split char string with multi-character delimiter in C

Question

I want to split a char *string based on multiple-character delimiter. I know that strtok() is used to split a string but it works with single character delimiter.

I want to split char *string based on a substring such as "abc" or any other sub-string. How that can be achieved?

Answer 1

Finding the point at which the desired sequence occurs is pretty easy: strstr supports that:

char str[] = "this is abc a big abc input string abc to split up";
char *pos = strstr(str, "abc");

So, at that point, pos points to the first location of abc in the larger string. Here's where things get a little ugly. strtok has a nasty design where it 1) modifies the original string, and 2) stores a pointer to the "current" location in the string internally.

If we didn't mind doing roughly the same, we could do something like this:

char *multi_tok(char *input, char *delimiter) {
    static char *string;
    if (input != NULL)
        string = input;

    if (string == NULL)
        return string;

    char *end = strstr(string, delimiter);
    if (end == NULL) {
        char *temp = string;
        string = NULL;
        return temp;
    }

    char *temp = string;

    *end = '\0';
    string = end + strlen(delimiter);
    return temp;
}

This does work. For example:

int main() {
    char input [] = "this is abc a big abc input string abc to split up";

    char *token = multi_tok(input, "abc");

    while (token != NULL) {
        printf("%s\n", token);
        token = multi_tok(NULL, "abc");
    }
}

produces roughly the expected output:

this is
 a big
 input string
 to split up

Nonetheless, it's clumsy, difficult to make thread-safe (you have to make its internal string variable thread-local) and generally just a crappy design. Using (for one example) an interface something like strtok_r, we can fix at least the thread-safety issue:

typedef char *multi_tok_t;

char *multi_tok(char *input, multi_tok_t *string, char *delimiter) {
    if (input != NULL)
        *string = input;

    if (*string == NULL)
        return *string;

    char *end = strstr(*string, delimiter);
    if (end == NULL) {
        char *temp = *string;
        *string = NULL;
        return temp;
    }

    char *temp = *string;

    *end = '\0';
    *string = end + strlen(delimiter);
    return temp;
}

multi_tok_t init() { return NULL; }

int main() {
    multi_tok_t s=init();

    char input [] = "this is abc a big abc input string abc to split up";

    char *token = multi_tok(input, &s, "abc");

    while (token != NULL) {
        printf("%s\n", token);
        token = multi_tok(NULL, &s, "abc");
    }
}

I guess I'll leave it at that for now though--to get a really clean interface, we really want to reinvent something like coroutines, and that's probably a bit much to post here.