Write your own strtok in C

This is just an interesting program I ever come across in an interview, and I almost spent 3-4 hours to completely understanding this after the interview. Yeah. I'm such a bad programmer. To make life easier, I added comments in the complex legs of the code.

Please leave a comment if I miss to add something or if you are not able to understand it.

#include <stdio.h>
#include <string.h>

#define SIZE_OF_STRING 50 
#define SIZE_OF_DELIMITER 5

/* Make the temp_ptr as static, so it will hold the previous pointing address */
static char *temp_ptr = NULL;

char *string_token(char *str, char *delimiter)
{
    char *final_ptr = NULL;  
    /* Flag has been defined as static to avoid the parent function loop
     * runs infinitely after reaching end of string.
     */ 
    static int flag = 0;
    int i, j;

    if (delimiter == NULL) {
        return NULL;
    }

    /* If flag is 1, it will denote the end of the string */
    if (flag == 1) {
        return NULL;
    }

    /* The below condition is to avoid temp_ptr is getting assigned 
     * with NULL string from the parent function main. Without
     * the below condition, NULL will be assigned to temp_ptr 
     * and final_ptr, so accessing these pointers will lead to
     * segmentation fault.
     */
    if (str != NULL) { 
        temp_ptr = str; 
    }

    /* Before function call ends, temp_ptr should move to the next char,
     * so we can't return the temp_ptr. Hence, we introduced a new pointer
     * final_ptr which points to temp_ptr.
     */
    final_ptr = temp_ptr;

    printf("%s %d str: %s delimiter: %s length: %ld temp_ptr: %s strlen: %ld"
           " final_ptr: %s \n",__func__, __LINE__, str, delimiter, 
           strlen(delimiter), temp_ptr, strlen(temp_ptr), final_ptr);

    for (i = 0; i <= strlen(temp_ptr); i++)
    {
        for (j = 0; j < strlen(delimiter); j++) {

            if (temp_ptr[i] == '\0') {
                /* If the flag is not set, both the temp_ptr and flag_ptr 
                 * will be holding string "Jasmine" which will make parent 
                 * to call this function string_token infinitely. 
                 */
                flag = 1;
                return final_ptr;
            }
            
            if ((temp_ptr[i] == delimiter[j])) {
                /* NULL character is assigned, so that final_ptr will return 
                 * til NULL character. Here, final_ptr points to temp_ptr.
                 */
                temp_ptr[i] = '\0';
                temp_ptr += i+1;
                return final_ptr;
            }
        }
    }
    /* You will somehow end up here if for loop condition fails.
     * If this function doesn't return any char pointer, it will 
     * lead to segmentation fault in the parent function.
     */
    return NULL;
}


int main()
{
    char str[SIZE_OF_STRING] = "shirley|Rose|Jasmine";
    char *token = NULL;
    char del[SIZE_OF_DELIMITER] = "|";

    token = string_token(str, del);
    while (token != NULL) {       
        printf("token %s\n", token);
        token = string_token(NULL, del);
    }
    return 0;
}

Output:

string_token 46 str: shirley|Rose|Jasmine delimiter: | length: 1 temp_ptr: shirley|Rose|Jasmine strlen: 20 final_ptr: shirley|Rose|Jasmine token shirley string_token 46 str: (null) delimiter: | length: 1 temp_ptr: Rose|Jasmine strlen: 12 final_ptr: Rose|Jasmine token Rose string_token 46 str: (null) delimiter: | length: 1 temp_ptr: Jasmine strlen: 7 final_ptr: Jasmine token Jasmine

Source: Stackoverflow