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### 2.35 `strtol`—string to long

Synopsis

         #include <stdlib.h>
         long strtol(const char *restrict s, char **restrict ptr,int base);

         long _strtol_r(void *reent,
             const char *restrict s, char **restrict ptr,int base);

Description
The function `strtol` converts the string `*`s to a `long`. First, it breaks down the string into three parts: leading whitespace, which is ignored; a subject string consisting of characters resembling an integer in the radix specified by base; and a trailing portion consisting of zero or more unparseable characters, and always including the terminating null character. Then, it attempts to convert the subject string into a `long` and returns the result.

If the value of base is 0, the subject string is expected to look like a normal C integer constant: an optional sign, a possible ``0x`' indicating a hexadecimal base, and a number. If base is between 2 and 36, the expected form of the subject is a sequence of letters and digits representing an integer in the radix specified by base, with an optional plus or minus sign. The letters `a`–`z` (or, equivalently, `A`–`Z`) are used to signify values from 10 to 35; only letters whose ascribed values are less than base are permitted. If base is 16, a leading `0x` is permitted.

The subject sequence is the longest initial sequence of the input string that has the expected form, starting with the first non-whitespace character. If the string is empty or consists entirely of whitespace, or if the first non-whitespace character is not a permissible letter or digit, the subject string is empty.

If the subject string is acceptable, and the value of base is zero, `strtol` attempts to determine the radix from the input string. A string with a leading `0x` is treated as a hexadecimal value; a string with a leading 0 and no `x` is treated as octal; all other strings are treated as decimal. If base is between 2 and 36, it is used as the conversion radix, as described above. If the subject string begins with a minus sign, the value is negated. Finally, a pointer to the first character past the converted subject string is stored in ptr, if ptr is not `NULL`.

If the subject string is empty (or not in acceptable form), no conversion is performed and the value of s is stored in ptr (if ptr is not `NULL`).

The alternate function `_strtol_r` is a reentrant version. The extra argument reent is a pointer to a reentrancy structure.

Returns
`strtol` returns the converted value, if any. If no conversion was made, 0 is returned.

`strtol` returns `LONG_MAX` or `LONG_MIN` if the magnitude of the converted value is too large, and sets `errno` to `ERANGE`.

Portability
`strtol` is ANSI.

No supporting OS subroutines are required.