Allocating array of structs dynamically with malloc

Question

I am trying to create dynamically an array of structs . Although with small sizes seems to work fine but the program crushes later(segmentation fault ) . When in have a big ipt_size it crushes very fast with nosense numbers so i assume the problem is in the way i allocate my array.

int create_ipt(int ipt_size,frame **ipt){
    int i;
    *ipt=malloc(ipt_size*sizeof(frame*));
 for(i=0;i<ipt_size;i++){
    ipt[i]=malloc(sizeof(frame));
     if (ipt[i]==NULL) return 1; //error
    ipt[i]->empty=1;
   }
 return 0; //success
}//create page table

I call the function with

     frame *ipt;
    create_ipt(ipt_size,&ipt);

Do you know what's happening ? frame has 3 ints inside

*ipt=malloc(ipt_size*sizeof(frame*)); --> *ipt=malloc(ipt_size*sizeof(frame));.. if (*ipt==NULL) return 1; for(i=0;i<ipt_size;i++){ (*ipt)[i].empty=1; } — BLUEPIXY
– BLUEPIXY, Commented Jan 13, 2017 at 16:31
Thank you man . That was the way . add your comment as an answer to make it green :) — panos
– panos, Commented Jan 13, 2017 at 16:46

Sourav Ghosh · Accepted Answer · 2017-01-13 16:46:46Z

2

Look at the type of *ipt, it is frame *, that means, it's a pointer to type frame, i.e., points to a type frame. So you need to allocate memory which is capable of holding type frame and allocate the address to *ipt.

Change the memory allocation to reflect the same. Change

 *ipt=malloc(ipt_size*sizeof(frame*));

to

*ipt=malloc(ipt_size*sizeof(frame));

Then, have a look and revise (remove) the following statement ipt[i]=malloc(sizeof(frame));, you have already allocated memory for holding ipt_size number of elements.

After that, look at the dereference statement. Instead of

ipt[i]->empty=1;

you need to write

(*ipt)[i].empty = 1;

and that explicit parenthesis is there because of (to avoid the unwanted effect of) operator precedence.

edited Jan 13, 2017 at 16:46

answered Jan 13, 2017 at 16:33

Sourav Ghosh

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Comments

cpatricio · Accepted Answer · 2017-01-13 17:13:51Z

0

In create_ipt, you use:

ipt to access **ipt (pointer to the pointer you created in the main)

*ipt to access *ipt (pointer to the struct frame)

**ipt to access ipt (struct frame itself)

int create_ipt(int ipt_size, frame **ipt)
{
        int i;
        *ipt = malloc(ipt_size*sizeof(frame*)); // *ipt is the pointer to the struct frame you want to allocate memory)
        for(i=0; i < ipt_size; i++)
        {
                // If you only have integers you don't need to allocate memory to the struct, it's already allocated, but if you have char* you should malloc that var
                (*ipt)[i].text = malloc(text_size*sizeof(char));
                (*ipt)[i].empty = 1;
        }
        return 0;
}

answered Jan 13, 2017 at 17:13

cpatricio

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