How to Print a struct with Bitfields as an Unsigned Int in GDB

Learn how to display a C struct containing bitfields as a single unsigned integer in GDB for easier debugging.
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This video is based on the question https://stackoverflow.com/q/79421307/ asked by the user 'Max Sedlusch' ( https://stackoverflow.com/u/28496108/ ) and on the answer https://stackoverflow.com/a/79421394/ provided by the user 'dbush' ( https://stackoverflow.com/u/1687119/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Problem Overview

When debugging C code with GDB, you may encounter structs that use bitfields to pack multiple values into an integer. Printing these structs normally displays each bitfield separately, which can be hard to interpret if you want the entire packed value as a single unsigned integer.

For example, consider this struct:

[[See Video to Reveal this Text or Code Snippet]]

Inspecting an instance in GDB by default shows each bitfield individually. Often, it is more convenient to see the underlying packed integer value.

Solution in GDB

Casting the Struct Pointer

GDB allows casting of variable pointers to other types, so you can cast the address of the struct to an unsigned int pointer and dereference it.

If e is the instance of your struct, run:

[[See Video to Reveal this Text or Code Snippet]]

or simply:

[[See Video to Reveal this Text or Code Snippet]]

This prints the hexadecimal representation of the packed bits as a single unsigned integer.

Using the x Command to Examine Memory

Alternatively, you can examine the struct's memory with the x command:

To view the struct as one 4-byte hexadecimal word:

[[See Video to Reveal this Text or Code Snippet]]

To view as bytes (useful for byte-level checks):

[[See Video to Reveal this Text or Code Snippet]]

Summary

Use type casting in GDB to interpret the bitfield struct as an unsigned int.

Use p/x (unsigned int) e for a quick hexadecimal print.

The x command lets you peek into the memory representation directly.

These techniques allow you to see the packed bitfield values succinctly and aid debugging efforts.