malloc() with array of structures

Ok...its been two days now and I can't get it yet! I googled for long hours but couldn't find what exactly I was looking for. So, here it is -

Say, I have a structure with only one element. (I'll add more elements to the structure once I get it to work!)

Code:

---

typedef struct
{
  int roll;
}STUDENT;

---

Now when the program is run, I want the user to specify the number of students and then allocate the necessary memory using malloc(). So, I did this -

Code:

---

    int num;
    REC *pt;
    pt= (REC *) malloc (num*sizeof(REC));  // I also tried this without casting!

---

Now, I am having difficulties in stroing the values and then displaying those values back to the screen. I used the following code -

Code:

---

        printf("\n* * * * Enter roll no of students. * * * *\n");
        for (i=0;i<num;i++)
        {
                printf("-->");
              scanf("%d",(pt+i)->roll);
        }

        printf("\n* * * * Entered roll no. * * * *\n");
        for (i=0;i<num;i++)
        {       
              printf("%d, ",*(pt+i)->roll);    //This is where the error occures!
        }

---

when I try do compile it, I get the error "invalid type argument of `unary *'".

How can I solve this?

P.S. I am using wxdev C++.

Don't get too fancy with pointer arithmetic. It's pt[i].roll. If you really really really want to do the pointer arithmetic explicitly, you will have to do this:

Code:

---

*((REC*)(pt+i))->roll

---

This is read as "take pt + i, cast the result to pointer to REC, and dereference it to get the member called 'roll'"

Thank you very much QuadraticFighte for your ultra quick reply :)

I did try using pt[i].roll. It compiles with no errors but after taking the inputs, the program crashes! Not sure whether this is related with wxdev C++ or not.

Again, when I use

Code:

---

printf("%d, ", *((REC*)(pt+i))->roll);

---

I get the previously mentioned error i.e. "invalid type argument of `unary *'

So, still no sign of light :confused:

can you attach your code ?

This:

Code:

---

int num;
REC *pt;
pt= (REC *) malloc (num*sizeof(REC));  // I also tried this without casting!

---

should be:

Code:

---

int num = 10; /* or whatever number is required */
REC *pt = malloc(num * sizeof(*pt));

---

Quote:

---

Originally Posted by laserlight

This:

Code:

---

int num = 10; /* or whatever number is required */

---

---

num is assigned a value during runtime by the user.

Quote:

---

Code:

---

REC *pt = malloc(num * sizeof(*pt));

---

---

The program still crashes after taking the inputs :confused: (Perhaps its a problem of the IDE I am using!)

You should post the smallest and simplest compilable program that demonstrates the crash.

ok...it seems to work...FINALLY !

Here's the code

Code:

---

typedef struct
{
    int roll;
}REC;

int main()
{
    int i, n =3;
    REC *pt= malloc(n*sizeof(*pt));
    printf("\n* * * * Enter roll no of students. * * * *\n");
    for (i=0;i<n;i++)
  {
        printf("-->");
        scanf("%d", &pt[i].roll);
    }
    printf("\n* * * * Entered roll no. * * * *\n");
    for (i=0;i<n;i++)
  {
        printf("%d, ", pt[i].roll); 
    }
    free(pt);
    getche();
    return 0;
}

---

I changed the code so many times that I cannot remember what I did in the first place ! However, I have one question

Code:

---

REC *pt= malloc(n*sizeof(*pt));

---

Why use sizeof(*pt) instead of sizeof(REC) ? or is it same thing?

Thanks to everyone for your kind help :)

Quote:

---

Originally Posted by khpuce

Why use sizeof(*pt) instead of sizeof(REC) ? or is it same thing?

---

If you change the type of pt to another type, no change on sizeof parameter is needed.