Sizeof struct in Go

Question

I'm having a look at Go, which looks quite promising. I am trying to figure out how to get the size of a go struct, for example something like

type Coord3d struct {
    X, Y, Z int64
}

Of course I know that it's 24 bytes, but I'd like to know it programmatically..

Do you have any ideas how to do this ?

Answer 1

See the binary package. Specifically the TotalSize method

Answer 2

import unsafe "unsafe"

/* Structure describing an inotify event.  */
type INotifyInfo struct {
    Wd     int32  // Watch descriptor
    Mask   uint32 // Watch mask
    Cookie uint32 // Cookie to synchronize two events
    Len    uint32 // Length (including NULs) of name
}

func doSomething() {
    var info INotifyInfo
    const infoSize = unsafe.Sizeof(info)
    ...
}

NOTE: The OP is mistaken. The unsafe.Sizeof does return 24 on the example Coord3d struct. See comment below.

Answer 3

binary.TotalSize is also an option, but note there's a slight difference in behavior between that and unsafe.Sizeof: binary.TotalSize includes the size of the contents of slices, while unsafe.Sizeof only returns the size of the top level descriptor. Here's an example of how to use TotalSize.

package main

import (
    "encoding/binary"
    "fmt"
    "reflect"
)

type T struct {
    a uint32
    b int8
}

func main() {
    var t T
    r := reflect.NewValue( t )
    s := binary.TotalSize( r )

    fmt.Printf( "%v\n", s )
}

Answer 4

This is subject to change but last I looked there is an outstanding compiler bug (bug260.go) related to structure alignment. The end result is that packing a structure might not give the expected results. That was for compiler 6g version 5383 release.2010-04-27 release. It may not be affecting your results, but it's something to be aware of.

UPDATE: The only bug left in go test suite is bug260.go, mentioned above, as of release 2010-05-04.

Hotei