Error (incompatible types) when initialising struct array

Question

I've read some similar questions to this but I still can't see where I'm going wrong.

I malloc the pointers and that seems to work OK, but I get an error (incompatible types) on this line:

canArray [i] = (TinCan *) malloc(sizeof(TinCan))

Here is the complete code:

typedef struct TinCan
{
    int date;
    int time;
} TinCan;

int main ()
{
    int i;
    TinCan *canArray = malloc(10 * sizeof(TinCan));

    for (i =0; i < 9; i++ )
    {
        canArray [i] = (TinCan *) malloc(sizeof(TinCan));
    }
}

score 4 · Accepted Answer · edited May 23 '17 at 12:04

Guessing that you have a typedef struct {...} TinCan; somewhere, then here:

TinCan *canArray = malloc(10 * sizeof(TinCan));

You have enough space for 10 TinCan structures, but here:

canArray [i] = (TinCan *) malloc(sizeof(TinCan));

You are trying to allocate space for another TinCan struct.

Do you want:

An array of TinCans? If so, you don't need the loop - the space has already been allocated when you asked for 10 * sizeof(TinCan)
An array of pointers to TinCan structs? If so, change the first line to:
```
TinCan **canArray = malloc(10 * sizeof(canArray[0]));
```
and keep the loop.

Some general comments:

You don't need the cast before the malloc() call - see - Do I cast the result of malloc?
It's good practice to use sizeof(varname[0]) rather than sizeof(typename), to avoid (or make more obvious) silly mistakes.
With the current loop code, you will leave the last TinCan uninitialised - you're creating 10 entries in canArray, but only initialising 9 of them with i < 9. Change that to i < 10, or for extra credit, swap out both for a #define NUMBER_OF_CANS 10

Thanks, I understand now. If I want to pass that array now to a method for initialisation, would it look like this? intitialise(canArray**)? It's not working for me. — Dawson, May 01 '13 at 06:46
It doesn't matter. You can have a function initialise an array, or an array of pointers. I would probably go with the first approach (a pointer to 10 `TinCan`s), since the code will be cleaner. — Timothy Jones, May 01 '13 at 06:48
Also, it's important to remember that arrays and pointers aren't the same thing - if that sounds confusing, have a read of http://c-faq.com/aryptr/aryptr2.html — Timothy Jones, May 01 '13 at 06:50

score 3 · Answer 2 · answered May 01 '13 at 06:35

3

You have declared an array of TinCan structs, but your loop suggests you want an array of pointers to TinCan structs.

Change the declaration of canArray to:

TinCan *canArray[] = malloc(10 * sizeof(TinCan*));

answered May 01 '13 at 06:35

Paul Hankin

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Error (incompatible types) when initialising struct array

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