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STRTOUL(3) Library Functions Manual STRTOUL(3)

strtoul, strtoull, strtoumax, strtouq
convert a string to an unsigned long, unsigned long long, uintmax_t or uquad_t integer

library “libc”

#include <stdlib.h>
#include <limits.h>

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

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


#include <inttypes.h>

uintmax_t
strtoumax(const char * restrict nptr, char ** restrict endptr, int base);


#include <sys/types.h>
#include <stdlib.h>
#include <limits.h>

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

The strtoul() function converts the string in nptr to an unsigned long int value. The strtoull() function converts the string in nptr to an unsigned long long int value. The strtoumax() function converts the string in nptr to an uintmax_t value. The strtouq() function converts the string in nptr to a u_quad_t value.

The conversion is done 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 appropriate value in the obvious manner, stopping at the end of the string or at the first character that does not produce a valid digit in the given base. (In bases above 10, the letter ‘A’ in either upper or lower case represents 10, ‘B’ represents 11, and so forth, with ‘Z’ representing 35.)

If endptr is non-nil, the functions store the address of the first invalid character in *endptr. If there were no digits at all, however, the functions store the original value of nptr in *endptr. (Thus, if *nptr is not ‘\0’ but **endptr is ‘\0’ on return, the entire string was valid.)

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, unless the original (non-negated) value would overflow; in the latter case, strtoul() returns ULONG_MAX, strtoull() returns ULLONG_MAX, strtoumax() returns UINTMAX_MAX, strtouq() returns UQUAD_MAX, and the global variable errno is set to ERANGE.

There is no way to determine if strtoul() has processed a negative number (and returned an unsigned value) short of examining the string in nptr directly. If the base argument is not supported then errno is set to EINVAL and the functions return 0.

If no error occurs, errno is left unchanged. This behavior (which is unlike most library functions) is guaranteed by the pertinent standards.

Because the return value of strtoul() cannot be used unambiguously to detect an error, errno is left unchanged after a successful call. To ensure that a string is a valid number (i.e., in range and containing no trailing characters), clear errno beforehand explicitly, then check it afterwards:
char *ep;
unsigned long ulval;

...

errno = 0;
ulval = strtoul(buf, &ep, 10);
if (buf[0] == '\0' || *ep != '\0')
	goto not_a_number;
if (errno == ERANGE && ulval == ULONG_MAX)
	goto out_of_range;

This example will accept “12” but not “12foo” or “12\n”. If trailing whitespace is acceptable, further checks must be done on *ep; alternately, use sscanf(3).

[]
The base is not between 2 and 36 and does not contain the special value 0.
[]
The given string was out of range; the value converted has been clamped.

atof(3), atoi(3), atol(3), atoll(3), strtod(3), strtoi(3), strtoimax(3), strtol(3), strtoll(3), strtou(3)

The strtoul() function conforms to ANSI X3.159-1989 (“ANSI C89”). The strtoull() and strtoumax() functions conform to ISO/IEC 9899:1999 (“ISO C99”).

The strtouq() function is a BSD legacy function equivalent to strtoull() and should not be used in a new code.

Ignores the current locale.
April 30, 2015 NetBSD-7.0.1