Custom sizeof operator in C

Graphic showing code

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.

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


The code section, (char *)(&(X) + 1) adds 1 to the pointer according to the pointer type and the resultant address is typecasted 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 typecasted to a character pointer.

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

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David Law

David Law

Great information.
Lucky me I discovered your site by accident (stumbleupon).

I have book marked it
for later!