strtoul, strtoull

Defined in header <stdlib.h>
unsigned long      strtoul( const char          *str, char          **str_end, 
                            int base );
(until C99)
unsigned long      strtoul( const char *restrict str, char **restrict str_end, 
                            int base );
(since C99)
unsigned long long strtoull( const char *restrict str, char **restrict str_end,
                             int base );
(since C99)

Interprets an unsigned integer value in a byte string pointed to by str.

Discards any whitespace characters (as identified by calling isspace()) until the first non-whitespace character is found, then takes as many characters as possible to form a valid base-n (where n=base) unsigned integer number representation and converts them to an integer value. The valid unsigned integer value consists of the following parts:

  • (optional) plus or minus sign
  • (optional) prefix (0) indicating octal base (applies only when the base is 8 or ​0​)
  • (optional) prefix (0x or 0X) indicating hexadecimal base (applies only when the base is 16 or ​0​)
  • a sequence of digits

The set of valid values for base is {0,2,3,...,36}. The set of valid digits for base-2 integers is {0,1}, for base-3 integers is {0,1,2}, and so on. For bases larger than 10, valid digits include alphabetic characters, starting from Aa for base-11 integer, to Zz for base-36 integer. The case of the characters is ignored.

Additional numeric formats may be accepted by the currently installed C locale.

If the value of base is ​0​, the numeric base is auto-detected: if the prefix is 0, the base is octal, if the prefix is 0x or 0X, the base is hexadecimal, otherwise the base is decimal.

If the minus sign was part of the input sequence, the numeric value calculated from the sequence of digits is negated as if by unary minus in the result type, which applies unsigned integer wraparound rules.

The functions sets the pointer pointed to by str_end to point to the character past the last character interpreted. If str_end is NULL, it is ignored.


str - pointer to the null-terminated byte string to be interpreted
str_end - pointer to a pointer to character.
base - base of the interpreted integer value

Return value

Integer value corresponding to the contents of str on success. If the converted value falls out of range of corresponding return type, range error occurs (errno is set to ERANGE) and ULONG_MAX or ULLONG_MAX is returned. If no conversion can be performed, ​0​ is returned.


#include <stdio.h>
#include <errno.h>
#include <stdlib.h>
int main(void)
    const char *p = "10 200000000000000000000000000000 30 -40";
    printf("Parsing '%s':\n", p);
    char *end;
    for (unsigned long i = strtoul(p, &end, 10);
         p != end;
         i = strtoul(p, &end, 10))
        printf("'%.*s' -> ", (int)(end-p), p);
        p = end;
        if (errno == ERANGE){
            printf("range error, got ");
            errno = 0;
        printf("%lu\n", i);


Parsing '10 200000000000000000000000000000 30 -40':
'10' -> 10
' 200000000000000000000000000000' -> range error, got 18446744073709551615
' 30' -> 30
' -40' -> 18446744073709551576


  • C11 standard (ISO/IEC 9899:2011):
    • The strtol, strtoll, strtoul, and strtoull functions (p: 344-345)
  • C99 standard (ISO/IEC 9899:1999):
    • The strtol, strtoll, strtoul, and strtoull functions (p: 310-311)
  • C89/C90 standard (ISO/IEC 9899:1990):
    • The strtoul function

See also

converts a wide string to an unsigned integer value
converts a byte string to an integer value
converts a byte string to an integer value
C++ documentation for strtoul

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