Change value of golang C.String variable in C function

Question

i want to get an int and string from C function. int - error code, string - the data.

Tried that way:

package main
/*
#include <stdlib.h>
#include <string.h>
#include <stdio.h>

int test(char *out){
  snprintf(out, sizeof out, "1234567890abcd");
  return 0;
}

*/
import "C"

import (
  "fmt"
  "unsafe"
)

func main() {
  foo := C.CString("")
  defer C.free(unsafe.Pointer(foo))
  C.test(foo)
  fmt.Println("Output is: "+C.GoString(foo))
}

and it almost worked.

Output is: 1234567

but should be: 1234567890abcd

I think that size of C.String passed to C function is 8. But don't know how can I extend it (fixed size <> 8) or change into dynamic.

So the question is: How should I pass that variable to C?

Answer 1

After a research i found a solution:

package main


/*
#include <stdlib.h>
#include <string.h>
#include <stdio.h>

int test(char *out, int len){
  snprintf(out, len, "1234567890123456789012345671`iknvcap9nv1-93hbf1-39b");
  return 0;

}

*/
import "C"

import (
  "fmt"
  "unsafe"
)

func main() {
  buf := make([]byte, 8192)
  C.test((*C.char)(unsafe.Pointer(&buf[0])), C.int(len(buf)))
  fmt.Println(string(buf))
}

To receive 'string' from C function, i had to create a slice:

  buf := make([]byte, 8192)

and pass a pointer to it:

  C.test((*C.char)(unsafe.Pointer(&buf[0])), C.int(len(buf)))

Using C.String in that situation is not working.