proper way to change *C.char from go - go

Question

I am new with go and cgo and after browsing the internet for some time I have not figured out a good and fast way to change a char* from go.
What is the fastest way to change *C.char from go&
Here is my code and my attempt to change the string(It does not work)
package asciiEngine
// #include <windows.h>
import "C"
type Screen struct {
Width, Height int
Length C.ulong
Data *C.char
GoData string
HConsole C.HANDLE
BytesWritten C.DWORD
Start C.COORD
}
func (s Screen) Draw() {
C.WriteConsoleOutputCharacter(s.HConsole, s.Data, s.Length, s.Start, &s.BytesWritten)
}
func CreateScreen(width, height int) Screen {
screen := Screen{
Width: width,
Height: height,
Length: C.ulong(width * height),
Data: (*C.char)(C.malloc(C.ulonglong(width * height))),
HConsole: C.CreateConsoleScreenBuffer(C.GENERIC_READ|C.GENERIC_WRITE, 0, nil, C.CONSOLE_TEXTMODE_BUFFER, nil),
BytesWritten: 0,
}
screen.GoData = C.GoString(screen.Data) // my attempt to get a reference to the C string
//C.SetConsoleActiveScreenBuffer(screen.HConsole)
return screen
}
main.go:
package main
// #include "stdio.h"
// void print(char* data) {
// printf(data);
// }
import "C"
import (
"fmt"
"github.com/demantar/ascii-engine"
)
func main() {
screen := asciiEngine.CreateScreen(100, 50)
C.print((*C.char)(screen.Data))
fmt.Println()
screen.GoData = "askdssdfselkkskdkflsekfjdkjfksjeflsdkfjjekdjflskasdfkksdjjekdskdfjkskd"
C.print((*C.char)(screen.Data))
}
output
P
P
I'm also pretty new to C and am doing this because I could not find a library to do this

Answer 1

For example, use gData as a Go byte slice reference to the underlying cData C char array.
package main
import (
"fmt"
"unsafe"
)
/*
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
int printData(unsigned char *data) {
return printf("cData: %lu \"%s\"\n", (long unsigned int)strlen(data), data);
}
*/
import "C"
func main() {
// Allocate C data buffer.
width, height := 8, 2
lenData := width * height
// add string terminating null byte
cData := (*C.uchar)(C.calloc(C.size_t(lenData+1), C.sizeof_uchar))
// When no longer in use, free C allocations.
defer C.free(unsafe.Pointer(cData))
// Go slice reference to C data buffer,
// minus string terminating null byte
gData := (*[1 << 30]byte)(unsafe.Pointer(cData))[:lenData:lenData]
// Write and read cData via gData.
for i := range gData {
gData[i] = '.'
}
copy(gData[0:], "Data")
gData[len(gData)-1] = 'X'
fmt.Printf("gData: %d %q\n", len(gData), gData)
C.printData(cData)
}
Output:
gData: 16 "Data...........X"
cData: 16 "Data...........X"
Reference: Command cgo