C.malloc of Go type?
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Max
May 19, 2018, 4:42:12 PM5/19/18
to golang-nuts
After reading https://golang.org/cmd/cgo/#hdr-Passing_pointers and https://blog.golang.org/c-go-cgo,
I am trying to understand if there are valid ways to pass between Go and C complicated data structures
with (potentially) lots of pointers.
At the moment this is just a speculative question, but it could quickly become useful both for my Go interpreter gomacro
and for other similar tasks (I am thinking at least about https://github.com/gijit/gi) that need to pass arbitrary
data structures between compiled Go code and the interpreter itself (which may not be written in Go).
The question is: is it allowed to take the unsafe.Pointer returned by C.malloc() and convert it to a pointer to a Go struct type?
In practice: is the following code valid ?
Thanks,
Max
// -----------------------------------------------------------------------------------------
package main
/*
#include <string.h>
*/
import "C"
import (
"fmt"
"unsafe"
)
type List struct {
Node int
Next *List
}
func mallocList() *List {
const n = unsafe.Sizeof(List{})
p := C.malloc(C.ulong(n))
C.memset(p, C.int(n), 0)
return (*List)(p)
}
func main() {
// if this code is valid, x can contain C pointers and can be passed freely to C code,
// thus exchanging complicated data structured between Go and C
x := mallocList()
x.Next = mallocList()
fmt.Printf("%#v\n", x)
fmt.Printf("%#v\n", x.Next)
}
I am trying to understand if there are valid ways to pass between Go and C complicated data structures
with (potentially) lots of pointers.
At the moment this is just a speculative question, but it could quickly become useful both for my Go interpreter gomacro
and for other similar tasks (I am thinking at least about https://github.com/gijit/gi) that need to pass arbitrary
data structures between compiled Go code and the interpreter itself (which may not be written in Go).
The question is: is it allowed to take the unsafe.Pointer returned by C.malloc() and convert it to a pointer to a Go struct type?
In practice: is the following code valid ?
Thanks,
Max
// -----------------------------------------------------------------------------------------
package main
/*
#include <string.h>
*/
import "C"
import (
"fmt"
"unsafe"
)
type List struct {
Node int
Next *List
}
func mallocList() *List {
const n = unsafe.Sizeof(List{})
p := C.malloc(C.ulong(n))
C.memset(p, C.int(n), 0)
return (*List)(p)
}
func main() {
// if this code is valid, x can contain C pointers and can be passed freely to C code,
// thus exchanging complicated data structured between Go and C
x := mallocList()
x.Next = mallocList()
fmt.Printf("%#v\n", x)
fmt.Printf("%#v\n", x.Next)
}
Max
May 19, 2018, 4:48:08 PM5/19/18
to golang-nuts
On Saturday, May 19, 2018 at 10:42:12 PM UTC+2, Max wrote:
func mallocList() *List {
const n = unsafe.Sizeof(List{})
p := C.malloc(C.ulong(n))
C.memset(p, C.int(n), 0)
return (*List)(p)
}
I swapped the arguments to C.memset(). The correct version is:
func mallocList() *List {
const n = C.ulong(unsafe.Sizeof(List{}))
p := C.malloc(n)
C.memset(p, 0, n)
return (*List)(p)
}
Jan Mercl
May 19, 2018, 5:03:47 PM5/19/18
to Max, golang-nuts
On Sat, May 19, 2018 at 10:42 PM Max <massimilia...@gmail.com> wrote:
> The question is: is it allowed to take the unsafe.Pointer returned by C.malloc() and convert it to a pointer to a Go struct type?
-j
andrey mirtchovski
May 19, 2018, 8:10:14 PM5/19/18
to Jan Mercl, Max, golang-nuts
also useful if you're dealing with blobs of C memory:
On Sat, May 19, 2018 at 3:03 PM, Jan Mercl <0xj...@gmail.com> wrote:
--
On Sat, May 19, 2018 at 10:42 PM Max <massimiliano.ghilardi@gmail.com> wrote:
> The question is: is it allowed to take the unsafe.Pointer returned by C.malloc() and convert it to a pointer to a Go struct type?
See https://golang.org/pkg/unsafe/#Pointer-j
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