CODEWITHSUNDEEP
cpointers
bobby
bobby

Reputation: 2789

Allocating an array of structure pointers

typedef struct _X
{
   int a;
   int b;
}X;

X** arrayOfXPointers;
arrayOfXPointers  = malloc( 10 * sizeof(X*));

I have something like the code shown above. I know that I have to allocate the individual arrayOfXPointers array elements.
My question is that before I do this, will arrayOfXPointers[0], arrayOfXPointers[1] etc be null or point to some junk value.

How can I check if an individual array element has been allocated previously or not, given that its value can be junk. Is there a way to dynamically initialise all the array elements as null?


From the answers using calloc seems to be the best bet, but is not guaranteed by the C standard

Another idea which came to mind, is running a loop and assigning arrayOfXPointers[i] to be NULL
Is there any other better portable method?

Upvotes: 0

Views: 91

Answers (4)

Dabo
Dabo

Reputation: 2373

As you using malloc elements of array will have some junk values. If you want them to be NULLs you can use calloc instead which is actually malloc + memset which zero-initializes the allocated piece of memory.

Edit: As @rmartinjak mentioned, standard does not guarantee that NULL will be represented as all-bits-zero, but it's defined as ((void *)0) by most C standard library implementations

In order to check if an individual array element is null or not you just check it 

if (arrayOfXPointers[0] == NULL)

BTW NULL definition is in <stddef.h>, so don't forget to include it.

Upvotes: 2

Jeyaram
Jeyaram

Reputation: 9474

From malloc() man page.

The malloc() function allocates size bytes and returns a pointer to the allocated memory. The memory is not initialized. So it may be NULL or some garbage value. If you concerned with content as NULL, calloc() is better option.

Upvotes: 1

Kissiel
Kissiel

Reputation: 1955

It will have junk in the memory. To check if pointer is NULL do: if (pointer==NULL) or more idiomatically if (!pointer).

Upvotes: 2

Lee Duhem
Lee Duhem

Reputation: 15121

My question is that before I do this, will arrayOfXPointers[0], arrayOfXPointers[1] etc be null or point to some junk value.

They could be NULL, could have junk values, could also be a mix of both.

How can I check if an individual array element is null or not

Something like if (arrayOfXPointers[0] == NULL) should be enough.

Upvotes: 0

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