CODEWITHSUNDEEP
carrayspointerssizeofpointer-to-array
Pratik Singhal
Pratik Singhal

Reputation: 6492

How is pointer to array different from array names?

I was reading more about arrays vs pointers in C and wrote the following program. 

#include <stdio.h> 

int arr[10] = { } ; 
typedef int (*type)[10]  ;
int main()
{
   type val = &arr ; 
   printf("Size is %lu\n", sizeof(val)) ; 
   printf("Size of int is %lu\n", sizeof(int)) ;
}

If, I execute this program, then sizeof(val) is given to be 8 and sizeof(int) is given to be 4.

If val is a pointer to the array with 10 elements, shouldn't it's size be 40. Why is the sizeof(val) 8 ?

Upvotes: 2

Views: 94

Answers (6)

Vlad from Moscow
Vlad from Moscow

Reputation: 310980

First of all this initialization of an array

int arr[10] = { } ;

is invalid in C. You may not use emplty braces in C (in C++ they are allowed). You have to write

int arr[10] = { 0 } ;

In C the corresponding initializer is defined the following way

initializer:
    { initializer-list }
    { initializer-list , }

while in C++

braced-init-list:
    { initializer-list ,opt }
    { }

As for this statement

printf("Size is %lu\n", sizeof(val)) ;

then val is a pointer because it has type type defined like

typedef int (*type)[10]  ;

Change this statement to

printf( "Size is %zu\n",  sizeof( *val ) );

if you want to get the size of the object (that is of the array) pointed to by the pointer.

Upvotes: 3

Sourav Ghosh
Sourav Ghosh

Reputation: 134326

If val is a pointer to the array...

Yes, it is, and sizeof(val) produces the size for the "pointer to the array", not the array itself.

...shouldn't it's size be 40.?

No, sizeof(val) calculates the size of the operand, the "pointer" here. In your platform, the size of a pointer seems to be 64 bits, i.e., 8 bytes. So, it gives 8.

Also, as I mentioned, use %zu to print size_t, the type produced by sizeof operator.

Upvotes: 3

mazhar islam
mazhar islam

Reputation: 5619

Most probably you are in a 64 bit PC where memory address need 64 bits or 8 byte space for representation.

Now the pointer actually holds the memory address, whereas it may hold an address of int or maybe an address of int[], doesn't matter.

So, when you execute,

printf("Size is %lu\n", sizeof(val)) ;

it shows 8 as because the pointer holds address which need 64 bits space.

And 

printf("Size of int is %lu\n", sizeof(int)) ;

this line just print the size of int which is 4 bytes.

Upvotes: 0

NightWatcher
NightWatcher

Reputation: 148

val is pointer and its size is equivalent to address bus size of system. so, sizeof(val) or sizeof(anyPointer) will give you 8 as output. if you want 40 then try sizeof(arr).

Pointer to an array is simple pointer where you can write val++ while array name is constant pointer you cannot write arr++.

you can also check this link

Upvotes: 0

Mario The Spoon
Mario The Spoon

Reputation: 4859

A pointer is a distinct data type and always has a fixd size. Think of a pointer as a sign that points to the actual data, and the sign always the same size - regardless if it points to a single character or a larger array.

Upvotes: 0

Igal S.
Igal S.

Reputation: 14543

sizeof returns the size of the pointer itself. In 64bit system it is 8 bytes. Pointers have no knowledge of the size of buffer they points to.

Upvotes: 0

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