strtok doesn't return any value

Question

I want to write a programm that converts a string with numbers ("1 2 3") into an integer array. But strtok() doesn't return a value. Why not? My console output is just empty.

Edit: I get no error message.

This is my code:

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

int* string_to_array(char * string, int * myArray);

int main(void) {
    int* myArray = 0;
    myArray = (int*) malloc(10 * sizeof(int));
    myArray = string_to_array("1 2 3 4", myArray);

    printf("result: %d\n", myArray[0]);
    printf("result: %d\n", myArray[1]);
    return 0;
}

int* string_to_array(char * string, int * myArray){
    char delimiter[] = " ";
    char *ptr;
    char *ptr2;
    long ret;

    printf("string_to_array() was called.\n");
    printf("string is: %s\n", string);

    ptr = strtok(string, delimiter);            // here is the problem
    printf("strtok done; ptr: %s\n", ptr);

    int index = 0;
    while(ptr != NULL) {        
        ret = strtol(ptr, &ptr2, 10);
        printf("ret: %d\n", ret);
        myArray[index] = ret;
        ptr = strtok(NULL, delimiter);
        index++;
    }

    return myArray;
}

You cannot call `strtok` on string literals i.e.`"1 2 3 4"`. `strtok` modifies its first param. string literals are only to be read. Put `"1 2 3 4"` into char array. — rafix07, Sep 17 '19 at 07:54
@kame Use either sscanf or strspn and strcspn to extract numbers. — Vlad from Moscow, Sep 17 '19 at 07:57
@rafix07 Sorry. I am still a beginner. I assumed string and char array is the same. But if not how are they different? — kame, Sep 17 '19 at 07:58
Aside: Consider the tidy: `myArray = malloc(sizeof *myArray * 10);` than `myArray = (int*) malloc(10 * sizeof(int));`. — chux - Reinstate Monica, Sep 17 '19 at 11:51
kame, "string and char array is the same. But if not how are they different?" --> [`What is the difference between char s[] and char *s?`](https://stackoverflow.com/q/1704407/2410359) — chux - Reinstate Monica, Sep 17 '19 at 11:53
@kame [Do I cast the result of malloc?](https://stackoverflow.com/q/605845/2410359) is an interesting read. Yet I would not alter this question concerning that as it is a minor point. — chux - Reinstate Monica, Sep 17 '19 at 12:39

score 4 · Accepted Answer · answered Sep 17 '19 at 08:11

Since strtol is being use to parse the values, use ptr2 to iterate through string. After a conversion, ptr2 will point to the next character following the converted value. Use it as the starting point for the next conversion. More error checking can be done using strtol.

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

int* string_to_array(char * string, int * myArray);

int main(void) {
    int* myArray = 0;
    myArray = (int*) malloc(10 * sizeof(int));
    myArray = string_to_array("1 2 3 4", myArray);

    printf("result: %d\n", myArray[0]);
    printf("result: %d\n", myArray[1]);
    return 0;
}

int* string_to_array(char * string, int * myArray){
    char *ptr;
    char *ptr2;
    long ret;

    printf("string_to_array() was called.\n");
    printf("string is: %s\n", string);

    ptr = string;

    int index = 0;
    while(*ptr) {
        ret = strtol(ptr, &ptr2, 10);
        printf("ret: %ld\n", ret);
        myArray[index] = ret;
        ptr = ptr2;
        index++;
    }

    return myArray;
}

score 4 · Answer 2 · answered Sep 17 '19 at 08:36

As rafix07 wrote in his comment, you cannot use strtok with a string literal like "1 2 3 4". On my system I get a segmentation fault in strtok because of this.

To use an array instead of a string literal you can modify your program like this:

int main(void) {
    int* myArray = 0;
    char input[] = "1 2 3 4"; /* This initializes a writable array with the string literal */

    myArray = (int*) malloc(10 * sizeof(int));
    myArray = string_to_array(input, myArray); /* pass array instead of literal */

    printf("result: %d\n", myArray[0]);
    printf("result: %d\n", myArray[1]);
    return 0;
}

Not that strtok modifies your original data. If you don't want this you can make your function string_to_array work on a copy of the data. With this modification you can call it with a string literal as in your original main function.

int* string_to_array(char * string, int * myArray){
    char delimiter[] = " ";
    char *ptr;
    char *ptr2;
    long ret;
    char *data;
    data = strdup(string);

    /* FIXME error handling for strdup */

    printf("string_to_array() was called.\n");
    printf("string is: %s\n", string);

    ptr = strtok(data, delimiter);            // here is the problem
    printf("strtok done; ptr(%p): %s\n", ptr, ptr);

    int index = 0;
    while(ptr != NULL) {        
        ret = strtol(ptr, &ptr2, 10);
        printf("ret: %ld\n", ret);
        myArray[index] = ret;
        ptr = strtok(NULL, delimiter);
        index++;
    }

    free(data);
    return myArray;
}

Note that you don't need to return the input pointer myArray from string_to_array() and assign it to myArray in main() because the original array is modified.

Your function string_to_array() doesn't know how big you allocated your array. I suggest to pass the number of elements as a function parameter and check that you don't write behind the end of the array. You also don't return the number of elements in myArray you actually filled with data, so your main function might access uninitialized elements.

strtok doesn't return any value

2 Answers2