C | find the size of the value a void Pointer is pointing at using sizeof()

user_13416636 0
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Im trying to use sizeof() to find the size of the value a void pointer is pointing at, I want to use it to print the value.

this code

    int num = 5;
    void *voidPtr;
    voidPtr = #
    printf("%d", sizeof(*voidPtr));

Prints, 1. How can i get it to print 4?

If theres any other way to print the value of the void pointer, please let me know

Thanks in advance.

void-pointers void pointers sizeof c
5 Answers

The value of pointer merely conveys the starting address of an object, not any information about its size.

The type of a pointer inherently conveys information about the size of the object it points to, if it is a complete type. However, void is incomplete, and a void * provides no information about the size of the object it originated from (except possibly by extensions to standard C and by debugging features).

Generally, if your program needs to know the size of what a void * points to, it must track that information itself.

user_2982250

  1. *voidPtr is invalid in standard C. It is a GCC extension and that is the reason why sizeof(*voidptr) is 1. Any other (non GCC family) compiler will not compile this code.

  2. %d is the wrong format to print size_t. You should use %zu instead.

  3. There is no way in C to determine what size object is referenced by the void pointer

user_61100940

The type void is an incomplete type. It means that the size of an expression of the type void is undefined.

From the C Standard (6.2.5 Types)

19 The void type comprises an empty set of values; it is an incomplete object type that cannot be completed

and (6.5.3.4 The sizeof and alignof operators)

1 The sizeof operator shall not be applied to an expression that has function type or an incomplete type, ...

Some compilers for back compatibility make sizeof( void ) equal to sizeof( char ) that is to 1.

If you need to get the size of the pointed object using a pointer of the type void * then you need to cast the pointer to the pointer of the referenced object type as for example

printf("%zu", sizeof( *( int * )voidPtr));

Pay attention to that you need to use the conversion specifier %zu with expressions of the type size_t.

user_28772410

I guess you think for this:

printf( "%d", sizeof voidPtr );

It is the size of voidPtr variable, not the pointed value *voidPtr. sizeof voidPtr will be 4 if program is compiled as 32-bit. PS: sizeof does not need parenthesis when used with variable/object, not type. E.g. sizeof MyVariable vs sizeof (long).

user_24174590

You said, "I'm trying to find the size of the value a void pointer is pointing at", is pointing to an integer in your code, so use this:

printf("%d", sizeof(int));

Understand that 'sizeof()' must know the type at compile time.

user_81086370
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