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));
    }
}

Community · Accepted Answer · 2017-05-23 12:04:58Z

4

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

edited May 23, 2017 at 12:04

CommunityBot

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answered May 1, 2013 at 6:37

Timothy Jones

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3 Comments

Dawson Over a year ago

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.

Timothy Jones Over a year ago

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 TinCans), since the code will be cleaner.

Timothy Jones Over a year ago

Also, it's important to remember that arrays and pointers aren't the same thing - if that sounds confusing, have a read of c-faq.com/aryptr/aryptr2.html

Paul Hankin · Accepted Answer · 2013-05-01 06:35:13Z

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 1, 2013 at 6:35

Paul Hankin

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