Sample go code to call COM code, using IMalloc/CoGetMalloc as an example

imalloc.go

package main

import (
    "fmt"
    "log"
    "syscall"
    "unsafe"
)

type MallocVtbl struct {
    queryInterface uintptr 
    addref uintptr
    release uintptr
    alloc uintptr
    realloc uintptr
    free uintptr
    getSize uintptr
    didAlloc uintptr
    heapMinimize uintptr
}   

func main() {
    fmt.Printf("Loading ole32.dll...\n");

    handle, err := syscall.LoadLibrary("ole32.dll")
    if err != nil {
        log.Fatal(err)
        return
    }

    proc, err := syscall.GetProcAddress(handle, "CoGetMalloc")
    if err != nil {
        log.Fatal(err)
    }

    fmt.Printf("Calling CoGetMalloc\n");
    var malloc uintptr;
    ret, _, err := syscall.Syscall(proc, uintptr(2), uintptr(1), uintptr(unsafe.Pointer(&malloc)), uintptr(0))
    if ret < 0 {
        log.Fatal(err)
    }

    fmt.Printf("CoGetMalloc returned %x\n\n", malloc) 

    mallocVtblPtr := *(*uintptr)(unsafe.Pointer(malloc))
    mallocVtbl := (*MallocVtbl)(unsafe.Pointer(mallocVtblPtr))

    const memSize int = 100
    fmt.Printf("Calling IMalloc::Alloc(%v)\n", memSize)
    memPtr, _, err := syscall.Syscall(mallocVtbl.alloc, uintptr(2), malloc, uintptr(memSize), uintptr(0))
    
    if memPtr == 0 {
        log.Fatal(err)
    }
       
    fmt.Printf("IMalloc::Alloc returned %x\n\n", memPtr) 
 
    fmt.Printf("Calling IMalloc::GetSize with %x\n", memPtr)

    returnedSize, _, err := syscall.Syscall(mallocVtbl.getSize, uintptr(2), malloc, memPtr, uintptr(0))
    fmt.Printf("IMalloc::GetSize returned %v\n\n", returnedSize)

    fmt.Printf("Calling IMalloc::Free with %x\n", memPtr);
    syscall.Syscall(mallocVtbl.free, uintptr(2), malloc, memPtr, uintptr(0))
    fmt.Printf("IMalloc::Free succeeded\n\n");

    fmt.Printf("Calling IMalloc::DidAlloc with %x\n", memPtr)
    ret, _, err = syscall.Syscall(mallocVtbl.didAlloc, uintptr(2), malloc, memPtr, uintptr(0))
    didAlloc := bool(ret != 0)
    fmt.Printf("IMalloc::DidAlloc returned %v\n\n", didAlloc);

    fmt.Printf("Calling IMalloc::Release\n")
    ret, _, err = syscall.Syscall(mallocVtbl.release, uintptr(1), malloc, uintptr(0), uintptr(0))
    fmt.Printf("IMalloc::Release returned %v\n\n", ret);
}