Modifying a C void* byte array from Go code

Question

Basically, I'm trying to replace pread in a C program with a Go function. I've got most of the structure working, but I can't figure out how to populate the buffer argument (buf unsafe.Pointer) with the desired bytes.

Part of daemon.c:

static int preadShim(int fd, void *buf, size_t count, off_t offset) {
    //return pread(fd, buf, count, offset);
    return ReadOver(fd, buf, count, offset);
}

In a Golang file:

package main

import "C"
import "fmt"
import "unsafe"

//export ReadOver
func ReadOver(fd C.int, buf unsafe.Pointer, count C.int, offset C.int) C.int{
    fmt.Println("ReadOver got called")  
    newBuf := []byte("injected bytes")
    //TODO: copy newBuf over buf
    return count
}

Another Golang file:

package main

// #include "daemon.c"
import "C"
import "fmt"

func main() {
    //Start C program
    C.start()
}

Any suggestions? I've tried nothing and I'm all out of ideas.

Answer 1

You just have to create your own slice from it, for example:

func ReadOver(fd C.int, buf unsafe.Pointer, count C.int, offset C.int) C.int {
    fmt.Println("ReadOver got called")
    newBuf := []byte("injected bytes")
    slice := &reflect.SliceHeader{Data: uintptr(buf), Len: int(count), Cap: int(count)}
    rbuf := *(*[]byte)(unsafe.Pointer(slice))
    return C.int(copy(rbuf, newBuf))
}