How Does Go Handle Stack vs. Heap Allocation of Structs, and What Role Does Garbage Collection Play?

Stack vs Heap Allocation of Structs in Go, and How They Relate to Garbage Collection

Introduction

Programming languages allocate objects in memory, and different languages use different strategies to do this. In C, stack-allocated variables reside on the stack, while heap-allocated memory is allocated dynamically. Python, on the other hand, allocates all objects on the heap, using references to access them.

Stack vs Heap Allocation in Go

Go's approach to memory allocation differs from both C and Python. While Go does not explicitly specify where objects are allocated, it employs garbage collection to reclaim unused memory. This raises questions about how structs, which are composite types, are allocated in Go.

Sample Functions

Consider the following two functions, which appear to perform the same task:

func myFunction() (*MyStructType, error) {
    var chunk *MyStructType = new(HeaderChunk)

    ...

    return chunk, nil
}


func myFunction() (*MyStructType, error) {
    var chunk MyStructType

    ...

    return &chunk, nil
}

Questions

These sample functions raise several questions:

1. Allocation Location: Is chunk declared on the stack or heap in both functions?
2. Post-Return Availability: If chunk is declared on the stack in the second function, how does it remain accessible after the function returns?
3. Pointers and Value Passing: If chunk is declared on the stack in the second function, why are structs passed by value instead of reference? What is the purpose of pointers in this case?

Answer

Allocation Location

In Go, the stack is used for storing function arguments, local variables, and return values. The heap is used for allocating objects that have been created using new or that have escaped the stack.

In the first function, chunk is allocated on the heap using new. In the second function, Go's escape analysis determines that &chunk escapes the stack, so it is also allocated on the heap.

Post-Return Availability

Even though chunk is allocated on the stack in the second function, Go's garbage collector ensures that it remains accessible after the function returns. This is because escape analysis determines that chunk escapes the function scope, and the garbage collector will not reclaim it as long as it is referenced by another object.

Pointers and Value Passing

Structs are passed by value in Go because they are composite types. Passing a pointer instead would allow the callee to modify the original struct. In the case of the second function, chunk is allocated on the heap, so there is no need to pass a pointer to it. However, pointers are used when the caller needs to access a struct that has been allocated on the heap.

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