devxlogo

Find an Offset into a Structure

Why Trust Us

Here’s the code:

#define offset(a,b)  ((int) (&( ((a*)(0)) -> b)))

Step by step, here’s what it does:

  1. Type-cast 0, or NULL, into a pointer of type a. So a must be a typename.
  2. Member-select b from that pointer. So a must be a structure typename, and b some member property.
  3. Take the address of b. Since the a* used points to address 0, and b is some offset above the start of the a-structure, this means you’re getting the offset of b within a plus the address of a, which is zero. That is, you get the offset of b within a.
  4. Typecast that number to an integer.
Disclosure
devxblackblue

About Our Editorial Process

At DevX, we’re dedicated to tech entrepreneurship. Our team closely follows industry shifts, new products, AI breakthroughs, technology trends, and funding announcements. Articles undergo thorough editing to ensure accuracy and clarity, reflecting DevX’s style and supporting entrepreneurs in the tech sphere.

See our full editorial policy.

About Our Journalist

Charlie Frank

Charlie has over a decade of experience in website administration and technology management. As the site admin, he oversees all technical aspects of running a high-traffic online platform, ensuring optimal performance, security, and user experience.
View Author
Show More