# strtok, strtok_s

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< c‎ | string‎ | byte

Defined in header `<string.h>` |  |
| (1) |
char *strtok( char *str, const char *delim ); |  | (until C99)
char *strtok( char *restrict str, const char *restrict delim ); |  | (since C99)
char *strtok_s(char *restrict str, rsize_t *restrict strmax, const char *restrict delim, char **restrict ptr); | (2) | (since C11)

1) Finds the next token in a null-terminated byte string pointed to by `str`. The separator characters are identified by null-terminated byte string pointed to by `delim`.
This function is designed to be called multiple times to obtain successive tokens from the same string.

  - If `str` is not a null pointer, the call is treated as the first call to `strtok` for this particular string. The function searches for the first character which is not contained in `delim`.
    - If no such character was found, there are no tokens in `str` at all, and the function returns a null pointer.
    - If such character was found, it is the beginning of the token. The function then searches from that point on for the first character that is contained in `delim`.
      - If no such character was found, `str` has only one token, and future calls to `strtok` will return a null pointer
      - If such character was found, it is replaced by the null character '\0' and the pointer to the following character is stored in a static location for subsequent invocations.
    - The function then returns the pointer to the beginning of the token
  - If `str` is a null pointer, the call is treated as a subsequent call to `strtok`: the function continues from where it left in previous invocation. The behavior is the same as if the previously stored pointer is passed as str.

The behavior is undefined if either `str` or `delim` is not a pointer to a null-terminated byte string.
2) Same as (1), except that on every step, writes the number of characters left to see in `str` into *strmax and writes the tokenizer's internal state to *ptr. Repeat calls (with null `str`) must pass `strmax` and `ptr` with the values stored by the previous call. Also, the following errors are detected at runtime and call the currently installed constraint handler function, without storing anything in the object pointed to by `ptr`

  - `strmax`, `delim`, or `ptr` is a null pointer
  - on a non-initial call (with null `str`), `*ptr` is a null pointer
  - on the first call, *strmax is zero or greater than RSIZE_MAX
  - search for the end of a token reaches the end of the source string (as measured by the initial value of *strmax) without encountering the null terminator

The behavior is undefined if both `str` points to a character array which lacks the null character and `strmax` points to a value which is greater than the size of that character array.

  As with all bounds-checked functions, `strtok_s` only guaranteed to be available if __STDC_LIB_EXT1__ is defined by the implementation and if the user defines __STDC_WANT_LIB_EXT1__ to the integer constant 1 before including `<string.h>`.

### Parameters

str | - | pointer to the null-terminated byte string to tokenize
delim | - | pointer to the null-terminated byte string identifying delimiters
strmax | - | pointer to an object which initially holds the size of `str`: strtok_s stores the number of characters that remain to be examined
ptr | - | pointer to an object of type char*, which is used by strtok_s to store its internal state

### Return value

Returns pointer to the beginning of the next token or a null pointer if there are no more tokens.

### Note

This function is destructive: it writes the '\0' characters in the elements of the string `str`. In particular, a string literal cannot be used as the first argument of `strtok`.

Each call to `strtok` modifies a static variable: is not thread safe.

Unlike most other tokenizers, the delimiters in `strtok` can be different for each subsequent token, and can even depend on the contents of the previous tokens.

The `strtok_s` function differs from the POSIX `strtok_r` function by guarding against storing outside of the string being tokenized, and by checking runtime constraints. The Microsoft CRT `strtok_s` signature matches this POSIX `strtok_r` definition, not the C11 `strtok_s`.

### Example

Run this code

    #define __STDC_WANT_LIB_EXT1__ 1
    #include <string.h>
    #include <stdio.h>

    int main(void)
    {
        char input[] = "A bird came down the walk";
        printf("Parsing the input string '%s'\n", input);
        char *token = strtok(input, " ");
        while(token) {
            puts(token);
            token = strtok(NULL, " ");
        }

        printf("Contents of the input string now: '");
        for(size_t n = 0; n < sizeof input; ++n)
            input[n] ? putchar(input[n]) : fputs("\\0", stdout);
        puts("'");

    #ifdef __STDC_LIB_EXT1__
        char str[] = "A bird came down the walk";
        rsize_t strmax = sizeof str;
        const char *delim = " ";
        char *next_token;
        printf("Parsing the input string '%s'\n", str);
        token = strtok_s(str, &strmax, delim, &next_token);
        while(token) {
            puts(token);
            token = strtok_s(NULL, &strmax, delim, &next_token);
        }

        printf("Contents of the input string now: '");
        for(size_t n = 0; n < sizeof str; ++n)
            str[n] ? putchar(str[n]) : fputs("\\0", stdout);
        puts("'");
    #endif
    }

Possible output:

    Parsing the input string 'A bird came down the walk'
    A
    bird
    came
    down
    the
    walk
    Contents of the input string now: 'A\0bird\0came\0down\0the\0walk\0'
    Parsing the input string 'A bird came down the walk'
    A
    bird
    came
    down
    the
    walk
    Contents of the input string now: 'A\0bird\0came\0down\0the\0walk\0'

### References

- C11 standard (ISO/IEC 9899:2011):

  - 7.24.5.8 The strtok function (p: 369-370)

  - K.3.7.3.1 The strtok_s function (p: 620-621)

- C99 standard (ISO/IEC 9899:1999):

  - 7.21.5.8 The strtok function (p: 332-333)

- C89/C90 standard (ISO/IEC 9899:1990):

  - 4.11.5.8 The strtok function

### See also

strpbrk | finds the first location of any character in one string, in another string (function)
strcspn | returns the length of the maximum initial segment that consists of only the characters not found in another byte string (function)
strspn | returns the length of the maximum initial segment that consists of only the characters found in another byte string (function)
wcstok wcstok_s (C95)(C11) | finds the next token in a wide string (function)

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