Custom sizeof operator in C

The unary operator ‘sizeof’ is a very handy tool in C as well as in C++. It gives the storage size of a data-type/variable. This article describes an example C program showing the custom implementation of the ‘sizeof’ operator using macros.

#include <stdio.h>
#include <stddef.h>

#define MYSIZEOF(X) ((size_t)((char *)(&(X) + 1) - (char *)(&(X))))

int main(void)
{
  int x;
  
  printf("%zu\n", MYSIZEOF(x));

  return 0;
}

This example is an implementation of a custom sizeof() operator in C. The sizeof() operator functionality is implemented here using the macro function MYSIZEOF().

Description

The code section, (char )(&(X) + 1)* adds 1 to the pointer according to the pointer type and the resultant address is type-casted to a character pointer. Correlating with the example, when the address of the integer variable x is added with 1 then the address is advanced by 4 bytes (considering 4 bytes for int). In the right hand side of the - operator, the address of the variable is type-casted to a character pointer.

Now the expression ((char )(&(X) + 1) - (char )(&(X))) gives the size of the variable.