Total bytes allocated on dynamic heap

Question

Assume sizeof(int). Then, what's the total size of bytes that will be allocated on the dynamic heap? And can you please explain why?

#include<stdio.h>
#include<stdlib.h>
#define MAXROW 8
#define MAXCOL 27

int main()
{
    int (*p)[MAXCOL];
    p = (int (*) [MAXCOL])malloc(MAXROW *sizeof(*p));
    return0;
}

Well, I don't really care what it does it's just a question from a quiz... — dor132, Nov 26 '15 at 12:11
What? How did you make this conclusion? I don't need to know what it does further than that function. It just allocates the memory and that's it, and the question is how many bytes were allocated on the heap... — dor132, Nov 26 '15 at 12:15
Hint: the total size allocated from the heap depends on the size of the `int` type and available memory. — chqrlie, Nov 26 '15 at 13:55

score 0 · Answer 1 · edited May 23 '17 at 12:07

0

In your code,

int (*p)[MAXCOL];

is eqivalent to saying, declare p as a pointer to an array of MAXCOL number of ints.

So, considering sizeof(int) is 4 bytes (32 bit compiler / platform), sizeof(*p) is 108, and MAXROW *sizeof(*p) is 8 * 108, and malloc() allocate that many bytes, if successful.

Also, please see this discussion on why not to cast the return value of malloc() and family in C..

edited May 23 '17 at 12:07

Community

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answered Nov 26 '15 at 12:10

Sourav Ghosh

133,132
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@user3741208 What happened when you tried to print `MAXROW *sizeof(*p)` as an integer? – Lundin Nov 26 '15 at 12:14
Lundin, it prints 864 – dor132 Nov 26 '15 at 12:18
@user3741208 There you go then. `8 * (27*4)` – Lundin Nov 26 '15 at 12:21
@user3741208 Ok, my bad, I confused `MAXROW` and `MAXCOL` – Sourav Ghosh Nov 26 '15 at 12:21
doesn't work either... well I guess their computers have weird CPUs, thanks anyway guys! – dor132 Nov 26 '15 at 12:25
the answer is 432, MAXROW*MAXCOL*2 – dor132 Nov 26 '15 at 12:26

dod_moshe · Accepted Answer · 2015-12-18T19:25:34.410

0

Assume "sizeof(int) is "(what?)... I guess you meant 4. In the first line you declare p to be a pointer to an array of 27 integers. In the second line you allocate memory in the heap for size of dereferenced p - which is 27 integers times 8 - that's 27*4*8 so the number of bytes allocated is 864.

edited Dec 18 '15 at 19:25

answered Nov 26 '15 at 12:25

dod_moshe

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2
13

score 0 · Answer 3 · answered Nov 26 '15 at 14:01

0

The answer should be MAXROW*MAXCOL*sizeof(int). The size of int cannot be determined from the code shown. It can be 2, 4, 8... or even 42, pretty much anything greater than 0.

If your teacher or course expects 432, they rely on extra context you failed to provide. Re-reading your question, you write assume sizeof(int). You need to say what should be assumed precisely.

answered Nov 26 '15 at 14:01

chqrlie

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Sorry, I thought I mentioned it correctly. It's supposed to `sizeof(int)=4` – dor132 Nov 27 '15 at 10:37
Then `MAXROW * MAXCOL * sizeof(int)` = `27 * 8 * 4` = `864`. – chqrlie Nov 27 '15 at 16:07

Total bytes allocated on dynamic heap

3 Answers3