strtok doesn't work with non-const char argument

Question

I have this function that take a sentence and reverse each words. I have to modify the value in-place and the return value should be Null. I can't modify the main:

int main()
{
    char *string= "hello";
    reverser(string);
    printf("%s\n", string);
}

In my reverser function i use strtok which require a non-const char*

char* reverser(char *sentence) {
    char *copy = strdup(sentence);
    char *string;
    int i, j;

    for(j = 0; (string = strtok(j ? NULL : copy, " ")) != NULL; j++)
        for(i = strlen(string) - 1; i >= 0; --i, j++)
            sentence[j] = string[i];
    return NULL;
}

Even using strdup it doesn't work and I can't figure out why... Someone has any suggestion to make it work? thank you

Answer 1

Replace char *string= "hello"; with char string[] = "hello";

Otherwise string will be unmodifiable, and it would be impossible for the reverser function to work. (given that it will always return NULL)