Go by Example: Memory and Sizeof

Let’s try to figure out memory usage of Go.

package main
import (

Unlike C, there is no #pragma pack in Go, the real memory allocation depends on its implementation.

type T struct {
	B  uint8 // is a byte
	I  int   // it is int32 on my x86 32 bit PC
	P  *int  // it is int32 on my x86 32 bit PC
	S  string
	SS []string
var p = fmt.Println

In this case, the t := T{} can not measured by this method.

func memUsage(m1, m2 *runtime.MemStats) {
	p("Alloc:", m2.Alloc-m1.Alloc,
		"TotalAlloc:", m2.TotalAlloc-m1.TotalAlloc,
		"HeapAlloc:", m2.HeapAlloc-m1.HeapAlloc)
func main() {

Here is a tricky to get pointer size

	const PtrSize = 32 << uintptr(^uintptr(0)>>63)
	p("PtrSize=", PtrSize)
	p("IntSize=", strconv.IntSize)
	var m1, m2, m3, m4, m5, m6 runtime.MemStats
	t := T{}
	p("sizeof(uint8)", unsafe.Sizeof(t.B),
		"offset=", unsafe.Offsetof(t.B))
	p("sizeof(int)", unsafe.Sizeof(t.I),
		"offset=", unsafe.Offsetof(t.I))
	p("sizeof(*int)", unsafe.Sizeof(t.P),
		"offset=", unsafe.Offsetof(t.P))
	p("sizeof(string)", unsafe.Sizeof(t.S),
		"offset=", unsafe.Offsetof(t.S))

Slice is a structure of Pointer, Len and Cap. Detail here

	p("sizeof([]string)", unsafe.Sizeof(t.SS),
		"offset=", unsafe.Offsetof(t.SS))

We can see the this structure is 4 + 4 + 4 + 8 + 12 = 32 bytes There are 3 padding bytes of first t.B expanded to 4 bytes.

	p("sizeof(T)", unsafe.Sizeof(t))

We will see 0 bytes, because it is on stack, so sizeof is the proper method to tell how much memory allocated.

	memUsage(&m1, &m2)

Even string assignment is in stack.

	t2 := "abc"
	memUsage(&m3, &m4)

map will alloc memory in heap

	t3 := map[int]string{1: "x"}
	memUsage(&m5, &m6)
	fmt.Println(t2, t3) // prevent compiler error
$ go run memory-and-sizeof.go
PtrSize= 32
IntSize= 32
sizeof(uint8) 1 offset= 0
sizeof(int) 4 offset= 4
sizeof(*int) 4 offset= 8
sizeof(string) 8 offset= 12
sizeof([]string) 12 offset= 20
sizeof(T) 32
Alloc: 0 TotalAlloc: 0 HeapAlloc: 0
Alloc: 0 TotalAlloc: 0 HeapAlloc: 0
Alloc: 144 TotalAlloc: 144 HeapAlloc: 144
abc map[1:x]

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