I'm playing around with wrapping FUSE with Go. However I've come stuck with how to deal with struct fuse_operations. I can't seem to expose the operations struct by declaring type Operations C.struct_fuse_operations as the members are lower case, and my pure-Go sources would have to use C-hackery to set the members anyway. My first error in this case is "can't set getattr" in what looks to be the Go equivalent of a default copy constructor. My next attempt is to expose an interface that expects GetAttr, ReadLink etc, and then generate C.struct_fuse_operations and bind the function pointers to closures that call the given interface.
This is what I've got (explanation continues after code):
package fuse
// #include <fuse.h>
// #include <stdlib.h>
import "C"
import (
//"fmt"
"os"
"unsafe"
)
type Operations interface {
GetAttr(string, *os.FileInfo) int
}
func Main(args []string, ops Operations) int {
argv := make([]*C.char, len(args) + 1)
for i, s := range args {
p := C.CString(s)
defer C.free(unsafe.Pointer(p))
argv[i] = p
}
cop := new(C.struct_fuse_operations)
cop.getattr = func(*C.char, *C.struct_stat) int {}
argc := C.int(len(args))
return int(C.fuse_main_real(argc, &argv[0], cop, C.size_t(unsafe.Sizeof(cop)), nil))
}
package main
import (
"fmt"
"fuse"
"os"
)
type CpfsOps struct {
a int
}
func (me *CpfsOps) GetAttr(string, *os.FileInfo) int {
return -1;
}
func main() {
fmt.Println(os.Args)
ops := &CpfsOps{}
fmt.Println("fuse main returned", fuse.Main(os.Args, ops))
}
This gives the following error:
fuse.go:21[fuse.cgo1.go:23]: cannot use func literal (type func(*_Ctype_char, *_Ctype_struct_stat) int) as type *[0]uint8 in assignment
I'm not sure what to pass to these members of C.struct_fuse_operations, and I've seen mention in a few places it's not possible to call from C back into Go code.
If it is possible, what should I do? How can I provide the "default" values for interface functions that acts as though the corresponding C.struct_fuse_operations member is set to NULL?
http://cheesesun.blogspot.com/2010/04/callbacks-in-cgo.html describes a general - if somewhat convoluted - technique. http://groups.google.com/group/golang-nuts/browse_thread/thread/abd0e30dafdbf297?tvc=2&pli=1 describes the problem.
I suspect that if you want to handle the function pointers as anything other than uintptrs, you'll have to hack on cgo.
Related
Please help to call the variable from another variable.
I have the script which is taking EC2 instances and return in "*ec2.Instance" variable.
I can print information from static text, for example :
fmt.Println(instance.InstanceType) // t3.small
But I have the list of reqired fields like and don't know how dynamic use name from this list :
fields := []string{"InstanceId", "InstanceType", "PrivateIpAddress"}
for i := range fields {
fmt.Println(fields[i])
fmt.Println(instance.fields[i]) // Not correct ... :(
}
You need to use reflection to do this in go.
The key takeaway is you need to "analyze" the returned value at runtime and access properties by name from the "reflected" structure. Reflection basically means analyzing objects at runtime.
package main
import (
"fmt"
"reflect"
)
type whatever struct {
cat string
dog string
animals int
something string
}
func main() {
wantProps := []string{ "cat", "animals"}
we := whatever{cat: "meow", animals: 22}
r := reflect.ValueOf(we)
for _, propName := range wantProps {
prop := r.FieldByName(propName)
fmt.Println(propName, prop)
}
}
More details:
Golang dynamic access to a struct property
Suppose I have a C library with code like this:
typedef int (callback_t)(int);
void register_callback(callback_t cb);
I want to write go bindings for this function and pass arbitrary go callbacks.
I found an excellent answer here. However, it is a trick that makes use of the fact that the callback accepts a void *, through which Go passes the function pointer to C and receives it back. However, this can't be applied in my example since there isn't a user void *.
The best I could do is:
/*
extern int gobridge(int data);
static int cbridge(void)
{
register_callback(gobridge);
}
*/
import "C"
type Callback func (int) int
var g_cb Callback
//export gobridge
func gobridge(data C.int) C.int {
return C.int(g_cb(int(data)))
}
func RegisterCallback(cb Callback) {
g_cb = cb //save callback in a global
C.cbridge()
}
func Count(left, right int) {
C.count(C.int(left), C.int(right))
}
This only works for a single register_callback (because of the global variable), but the C library can register many callbacks and invoke them all.
I hoped to achieve something like this, but this doesn't compile (put aside the clojure, it can't convert the go function to c function even though the signature is the same):
import "C"
func RegisterCallback(cb Callback) {
f := func (d C.int) C.int {
return C.int(cb(int(d)))
}
C.register_callback(f)
}
func Count(left, right int) {
C.count(C.int(left), C.int(right))
}
Is there a way to properly bind the C library?
You could create an array of callbacks and have the bridge iterate the array of callbacks if the there is no return value (void function).
Oops, there was one thing I forgot when I made this answer, and it's something that I'm both not quite sure on myself and that I can't seem to find information for on MSDN and Google and the Stack Overflow search.
There are a number of places in the Windows API where you use a negative number, or a number too large to fit in a signed integer; for instance, CW_USEDEFAULT, INVALID_HANDLE_VALUE, GWLP_USERDATA, and so on. In the world of C, everything is all fine and dandy: the language's integer promotion rules come to the rescue.
But in Go, I have to pass all my arguments to functions as uintptr (which is equivalent to C's uintptr_t). The return value from the function is also returned this way, and then I will need to compare. Go doesn't allow integer promotion, and it doesn't allow you to convert a signed constant expression into an unsigned one at compile-time.
Right now, I have a bit of a jerry-rig set up for handling these constants in my UI library. (Here's an example of what this solution looks like in action.) However, I'm not quite satisfied with this solution; it feels to me like it's assuming things about the ABI, and I want to be absolutely sure of what I'm doing.
So my question is: how are signed values handled when passing them to Windows API functions and how are they handled when returning?
All my constants are autogenerated (example output). The autogenerator uses a C ffi, which I'd rather not use for the main project since I can call the DLLs directly (this also makes cross-compilation easier at least for the rest of the year). If I could somehow leverage that, for instance by making everything into a C-side variable of the form
uintptr_t x_CONST_NAME = (uintptr_t) (CONST_NAME);
that would be helpful. But I can't do that without this answer.
Thanks!
Update
Someone on IRC put it differently (reformatted to avoid horizontal scrolling):
[19:13] <FraGag> basically, you're asking whether an int with a value of -1
will be returned as 0x00000000FFFFFFFF or as 0xFFFFFFFFFFFFFFFF
if an int is 4 bytes and an uintptr is 8 bytes
Basically this, but specifically for Windows API interop, for parameters passed in, and regardless of uintptr size.
#twotwotwo's comments to my question pointed me in the right direction. If Stack Overflow allowed marking comments as answers and having multiple answers marked, I'd do that.
tl;dr version: what I have now is correct after all.
I wrote a program (below) that simply dumped all the constants from package syscall and looked for constants that were negative, but not == -1 (as that would just be ^0). The standard file handles (STD_ERROR_HANDLE, STD_INPUT_HANDLE, and STD_OUTPUT_HANDLE) are (-12, -10, and -11, respectively). The code in package syscall passes these constants as the sole argument of getStdHandle(h int), which produces the required file handle for package os. getStdHandle() passes this int to an autogenerated function GetStdHandle(stdhandle int) that wraps a call to the GetStdHandle() system call. GetStdHandle() takes the int and merely converts it to uintptr for passing into syscall.Syscall(). Though no explanation is given in the autogenerator's source (mksyscall_windows.go), if this didn't work, neither would fmt.Println() =P
All of the above is identical on both windows/386 and windows/amd64; the only thing in a processor-specific file is GetStdHandle(), but the relevant code is identical.
My negConst() function is already doing the same thing, just more directly. As such, I can safely assume that it is correct.
Thanks!
// 4 june 2014
// based on code from 24 may 2014
package main
import (
"fmt"
"os"
"strings"
"go/token"
"go/ast"
"go/parser"
"code.google.com/p/go.tools/go/types"
_ "code.google.com/p/go.tools/go/gcimporter"
)
var arch string
func getPackage(path string) (typespkg *types.Package, pkginfo types.Info) {
var pkg *ast.Package
fileset := token.NewFileSet() // parser.ParseDir() actually writes to this; not sure why it doesn't return one instead
filter := func(i os.FileInfo) bool {
if strings.Contains(i.Name(), "_windows") &&
strings.Contains(i.Name(), "_" + arch) &&
strings.HasSuffix(i.Name(), ".go") {
return true
}
if i.Name() == "race.go" || // skip these
i.Name() == "flock.go" {
return false
}
return strings.HasSuffix(i.Name(), "_windows.go") ||
(!strings.Contains(i.Name(), "_"))
}
pkgs, err := parser.ParseDir(fileset, path, filter, parser.AllErrors)
if err != nil {
panic(err)
}
for k, _ := range pkgs { // get the sole key
if pkgs[k].Name == "syscall" {
pkg = pkgs[k]
break
}
}
if pkg == nil {
panic("package syscall not found")
}
// we can't pass pkg.Files directly to types.Check() because the former is a map and the latter is a slice
ff := make([]*ast.File, 0, len(pkg.Files))
for _, v := range pkg.Files {
ff = append(ff, v)
}
// if we don't make() each map, package types won't fill the structure
pkginfo.Defs = make(map[*ast.Ident]types.Object)
pkginfo.Scopes = make(map[ast.Node]*types.Scope)
typespkg, err = new(types.Config).Check(path, fileset, ff, &pkginfo)
if err != nil {
panic(err)
}
return typespkg, pkginfo
}
func main() {
pkgpath := "/home/pietro/go/src/pkg/syscall"
arch = os.Args[1]
pkg, _ := getPackage(pkgpath)
scope := pkg.Scope()
for _, name := range scope.Names() {
obj := scope.Lookup(name)
if obj == nil {
panic(fmt.Errorf("nil object %q from scope %v", name, scope))
}
if !obj.Exported() { // exported names only
continue
}
if _, ok := obj.(*types.Const); ok {
fmt.Printf("egrep -rh '#define[ ]+%s' ~/winshare/Include/ 2>/dev/null\n", obj.Name())
}
// otherwise skip
}
}
So, I'm trying to get used to Go! and I've come up to a problem where I try making a new data type "RandomType" which contains a slice.
package main
type RandomType struct {
RandomSlice []int
}
func main() {
r := new(RandomType)
r.RandomSlice = make([]int, 0)
append(r.RandomSlice, 5)
}
This bit of code yields an error:
append(r.RandomSlice, 5) not used
However for instance if I try with
type RandomType struct {
RandomInt int
}
func main() {
r := new(RandomType)
r.RandomInt = 5
}
this works fine.
Not sure what I'm doing wrong.
append doesn't change the slice you provide but builds a new one.
You must use the returned slice :
r.RandomSlice = append(r.RandomSlice, 5)
More details about append in Effective Go and in the Go blog.
I have an instance of a struct that I defined and I would like to convert it to an array of bytes. I tried []byte(my_struct), but that did not work. Also, I was pointed to the binary package, but I am not sure which function I should use and how I should use it. An example would be greatly appreciated.
One possible solution is the "encoding/gob" standard package. The gob package creates an encoder/decoder that can encode any struct into an array of bytes and then decode that array back into a struct. There's a great post, here.
As others have pointed out, it's necessary to use a package like this because structs, by their nature, have unknown sizes and cannot be converted into arrays of bytes.
I've included some code and a play.
package main
import (
"bytes"
"encoding/gob"
"fmt"
"log"
)
type P struct {
X, Y, Z int
Name string
}
type Q struct {
X, Y *int32
Name string
}
func main() {
// Initialize the encoder and decoder. Normally enc and dec would be
// bound to network connections and the encoder and decoder would
// run in different processes.
var network bytes.Buffer // Stand-in for a network connection
enc := gob.NewEncoder(&network) // Will write to network.
dec := gob.NewDecoder(&network) // Will read from network.
// Encode (send) the value.
err := enc.Encode(P{3, 4, 5, "Pythagoras"})
if err != nil {
log.Fatal("encode error:", err)
}
// HERE ARE YOUR BYTES!!!!
fmt.Println(network.Bytes())
// Decode (receive) the value.
var q Q
err = dec.Decode(&q)
if err != nil {
log.Fatal("decode error:", err)
}
fmt.Printf("%q: {%d,%d}\n", q.Name, *q.X, *q.Y)
}
I assume you want something like the way C handles this. There is no built in way to do that. You will have to define your own serialization and deserialization to and from bytes for your struct. The binary package will help you encode
the fields in your struct to bytes that you can add to the byte array but you will be responsible for specifying the lengths and offsets in the byte array that will hold the fields from your struct.
Your other options are to use one of the encoding packages: http://golang.org/pkg/encoding/ such as gob or json.
EDIT:
Since you want this for making a hash as you say in your comment the easisest thing to do is use []byte(fmt.Sprintf("%v", struct)) like so: http://play.golang.org/p/yY8mSdZ_kf
Just use json marshal， this is a very simple way.
newFsConfig := dao.ConfigEntity{EnterpriseId:"testing"}
newFsConfigBytes, _ := json.Marshal(newFsConfig)
I know this thread is old, but none of the answers were accepted, and there's a pretty simple way to do this.
https://play.golang.org/p/TedsY455EBD
important code from playground
import (
"bytes"
"fmt"
"encoding/json"
)
type MyStruct struct {
Name string `json:"name"`
}
testStruct := MyStruct{"hello world"}
reqBodyBytes := new(bytes.Buffer)
json.NewEncoder(reqBodyBytes).Encode(testStruct)
reqBodyBytes.Bytes() // this is the []byte
Serialization is likely proper answer.
But if you consent to unsafety and actually need to read struct as bytes, then relying on byte array memory representation might be a bit better than relying on byte slice internal structure.
type Struct struct {
Src int32
Dst int32
SrcPort uint16
DstPort uint16
}
const sz = int(unsafe.SizeOf(Struct{}))
var asByteSlice []byte = (*(*[sz]byte)(unsafe.Pointer(&struct_value)))[:]
Works and provides read-write view into struct, zero-copy. Two "unsafe" should hint enough that it may break badly.
You should use a bytes buffer instead of a string, the other suggested methods create a SHA1 of variable length, the SHA1 standard length must be 20 bytes (160 bits)
package main
import (
"crypto/sha1"
"fmt"
"encoding/binary"
"bytes"
)
type myStruct struct {
ID string
Data string
}
func main() {
var bin_buf bytes.Buffer
x := myStruct{"1", "Hello"}
binary.Write(&bin_buf, binary.BigEndian, x)
fmt.Printf("% x", sha1.Sum(bin_buf.Bytes()))
}
Try it yourself: http://play.golang.org/p/8YuM6VIlLV
It's a really easy method and it works great.
package main
import (
"crypto/sha1"
"fmt"
"encoding/binary"
"bytes"
)
type myStruct struct {
ID [10]byte
Data [10]byte
}
func main() {
var bin_buf bytes.Buffer
x := myStruct{"1", "Hello"}
binary.Write(&bin_buf, binary.BigEndian, x)
fmt.Printf("% x", sha1.Sum(bin_buf.Bytes()))
}
binary.Write takes a struct which has fixed length memory allocated datatype.
json.Marshal is the best option to convert a struct to []byte, see example below:
package main
import (
"encoding/json"
"fmt"
)
type ExampleConvertToByteArray struct {
Name string
SurName string
}
func main() {
example := ExampleConvertToByteArray{
Name: "James",
SurName: "Camara",
}
var exampleBytes []byte
var err error
exampleBytes, err := json.Marshal(example)
if err != nil {
print(err)
return
}
fmt.Println(string(exampleBytes))
}
Go playground -> https://play.golang.org/p/mnB9Cxy-2H3
Take a look at https://blog.golang.org/go-slices-usage-and-internals
Specifically slice internals. The idea is to mimic slice's internal structure and point to our struct instead of a byte sequence:
package main
import (
"fmt"
"unsafe"
)
// our structure
type A struct {
Src int32
Dst int32
SrcPort uint16
DstPort uint16
}
// that is how we mimic a slice
type ByteSliceA struct {
Addr *A
Len int
Cap int
}
func main() {
// structure with some data
a := A{0x04030201,0x08070605,0x0A09, 0x0C0B}
// create a slice structure
sb := &ByteSliceA{&a, 12, 12} // struct is 12 bytes long, e.g. unsafe.Sizeof(a) is 12
// take a pointer of our slice mimicking struct and cast *[]byte on it:
var byteSlice []byte = *(*[]byte)(unsafe.Pointer(sb))
fmt.Printf("%v\n", byteSlice)
}
Output:
[1 2 3 4 5 6 7 8 9 10 11 12]
https://play.golang.org/p/Rh_yrscRDV6
Have you considered serializing it to bson? http://labix.org/gobson
var v any
b := (*[unsafe.Sizeof(v)]byte)(unsafe.Pointer(&v))
c := b[:]