| STRTOUL(3) | Library Functions Manual | STRTOUL(3) | 
strtoul, strtoull,
  strtoumax, strtouq —
#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);
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’ or
    ‘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.)
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.
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 (ep == buf)
	goto not_a_number;
if (*ep != '\0')
	goto trailing_garbage;
if (errno) {
	assert(errno == ERANGE);
	assert(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).
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.
| November 13, 2015 | NetBSD 9.4 |