What Limits the Maximum Size of a Go Slice, and Why Do Empty Structs Make a Difference?

Maximum Length of a Slice in Go

You've encountered memory allocation issues when attempting to create slices with various sizes in Go. Let's delve deeper into the reasons behind these limitations.

According to the Go documentation, slice elements have index values ranging from 0 to len(s)-1, indicating that the maximum capacity for a slice is determined by the size of the native integer on the target build.

Upon examining the source code, we discover a safety check that ensures that creating slices of certain sizes is feasible:

if len64 < 0 || int64(len) != len64 || t.elem.size > 0 && uintptr(len) > maxmem/uintptr(t.elem.size) {
    panic(errorString("makeslice: len out of range"))
}

In your case, the condition uintptr(len) > maxmem/uintptr(t.elem.size) becomes true, prohibiting the requested allocation. This is because the memory required for the slice would exceed the maximum memory limit.

However, when you allocate a slice of empty structs, which consume no memory, the same allocation size becomes permissible:

r := make([]struct{}, math.MaxInt64)

This is because the condition evaluating the memory requirement becomes false since the empty structs consume no memory. Go allows for the creation of slices with very large capacities for such types.

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