle_quotesz_ in either uppercase or lowercase represents 10, _zsingle_quotesz_B_zsingle_quotesz_ represents 11, and so forth, with _zsingle_quotesz_Z_zsingle_quotesz_ representing 35.)

If endptr is not NULL, 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 _zsingle_quotesz__zsingle_quotesz_ but **endptr is _zsingle_quotesz__zsingle_quotesz_ on return, the entire string is valid.

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



(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


strtoul(): POSIX.1-2001, POSIX.1-2008, C89, C99 SVr4.

strtoull(): POSIX.1-2001, POSIX.1-2008, 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.

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().

Negative values are considered valid input and are silently converted to the equivalent unsigned long int 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)


This page is part of release 5.04 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at−pages/.

  Copyright 1993 David Metcalfe (

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References consulted:
    Linux libc source code
    Lewine_zsingle_quotesz_s _POSIX Programmer_zsingle_quotesz_s Guide_ (O_zsingle_quotesz_Reilly & Associates, 1991)
    386BSD man pages
Modified Sun Jul 25 10:54:03 1993 by Rik Faith (
Fixed typo, aeb, 950823
2002-02-22, joey, mihtjel: Added strtoull()