C Programming - Memory Allocation - Discussion

Here p is a pointer.so sizeof(p) is either become 4 bytes or 8 bytes.But here how sizeof(p) is 2 bytes.

int is 2 bytes with 4 elements so sizeof(*p) is 8 bytes (2*4). So the answer is 4(or)8, 8.

Here:

P = (int (*) [MAXCOL]) malloc (MAXROW *sizeof (*p)); is allocating a memory of 24 bytes to the pointer P but as it is a pointer to 2D array it will hold the base Row address i.e. 8.
Therefore *P holds up 8*3 bytes but will show 8 bytes i.e the size of its row in sizeof(*P).
And P is itself a pointer to an array of 2*4 bytes and its row holds 2 bytes each.

Actual memory consumed by both members is 2*4 and 8*3 bytes but a pointer holds the base address and thus the first element's address will be shown in sizeof() i.e. 2 and 8 bytes.

I don't understand.

P = (int (*) [MAXCOL]) malloc (MAXROW *sizeof (*p));

So, sizeof (*p) 4 * 2 and malloc gives 3 * 8.

So, the sizeof (*p) will be 24.

What is the difference between the two programs given below?

1.

#include<stdio.h>
#include<stdlib.h>
#define MAXROW 3
#define MAXCOL 4

int main()
{
int (*p)[MAXCOL];
printf("%d %d\n",sizeof(p),sizeof(*p));
// p = (int (*) [MAXCOL])malloc(MAXROW *sizeof(*p));
printf("%d %d\n",sizeof(p),sizeof(*p));
return 0;
}

2.

#include<stdio.h>
#include<stdlib.h>
#define MAXROW 3
#define MAXCOL 4

int main()
{
int (*p)[MAXCOL];
printf("%d %d\n",sizeof(p),sizeof(*p));
p = (int (*) [MAXCOL])malloc(MAXROW *sizeof(*p));
printf("%d %d\n",sizeof(p),sizeof(*p));
return 0;
}

#include<stdio.h>
#include<stdlib.h>
#define MAXROW 3
#define MAXCOL 4

int main()
{
int (*p)[MAXCOL];
p = (int (*) [MAXCOL])malloc(MAXROW *sizeof(*p));
return 0;
}

What is the difference with this code? It gives the output 24. Please explain.

Sizeof (p) = 2 because in p address of allocated memory is stored which is 2 Bytes.

Sizeof (*p) = 8 because *p is pointer to array of 4 Integers 4*2 = 8.

If you compile this code you get output 4, 16. And in answer they put 2, 8.

#include<stdio.h>
#include<stdlib.h>
#define MAXROW 3
#define MAXCOL 4

int main()
{
int (*p)[MAXCOL];
p = (int (*) [MAXCOL])malloc(MAXROW *sizeof(*p));
printf("%d, %d\n", sizeof(p), sizeof(*p));
return 0;
}

Could someone explain this code to me?

Integer takes a 4 bytes and 2 bytes sizeof operator than

2*4 = 8 bytes.

sizeof(*p) = 8 bytes.

Why does the line p = (int (*) [MAXCOL])malloc(MAXROW *sizeof(*p)); has no effect?

Can anyone explain its reason in detail. Please?