How to return char *arr[] with variable length in C?

Question

I want to refactor a function so that I can use it for arrays of different lengths and return that newly created array so that other functions can access it. I can't make the array static since you can't have static arrays with dynamic length. I also can't use a global struct because that needs to take the length of the array and that has to be hardcoded I think. So the question is whether it is even possible to do something like this:

char* splitElementsArr(FILE* file){
    int length = countBlankLines(file);
    char *arr[length] // or maybe use malloc here

    ...Some operations to fill array

    return arr;

Are you sure it needs to be an array of pointers, not an array of `char`? — Barmar, Feb 02 '21 at 16:31
@Barmar The function is assumed to split the file into chunks, so I'm not suprised it returns an array of pointers. — xhienne, Feb 02 '21 at 16:39
@xhienne Good point. It was the fact that the array is sized based on the file length that confused me. But I guess that's just an upper bound. — Barmar, Feb 02 '21 at 16:40
@Barmar Right, the function `countFileLength` is probably ill-named. — xhienne, Feb 02 '21 at 16:41
@Barmar yes I accidentally used the wrong function ... may bad — Phil, Feb 02 '21 at 16:43

score 0 · Answer 1 · answered Feb 02 '21 at 16:30

0

No, you can't. You are returning a pointer to local data that were dynamically allocated on the stack. As soon as you return, those data no longer exist.

Solution : use malloc().

answered Feb 02 '21 at 16:30

xhienne

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And then just call free() at the end of whole program? – Phil Feb 02 '21 at 16:34
Whenever you don't need those data any longer, you call `free()`. One `free()` for each `malloc()`. – xhienne Feb 02 '21 at 16:35
You'll also need to free all the `arr[i]`. @Phil – Barmar Feb 02 '21 at 16:35
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@Phil And do all `free(arr[i])` before `free(arr)` :-) – xhienne Feb 02 '21 at 16:45

How to return char *arr[] with variable length in C?

1 Answers1