How to memory allocate a pointer inside a pointer?

Question

I'm new in C programming (my main area is java) and in java there exists the ArrayList which I can make like

    ArrayList<Class> arraylist = new ArrayList<Class>();

where my class Class can have several items, such as int, or string.

In c, I found I cannot do that but need to do something like that, so I did this

    typedef struct vectorObject {
      int myInt;
      char *charPointer;
    } vectorObject;

I define a pointer of my structure like this:

    vectorObject *listVectorObject;

and using

    #define MAX_FILE_SIZE   50000

when I want to allocate the memory of that, I use this:

    int i;
    listVectorObject = malloc(MAX_FILE_SIZE);
    if (listVectorObject == NULL ) {
         printf("Out of memory1\n");
         exit(1);
    }
    for (i = 0; i < MAX_FILE_SIZE; i++) {
         listVectorObject[i].charPointer= malloc(MAX_FILE_SIZE);
         if (listVectorObject[i].charPointer == NULL ) {
              printf("Out of memory2\n");
              exit(1);
         }
    }

The problem is I always get a

    Out of Memory2

I already tried everything and I cannot find where my mistake is. Could you please help me? Thanks!!

perhaps you have to cast `listVectorObject =(VectorObject *) malloc(MAX_FILE_SIZE);` ? — pinkpanther, Jun 04 '13 at 15:22
Yeah, do you realize you're allocating approximately 2.5GB of memory here? `MAX_FILE_SIZE * MAX_FILE_SIZE = 2500000000`. — aardvarkk, Jun 04 '13 at 15:23
For starters, you aren't using `malloc` correctly. Shouldn't it be `listVectorOject = malloc(MAX_FILE_SIZE * sizeof(VectorObject));`? — GWW, Jun 04 '13 at 15:23
[Please don't cast the return value of `malloc()` in C](http://stackoverflow.com/a/605858/28169). — unwind, Jun 04 '13 at 15:24
@unwind: Sorry I fixed my comment. Old habits die hard I guess. — GWW, Jun 04 '13 at 15:25
@aardvarkk this is the reason you're getting this error. You're using too much memory. — KrisSodroski, Jun 04 '13 at 15:25

Nicholaz · Accepted Answer · 2013-06-04T15:37:17.587

3

I don't think you want 50000 vectorObjects each with 50000 bytes char-buffer.

So look at this:

first making a list of 1000 vectorObjects
then for each of those allocating a memory chunk to the char-pointer

.

int i, howmany= 1000;
vectorObject *listVectorObject = malloc(sizeof(vectorObject)*howmany);  // list of 1000 vectorObjects
if (listVectorObject == NULL ) {
     printf("Out of memory1\n");
     exit(1);
}

// one file-size char-pointer for each vector object above
for (i = 0; i<howmany; i++) {
     listVectorObject[i].charPointer= malloc(MAX_FILE_SIZE);
     if (listVectorObject[i].charPointer == NULL ) {
          printf("Out of memory2\n");
          exit(1);
     }
}

edited Jun 04 '13 at 15:37

answered Jun 04 '13 at 15:27

Nicholaz

1,419
9
11

Thanks everyone for your response! It was the memory! (I stupidly thought it would not affect it... changed it to a smaller number and it worked like a charm!) Thanks again!! – liwuen Jun 04 '13 at 15:32
1

Glad to hear. Pls make sure to change the "sizeof(vectorObject)*howmany" in the first alloc also. – Nicholaz Jun 04 '13 at 15:33
Already did it! Wow I love the speed of your responses! Thanks again and sorry to bother you with such a dumb question (In java is not necessary to do this so I was a little bit confused) – liwuen Jun 04 '13 at 15:37
2

No worries, we already started there sometimes (and going fromm Java to C will be a path full of traps). – Nicholaz Jun 04 '13 at 15:38

How to memory allocate a pointer inside a pointer?

1 Answers1