How to Convert a C Union Field to a Go Type in Golang CGo?

Golang CGo: Converting Union Field to Go Type

Problem Statement

In C, a union allows multiple data members to occupy the same memory location. When working with unions in CGo, it's often necessary to convert the union field to a suitable Go type for further processing.

Consider the following C struct with a union:

Let's assume you want to access the ui32v field in the value union on a 64-bit platform.

Solution

Normally, one would write a C wrapper function for each union element. However, for educational purposes, let's explore how to do it in Go.

Conversion from CGo Union Array to Go Pointer

Initially, the union is exposed as a Go byte array [8]byte. The goal is to convert this array to a Go type that points to the C guint32 array.

Traditionally, this can be done as follows:

This approach, however, encounters a conversion error between uint64 (the result of binary.Read) and unsafe.Pointer.

Simplified Conversion Method

A more straightforward solution is to use the address of the byte array itself, which is a pointer to the union field's memory location:

This technique effectively translates the byte array address to a pointer to the desired C type, bypassing the need for intermediary conversions.

Usage in Conjunction with Existing Functionality

This pointer can then be used in conjunction with existing functionality, such as converting a C array of guint32s to a string:

This approach greatly simplifies union field access and manipulation in Golang CGo applications.

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