从Golang中的C函数返回字符串

Question

I am trying to a call C function in GO. This works to some extends (for integers). However, there are issues A minimal example is given below

package main

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

void my_reverse(char* src, int len, char *dst){
  dst = malloc(sizeof(char) * (len + 1));
  printf("[c-part] src=%s
", src);
  for (int i = 0; i < len; ++i)
  {
    dst[i] = src[len - 1 - i];
  }
  dst[len] = 0;
  printf("[c-part] dst=%s
", dst);
}

void some_text(char* buffer, unsigned long long int *year){
  buffer = malloc(200 * sizeof(char));
  sscanf("year 2018d", "%s %16llu", buffer, year);
  printf("will return (%s, %16llu)
", buffer, *year);

}
*/
import "C"

import "unsafe"
import "fmt"

func Reverse(a string) (dst string) {

    c_src := C.CString(a)
    defer C.free(unsafe.Pointer(c_src))
    c_len := C.int(len(a))
    c_dst := C.CString(dst)
    defer C.free(unsafe.Pointer(c_dst))

    C.my_reverse(c_src, c_len, c_dst)

    return string(*c_dst)

}

func Sometext() (dst string, year int64) {

    c_dst := C.CString("")
    c_year := C.ulonglong(0)
    defer C.free(unsafe.Pointer(c_dst))

    C.some_text(c_dst, &c_year)

    return string(*c_dst), int64(c_year)

}

func main() {
    fmt.Printf("[gopart] dst=%v


", Reverse("Hello World"))

    buf, year := Sometext()
    fmt.Printf("received (%v, %v)
", buf, year)
}

These are two c-function, which allocate a new buffer in c. However, I get the output

[c-part] src=Hello World
[c-part] dst=dlroW olleH
[gopart] dst=


will return (year,             2018)
received (, 2018)

This means, these strings are available in C.But I can never use these strings in GO afterwards. Does the Go-Garbage collector remove the string before I can use it?

I expect to see

[gopart] dst=dlroW olleH
received (year, 2018)

there as well.

Answer 1

You don't need to handle memory allocation in c if you initialize the strings in Go using C.String

// Go string to C string // The C string is allocated in the C heap using malloc. // It is the caller's responsibility to arrange for it to be // freed, such as by calling C.free (be sure to include stdlib.h // if C.free is needed). func C.CString(string) *C.char

You can use c.GoString() to convert the C.String back in to go string. Below is a working code snippet of your sample.

package main

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

void my_reverse(char* src, char *dst){
  printf("[c-part] src=%s
", src);
  int n = strlen(src);
  for (int i = 0; i < n ; ++i)
  {
    dst[i] = src[n - 1 - i];
  }
  dst[n] = 0;
  printf("[c-part] dst=%s
", dst);
}

void some_text(char* buffer, unsigned long long int *year){
  sscanf("year 2018d", "%s %16llu", buffer, year);
  printf("will return (%s, %16llu)
", buffer, *year);
}
*/
import "C"

import "unsafe"
import "fmt"

func Reverse(a string) (dst string) {
    c_src := C.CString(a)
    defer C.free(unsafe.Pointer(c_src))
    c_dst := C.CString(dst)
    defer C.free(unsafe.Pointer(c_dst))
    C.my_reverse(c_src, c_dst)
    return C.GoString(c_dst)
}

func Sometext() (dst string, year int64) {
    c_dst := C.CString("")
    c_year := C.ulonglong(0)
    defer C.free(unsafe.Pointer(c_dst))
    C.some_text(c_dst, &c_year)
    return C.GoString(c_dst), int64(c_year)
}

func main() {
    fmt.Printf("[gopart] dst=%v


", Reverse("Hello World"))

    buf, year := Sometext()
    fmt.Printf("received (%v, %v)
", buf, year)
}