strtoul
Section: C Library Functions (3)
Updated: 2026-02-08
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NAME
strtoul, strtoull, strtouq - convert a string to an unsigned long integer
LIBRARY
Standard C library
(
libc,~
-lc)
SYNOPSIS
#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)):
strtoull():
_ISOC99_SOURCE
|| /* glibc <= 2.19: */ _SVID_SOURCE || _BSD_SOURCE
DESCRIPTION
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 [aq]+[aq] or [aq]-[aq]
sign.
If
base
is zero or 16, the string may then include a
"0x" or "0X" prefix, and the number will be read in base 16;
if
base
is zero or 2, the string may then include a
"0b" or "0B" prefix, and the number will be read in base 2;
otherwise,
a zero
base
is taken as 10 (decimal) unless the next character
is [aq]0[aq], 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 [aq]A[aq] in
either uppercase or lowercase represents 10, [aq]B[aq] represents 11, and so
forth, with [aq]Z[aq] 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 [aq][rs]0[aq] but
**endptr
is [aq][rs]0[aq] on return, the entire string is valid.
The
strtoull()
function works just like the
strtoul()
function but returns an
unsigned long long
value.
RETURN 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).
ERRORS
This function does not modify
errno
on success.
- EINVAL
-
(not in C99)
The given
base
contains an unsupported value.
- ERANGE
-
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).
ATTRIBUTES
For an explanation of the terms used in this section, see
attributes(7).
| Interface | Attribute | Value
|
|
strtoul(),
strtoull(),
strtouq()
| Thread safety | MT-Safe locale
|
VERSIONS
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().
STANDARDS
C23, POSIX.1-2024.
HISTORY
- strtoul()
-
POSIX.1-2001, C89, SVr4.
- strtoull()
-
POSIX.1-2001, C99.
- "0b", "0B"
-
C23.
glibc 2.38.
(Not in POSIX.)
CAVEATS
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.
BUGS
Signed numbers
Some negative values
are considered valid input and
are silently converted to
unsigned long.
White space
These functions silently accept leading whitespace.
To reject white space and/or a sign, call
isalnum(3)
before
strtoul().
EXAMPLES
See the example on the
strtol(3)
manual page;
the use of the functions described in this manual page is similar.
SEE ALSO
a64l(3),
atof(3),
atoi(3),
atol(3),
strtod(3),
strtol(3),
strtoumax(3)
Index
- NAME
-
- LIBRARY
-
- SYNOPSIS
-
- DESCRIPTION
-
- RETURN VALUE
-
- ERRORS
-
- ATTRIBUTES
-
- VERSIONS
-
- STANDARDS
-
- HISTORY
-
- CAVEATS
-
- BUGS
-
- Signed numbers
-
- White space
-
- isalnum(3)
-
- EXAMPLES
-
- SEE ALSO
-
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