Issue with dynamic C array

Question

I'm trying to make a dynamic char array, but I'm running into issues. I get segmentation fault when I try to add anything to the array.

static int arrays()
   {
    int INITIAL = 1;
    char **all_words;
    all_words = (char**)malloc(INITIAL*sizeof(*all_words));
    int currentSize = INITIAL;

    for(int i = 0; i < counter; i++){
        all_words = realloc(all_words,currentSize*sizeof(*all_words));
        strcpy(all_words[i], "hello");
        currentSize++;
    }
    for(int i = 0; i < counter; i++){
        printf("%s ", all_words[i]);
    }

}

I pretty much copied this from a guide online, so I'm not sure why it wouldn't work.

Don't forget to check that your malloc and realloc succeeded. — Fred Larson
– Fred Larson, Commented Mar 10, 2022 at 18:50
Are you trying to have a dynamic array of strings, or a dynamic array of char (holding one string)? — hyde
– hyde, Commented Mar 10, 2022 at 18:51
Why is INITIAL = 1 when you put nothing in the array? You should start at 0 with all_words = NULL. Increment currentSize before the realloc. Next replace strcpy with strdup so you actually allocate memory for the strings. — Goswin von Brederlow
– Goswin von Brederlow, Commented Mar 10, 2022 at 23:25

dbush · Accepted Answer · 2022-03-10 18:48:58Z

3

You've correctly allocated an array of char *, however those pointers remain uninitiaized. So when you then do this:

strcpy(all_words[i], "hello");

You're dereferencing an invalid pointer.

Each element of all_words needs to point to allocated space. This simplest way to do this is to use strdup if your system supports it instead of strcpy:

all_words[i] = strdup("hello");

Otherwise you would use malloc to allocate the space, then use strcpy

all_words[i] = malloc(sizeof("hello"));
strcpy(all_words[i], "hello");

answered Mar 10, 2022 at 18:48

dbush

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2 Comments

code1331 Over a year ago

Interesting. I recently did this with a struct called Words. Why didn't I need to allocate for every single index for that? array = (Words*)realloc(array, sizeof(Words)*++n); This was enough to do it.

dbush Over a year ago

@code1331 Because a struct (assuming Words is a struct) is not a pointer. When you have an array of pointers, those pointers still need to be set to a valid value before they can be dereferenced.

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