CODEWITHSUNDEEP
csizeof
Je Rog
Je Rog

Reputation: 5993

Interview question about various pointer size under 32bit architecture

char str[] = " http://www.ibegroup.com/";

char *p = str ;

void Foo ( char str[100]){

}

void *p = malloc( 100 );

What's the sizeof str,p,str,p in the above 4 case in turn?

I've tested it under my machine(which seems to be 64bit) with these results:

25 8 8 8

But don't understand the reason yet.

Upvotes: 2

Views: 1429

Answers (4)

phihag
phihag

Reputation: 287835

sizeof(char[]) returns the number of bytes in the string, i.e. strlen()+1 for null-terminated C strings filling the entire array. Arrays don't decay to pointers in sizeof. str is an array, and the string has 25 characters plus a null byte, so sizeof(str) should be 26. Did you add a space to the value?

The size of a pointer is of course always determined just by the machine architecture, so both instances of p are 8 bytes on 64-bit architectures and 4 bytes on 32-bit architectures.

In function arguments, arrays do decay to pointers, so you're getting the same result that you get for a pointer. Therefore, the following definitions are equivalent:

void foo(char s[42]) {};
void foo(char s[100]) {};
void foo(char* s) {};

Upvotes: 4

vsz
vsz

Reputation: 4909

in the cases the size of 

char str[] = “ http://www.ibegroup.com/”

is known to be 25 (24+1), because that much memory is actually allocated.

In the case of 

void Foo ( char str[100]){

no memory is allocated

Upvotes: -1

orlp
orlp

Reputation: 117681

The first is the sizeof of an built-in array, which is the amount of elements (24 + null on the end of the string).

The second is the sizeof of a pointer which is the native word size of your system, in your case 64 bit or 8 bytes.

The third is the sizeof of a pointer to the first element of an array which has the same size as any other pointer, the native word size of your system. Why a pointer to the first element of an array? Because size information of an array goes lost when passed to a function and it gets implicitly converted to a pointer to the first element instead.

The fourth is the sizeof of a pointer which has the same size as any other pointer.

Upvotes: 1

Tony the Pony
Tony the Pony

Reputation: 41357

str is an array of 8-bit characters, including null terminator.

p is a pointer, which is typically the size of the machine's native word size (32 bit or 64 bit).

The size taken up by a pointer stays constant, regardless of the size of the memory to which it points.

EDIT

In c++, arguments that are arrays are passed by reference (which internally is a pointer type), that's why the second instance of str has sizeof 8.

Upvotes: 0

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