goinvoke

goinvoke

Load DLLs and import functions with ease.

If all you need is an equivalent of LoadLibrary and GetProcAddress in Go, this library is a lot easier to work with than cgo. It does not require a C header to start with, and allows you to dynamically load different DLLs exposing the same set of functions.

Usage

Simply define a struct with attributes in the type of *windows.Proc or *windows.LazyProc, and call goinvoke.Unmarshal("path\\to\\file.dll", pointerToStruct).

package main

import (
	"errors"
	"fmt"
	"github.com/jamesits/goinvoke"
	"golang.org/x/sys/windows"
)

type Kernel32 struct {
	GetTickCount *windows.Proc

	// you can override the function name with a tag
	GetStartupInfo *windows.Proc `func:"GetStartupInfoW"`
}

func main() {
	k := Kernel32{}
	err := goinvoke.Unmarshal("kernel32.dll", &k)
	if err != nil {
		panic(err)
	}

	// a minimal example
	count, _, err := k.GetTickCount.Call()
	if !errors.Is(err, windows.ERROR_SUCCESS) {
		panic(err)
	}
	fmt.Printf("GetTickCount() = %d\n", count)

	// a more complete example
	startupInfo := windows.StartupInfo{}
	_, _, err = k.GetStartupInfo.Call(uintptr(unsafe.Pointer(&startupInfo)))
	if !errors.Is(err, windows.ERROR_SUCCESS) {
		panic(err)
	}
	lpTitle := windows.UTF16PtrToString(startupInfo.Title)
	fmt.Printf("lpTitle = %s\n", lpTitle)
}

For more examples of using this library, unmarshal_test.go is a good start point. If you need to define callback functions, see cgo_callback.go for an example.

Go: WindowsDLLs offers a great view of using the (*windows.Proc).Call() method.

Type Generator

Have a large DLL with a lot of functions and want to access all of them at once? Use our convenient invoker tool to generate the struct required! For example, if we want to call multiple functions in user32.dll, use the following commands to generate a "header":

go install github.com/jamesits/goinvoke/cmd/invoker@latest
invoker -dll "user32.dll" -generate

A file named user32_dll.go will be generated in the current directory with all the exports from that DLL. To use it in your code:

package main

import (
	"github.com/jamesits/goinvoke"
)

func main() {
	var err error
	
	k := User32{}

	// either use the object method
	err = k.Unmarshal("user32.dll")
	// or use the global Unmarshal function
	err = goinvoke.Unmarshal("user32.dll", &k)
	
    // ...
}

In the future, when your DLL is updated with new exported functions, just re-generate the file:

go generate .

For advanced usage of this tool, run invoker -help.

Caveats

Relative Import

Due to security concerns, if the path is relative and only contains a base name (e.g. "kernel32.dll"), file lookup is limited to only %WINDIR%\System32.

If you want to load a DLL packaged with your program (the DLL sits right beside your EXE, or under some sub-folder), the safe way is to get the directory where your program exists first:

package main

import (
	"github.com/jamesits/goinvoke"
	"github.com/jamesits/goinvoke/utils"
	"path/filepath"
)

type MyDll struct {
	// ...
}

func main() {
	var err error
	
	executableDir, err := utils.ExecutableDir()
	if err != nil {
		panic(err)
	}
	
	myDll := MyDll{}
	err = goinvoke.Unmarshal(filepath.Join(executableDir, /* optional */ "sub-folder", "MyDll.dll"), &myDll)
}

If you really want to load a DLL from your working directory, specify your intention explicitly with ".\\filename.dll". Loading a DLL from an arbitrary working directory might lead to serious security issues. DO NOT do this unless you know exactly what you are doing.

Error Processing

The Unmarshal() method returns an error with type (*multierror.Error) if any of the following case happens:

• DLL load fails (file does not exist, permission/ACL problem, WDAC/Code Integration policy, etc. )
• The DLL file exists, but a function defined in the struct is not exported by that DLL

It always trys to fill as much as function pointers it can find, and will not be stopped by non-critical errors. So, depending on your use case, you can ignore certain errors reported by Unmarshal(), and use whether the struct field is nil as an indicator of exported function existence of your loaded DLL file.

If you really want to decode individual errors, use err.(*multierror.Error).Errors. There are some examples in unmarshal_test.go.

Importing Functions by Ordinal

Importing functions by ordinal is fully supported, just use *windows.Proc and add a ordinal tag. The ordinal tag, if exists, always overrides the func tag.

package main

import (
	"github.com/jamesits/goinvoke"
	"golang.org/x/sys/windows"
)

type shlwapi struct {
	// function definition compatible with Windows XP or earlier
	SHCreateMemStream *windows.Proc `ordinal:"12"`
}

func main() {
	var err error

	s := shlwapi{}
	err = goinvoke.Unmarshal("shlwapi.dll", &s)
	if err != nil {
		panic(err)
	}
	
	// ...
}

*windows.LazyProc does not support a ordinal tag.

Performance

syscall.Syscall is somewhat slower due to it allocating heap twice more than a cgo call (variable length arguments, and another copy inside syscall.Syscall()). There is a internal/benchmark package to compare the performance of *windows.Proc, *windows.LazyProc and cgo. Example result under Go 1.19.1:

goos: windows
goarch: amd64
pkg: github.com/jamesits/goinvoke/internal/benchmark
cpu: AMD Ryzen 9 5900X 12-Core Processor
BenchmarkSyscallIsDebuggerPresent
BenchmarkSyscallIsDebuggerPresent-24            30003824                38.32 ns/op
BenchmarkSyscallIsDebuggerPresentLazy
BenchmarkSyscallIsDebuggerPresentLazy-24        29269077                41.75 ns/op
BenchmarkCgoIsDebuggerPresent
BenchmarkCgoIsDebuggerPresent-24                41355494                30.92 ns/op
PASS