How to manipulate a C character array inside a cgo function?

Question

I have a C function, that is calling a go-Function with an char array parameter. The go-Function has to modify the content of the parameter. How to accomplish this?

void cFunction() {
 char buffer[9] = "aaabbbccc"; // 9 is in this case correct, it is not a null-terminated-string
 goFunction(buffer);
 // buffer shall be modified
}

func goFunction(cBuffer *C.char) {
  // how to modify 3-5?
  //strncpy(cBuffer+3, "XXX")
}

EDIT: to be more precise. I have to implement a callbackfunction, that accepts an outparameter, which I have to manipulate.

  void callback(char outbuffer[9]) {
    goFunction(outbuffer);
  }

as I understand Franks answer, I should do something like

  allocate new go buffer
  convert C buffer to go buffer
  manipulate go buffer
  allocate new C buffer
  convert go buffer to C buffer
  memcpy C buffer into outbuffer

That is too much allocation and conversion for my taste

Maybe you can convert the cBuffer to a `[]byte` and then you can manipulate it however you want. — Giannis, Jan 28 '21 at 10:25
with `convert` you mean to allocate a new buffer? or just `cast` to `[]byte`? — Peter Miehle, Jan 28 '21 at 12:10
https://github.com/golang/go/wiki/cgo#turning-c-arrays-into-go-slices — JimB, Jan 28 '21 at 14:07

JimB · Accepted Answer · 2021-01-28T14:32:31.947

2

See the documentation for Turning C arrays into Go slices to get a indexable go slice containing the C data.

Because you are modifying the C buffer data in place, using the Go slice as a proxy, you can simply pass the same buffer to the callback. Note that using append may allocate a new Go array for your slice, so you need to avoid it and make sure you have sufficient space available in the buffer beforehand.

func goFunction(cBuffer *C.char, length int) {
    slice := (*[1 << 28]C.char)(unsafe.Pointer(cBuffer))[:length:length]
    // slice can now be modified using Go syntax, without pointer arithmetic
    
    C.callback(cBuffer)
}

edited Jan 28 '21 at 14:32

answered Jan 28 '21 at 14:17

JimB

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where do you get the 28 from? – Peter Miehle Jan 28 '21 at 14:18
@PeterMiehle: it's just an arbitrary size, see https://stackoverflow.com/questions/48756732/what-does-1-30c-yourtype-do-exactly-in-cgo/48756785#48756785 – JimB Jan 28 '21 at 14:19

score 1 · Answer 2 · answered Jan 28 '21 at 12:12

1

It is not recommanded to modify C struct in Go, or Go struct in C. Convert it at the interface, Ref1.

A few special functions convert between Go and C types by making copies of the data. In pseudo-Go definitions

More, give you a way to convert string with zero copy, Ref2.

func char2Slice(data unsafe.Pointer, len C.int) []byte {
    var value []byte
    sH := (*reflect.SliceHeader)(unsafe.Pointer(&value))
    sH.Cap, sH.Len, sH.Data = int(len), int(len), uintptr(data)
    return value
}

answered Jan 28 '21 at 12:12

Frank Wang

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what if I have to modify the C buffer directly (legacy-code, i have a callback-function that manipulates the data of the out-parameter)? – Peter Miehle Jan 28 '21 at 12:19
@PeterMiehle I can't get you right. Can you update your question and add this 'legacy-CallBack' in your description and pseudocode? – Frank Wang Jan 28 '21 at 12:25

How to manipulate a C character array inside a cgo function?

2 Answers2