Difference in between malloc and calloc in their memory layout?

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calloc allocates num blocks of memory, each of size size:

C++

void * calloc ( size_t num, size_t size );

Allocate space for array in memory. Allocates a block of memory for an array of num elements, each of them size bytes long, and initializes all its bits to zero.

In contrast, malloc allocates one block of memory of size size:

C++

void * malloc ( size_t size );

Allocate memory block. Allocates a block of size bytes of memory, returning a pointer to the beginning of the block.

Now my questions are:

1. Is there any difference between both (except for the zero-initialization by calloc)..?

2. Does calloc() allocate only contiguous memory Or it can allocate non-contiguous also..?

Posted 15-Nov-13 12:41pm

Atul Khanduri

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Albert Holguin 17-Nov-13 18:10pm

Not sure why you would want to allocate non-contiguous data. Remember that having data be contiguous makes handling and addressing a lot faster than non-contiguous segments.

Atul Khanduri 17-Nov-13 18:20pm

Ya i know this but just for knowledge i asked this question.....

Albert Holguin 18-Nov-13 11:21am

Ok, sounds good.

Atul Khanduri 18-Nov-13 13:17pm

:-)

Albert Holguin 18-Nov-13 13:26pm

I can both appreciate and relate to curiosity.... :)

Atul Khanduri 18-Nov-13 16:34pm

Thanks...:)

2 solutions

Solution 2

1. Actually, the memory created by calloc () is TOUCHED by the OS and initialized with zeros. There will be some performance compared to malloc( ) where the memory pointer is being returned which indicates the start of the memory block allocated.

The memory allocated is NOT TOUCHED unless it is being used by the OS.

2. Yes the memory allocated to calloc() is always contiguous by the OS. It swaps out the memory space if required, in order to assign it to calloc().

[brydon]capitalization and spelling fixes[/brydon]

Posted 18-Nov-13 1:39am

manem29

Updated 18-Nov-13 2:02am

v2

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PIEBALDconsult · Accepted Answer · 2013-11-15T13:23:00

Solution 1

1) No.

2) No. It's always contiguous.

Posted 15-Nov-13 13:23pm

PIEBALDconsult

Comments

Stefan_Lang 18-Nov-13 5:23am

I'm not quite sure about 1: What if you want to allocate an array of structs, where each struct contains one pointer and one char? Wouldn't the alignment requirement force padding between elements? How should the malloc syntax know or guess about that with only one parameter?

I think I need to do some testing...

Freak30 18-Nov-13 7:41am

You would obviously call malloc(sizeof(YourStruct) * numberOfElements);
The sizeof() does the padding for you.

Richard MacCutchan 18-Nov-13 8:38am

A simple test shows that it uses the packing default as chosen by the compiler.

nv3 18-Nov-13 7:48am

Correct on both accounts. That (2) is correct can already by seen by looking at the function signature. When only one pointer is returned, how could the allocated memory not be contiguous. My 5.

CPallini 18-Nov-13 7:53am

5

Atul Khanduri 18-Nov-13 13:22pm

Now, if we talk about performance then i think both have very less difference. Since, in calloc, after allocation all of the allocated memory initialized to 0. So, calloc have negotiable decrement in performance.
Correct me if i am wrong...

PIEBALDconsult 18-Nov-13 18:25pm

The question said to ignore that.