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strtok, strtok_s

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 multiples times to obtain successive tokens from the same string.

If str != NULL, 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 == NULL, the call is treated as a subsequent calls 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 all bounds-checked functions, strtok_s is 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 NULL 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.

Example

#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] ? printf("%c", input[n]) : printf("\\0");
    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] ? printf("%c", str[n]) : printf("\\0");
    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