#### 20.11.1 Parsing of Integers

The ‘`str`’ functions are declared in `stdlib.h` and those beginning with ‘`wcs`’ are declared in `wchar.h`. One might wonder about the use of `restrict` in the prototypes of the functions in this section. It is seemingly useless but the ISO C standard uses it (for the functions defined there) so we have to do it as well.

— Function: long int strtol (const char *restrict string, char **restrict tailptr, int base)

> The `strtol` (“string-to-long”) function converts the initial part of string to a signed integer, which is returned as a value of type `long int`.

> This function attempts to decompose string as follows:

> - A (possibly empty) sequence of whitespace characters. Which characters are whitespace is determined by the `isspace` function (see Classification of Characters). These are discarded.
> - An optional plus or minus sign (‘`+`’ or ‘`-`’).
> - A nonempty sequence of digits in the radix specified by base.

>   If base is zero, decimal radix is assumed unless the series of digits begins with ‘`0`’ (specifying octal radix), or ‘`0x`’ or ‘`0X`’ (specifying hexadecimal radix); in other words, the same syntax used for integer constants in C.

>   Otherwise base must have a value between `2` and `36`. If base is `16`, the digits may optionally be preceded by ‘`0x`’ or ‘`0X`’. If base has no legal value the value returned is `0l` and the global variable `errno` is set to `EINVAL`.

> - Any remaining characters in the string. If tailptr is not a null pointer, `strtol` stores a pointer to this tail in `*`tailptr.

> If the string is empty, contains only whitespace, or does not contain an initial substring that has the expected syntax for an integer in the specified base, no conversion is performed. In this case, `strtol` returns a value of zero and the value stored in `*`tailptr is the value of string.

> In a locale other than the standard `"C"` locale, this function may recognize additional implementation-dependent syntax.

> If the string has valid syntax for an integer but the value is not representable because of overflow, `strtol` returns either `LONG_MAX` or `LONG_MIN` (see Range of Type), as appropriate for the sign of the value. It also sets `errno` to `ERANGE` to indicate there was overflow.

> You should not check for errors by examining the return value of `strtol`, because the string might be a valid representation of `0l`, `LONG_MAX`, or `LONG_MIN`. Instead, check whether tailptr points to what you expect after the number (e.g. `'\0'` if the string should end after the number). You also need to clear errno before the call and check it afterward, in case there was overflow.

> There is an example at the end of this section.

— Function: long int wcstol (const wchar_t *restrict string, wchar_t **restrict tailptr, int base)

> The `wcstol` function is equivalent to the `strtol` function in nearly all aspects but handles wide character strings.

> The `wcstol` function was introduced in Amendment 1 of ISO C90.

— Function: unsigned long int strtoul (const char *retrict string, char **restrict tailptr, int base)

> The `strtoul` (“string-to-unsigned-long”) function is like `strtol` except it converts to an `unsigned long int` value. The syntax is the same as described above for `strtol`. The value returned on overflow is `ULONG_MAX` (see Range of Type).

> If string depicts a negative number, `strtoul` acts the same as strtol but casts the result to an unsigned integer. That means for example that `strtoul` on `"-1"` returns `ULONG_MAX` and an input more negative than `LONG_MIN` returns (`ULONG_MAX` + 1) / 2.

> `strtoul` sets errno to `EINVAL` if base is out of range, or `ERANGE` on overflow.

— Function: unsigned long int wcstoul (const wchar_t *restrict string, wchar_t **restrict tailptr, int base)

> The `wcstoul` function is equivalent to the `strtoul` function in nearly all aspects but handles wide character strings.

> The `wcstoul` function was introduced in Amendment 1 of ISO C90.

— Function: long long int strtoll (const char *restrict string, char **restrict tailptr, int base)

> The `strtoll` function is like `strtol` except that it returns a `long long int` value, and accepts numbers with a correspondingly larger range.

> If the string has valid syntax for an integer but the value is not representable because of overflow, `strtoll` returns either `LLONG_MAX` or `LLONG_MIN` (see Range of Type), as appropriate for the sign of the value. It also sets `errno` to `ERANGE` to indicate there was overflow.

> The `strtoll` function was introduced in ISO C99.

— Function: long long int wcstoll (const wchar_t *restrict string, wchar_t **restrict tailptr, int base)

> The `wcstoll` function is equivalent to the `strtoll` function in nearly all aspects but handles wide character strings.

> The `wcstoll` function was introduced in Amendment 1 of ISO C90.

— Function: long long int strtoq (const char *restrict string, char **restrict tailptr, int base)

> `strtoq` (“string-to-quad-word”) is the BSD name for `strtoll`.

— Function: long long int wcstoq (const wchar_t *restrict string, wchar_t **restrict tailptr, int base)

> The `wcstoq` function is equivalent to the `strtoq` function in nearly all aspects but handles wide character strings.

> The `wcstoq` function is a GNU extension.

— Function: unsigned long long int strtoull (const char *restrict string, char **restrict tailptr, int base)

> The `strtoull` function is related to `strtoll` the same way `strtoul` is related to `strtol`.

> The `strtoull` function was introduced in ISO C99.

— Function: unsigned long long int wcstoull (const wchar_t *restrict string, wchar_t **restrict tailptr, int base)

> The `wcstoull` function is equivalent to the `strtoull` function in nearly all aspects but handles wide character strings.

> The `wcstoull` function was introduced in Amendment 1 of ISO C90.

— Function: unsigned long long int strtouq (const char *restrict string, char **restrict tailptr, int base)

> `strtouq` is the BSD name for `strtoull`.

— Function: unsigned long long int wcstouq (const wchar_t *restrict string, wchar_t **restrict tailptr, int base)

> The `wcstouq` function is equivalent to the `strtouq` function in nearly all aspects but handles wide character strings.

> The `wcstouq` function is a GNU extension.

— Function: intmax_t strtoimax (const char *restrict string, char **restrict tailptr, int base)

> The `strtoimax` function is like `strtol` except that it returns a `intmax_t` value, and accepts numbers of a corresponding range.

> If the string has valid syntax for an integer but the value is not representable because of overflow, `strtoimax` returns either `INTMAX_MAX` or `INTMAX_MIN` (see Integers), as appropriate for the sign of the value. It also sets `errno` to `ERANGE` to indicate there was overflow.

> See Integers for a description of the `intmax_t` type. The `strtoimax` function was introduced in ISO C99.

— Function: intmax_t wcstoimax (const wchar_t *restrict string, wchar_t **restrict tailptr, int base)

> The `wcstoimax` function is equivalent to the `strtoimax` function in nearly all aspects but handles wide character strings.

> The `wcstoimax` function was introduced in ISO C99.

— Function: uintmax_t strtoumax (const char *restrict string, char **restrict tailptr, int base)

> The `strtoumax` function is related to `strtoimax` the same way that `strtoul` is related to `strtol`.

> See Integers for a description of the `intmax_t` type. The `strtoumax` function was introduced in ISO C99.

— Function: uintmax_t wcstoumax (const wchar_t *restrict string, wchar_t **restrict tailptr, int base)

> The `wcstoumax` function is equivalent to the `strtoumax` function in nearly all aspects but handles wide character strings.

> The `wcstoumax` function was introduced in ISO C99.

— Function: long int atol (const char *string)

> This function is similar to the `strtol` function with a base argument of `10`, except that it need not detect overflow errors. The `atol` function is provided mostly for compatibility with existing code; using `strtol` is more robust.

— Function: int atoi (const char *string)

> This function is like `atol`, except that it returns an `int`. The `atoi` function is also considered obsolete; use `strtol` instead.

— Function: long long int atoll (const char *string)

> This function is similar to `atol`, except it returns a `long long int`.

> The `atoll` function was introduced in ISO C99. It too is obsolete (despite having just been added); use `strtoll` instead.

All the functions mentioned in this section so far do not handle alternative representations of characters as described in the locale data. Some locales specify thousands separator and the way they have to be used which can help to make large numbers more readable. To read such numbers one has to use the `scanf` functions with the ‘`'`’ flag.

Here is a function which parses a string as a sequence of integers and returns the sum of them:

         int
         sum_ints_from_string (char *string)
         {
           int sum = 0;

           while (1) {
             char *tail;
             int next;

             /* Skip whitespace by hand, to detect the end.  */
             while (isspace (*string)) string++;
             if (*string == 0)
               break;

             /* There is more nonwhitespace,  */
             /* so it ought to be another number.  */
             errno = 0;
             /* Parse it.  */
             next = strtol (string, &tail, 0);
             /* Add it in, if not overflow.  */
             if (errno)
               printf ("Overflow\n");
             else
               sum += next;
             /* Advance past it.  */
             string = tail;
           }

           return sum;
         }