Pass a populated GO structure from GO source file to C : ctogo

Getting Help
1

Am trying to send a GO structure from from GO code to a C code and access the content of GO structure in C.
I was able to pass a GO int from GO code to C and access the GO int in C and also was able to modify, but am facing issues in passing a GO structure to C. Below is the code snippet.

go_func.go

package main

import (
    /*
        #include "cmain.h"
    */
    "C"
    "fmt"
)

type test struct {
    first_name string
    last_name  string
}

func main() {
    var p test
    p.first_name = "Hello"
    p.last_name = "How are you"
    C.cmain(C.test(&p))
}

cmain.h

typedef struct {
    char first_name[100];
    char last_name[100];
}test;

void cmain(test*);

cmain.c

#include <stdio.h>
#include "cmain.h"
#include "_cgo_export.h"

  void cmain(test *value) {
    printf("first_name:%s\n", value->first_name);
    printf("last_name:%s\n", value->last_name);
}

I get error when i try the above,
Adding to the above issue, i have a doubt, cant i have a single structure in GO and pass the same structure to C, is it required to have the similar replica of the GO structure even in C ?

2

Go strings are nothing like C strings. See https://golang.org/cmd/cgo/ for functions to convert between the two. You also can’t pass any Go pointers into the C world. Generally you’ll have to manage allocations and copies at the boundary.

3

Thank you @calmh, can you please let me know what is the problem in the code that i have written ?

4

You’re trying to convert a

type test struct {
    first_name string
    last_name  string
}

to a

typedef struct {
    char first_name[100];
    char last_name[100];
}test;

by C.cmain(C.test(&p)).

This is impossible because a Go string looks nothing like a C char[100] (nor a *char for that matter).

Which, I suspect, is what the error that you’re not showing us is also telling you.

What you need to do is malloc a new C.test, use the conversion functions to copy and convert the strings, and pass the pointer to that into C.

5

Can you please tell me what do you mean by malloc a new C.test. I need to copy the contents of GO Structure to C Structure, so for that i have used C.CStrings and C.GoStrings, but not able to find any success, I have re iterated the code below.

I want to copy contents of GOLang structure to C Structure.
Here i want the populated GO Struct (type test struct) to be copied to C structure test_c.

Have put the following logic. I have accessed C structure test_c in go file, as C.test_c, and tried to copy the content of test struct to C.test_c (p_c) by using C.GoString, but when ever i try to do this i get this error

Can anyone let me know what is the mistake that am doing, is there a better method to achive the same ?

./go_structure.go:30: cannot use _Cfunc_CString(p_go.a) (type *C.char) as type [10]C.char in assignment
./go_structure.go:31: cannot use _Cfunc_CString(p_go.b) (type *C.char) as type [10]C.uchar in assignment

Below is the code,

go_stucture.go

package main

import (
    /*
        #include "cmain.h"
    */
    "C"
    "fmt"
    "unsafe"
)

type test struct {
    a string
    b string
}

func main() {

    var p_go test
    var p_c C.test_c
    p_go.a = "ABCDEFGHIJ"
    p_go.b = "QRSTUVXWYZ"

    //fmt.Println(unsafe.Sizeof(p_c.a))

    fmt.Println("In GO code\n")
    fmt.Println("GO code structure Member:a=%s", p_go.a)
    fmt.Println("GO code structure Member:b=%s", p_go.b)

    p_c.a = C.GoString(p_go.a)
    p_c.b = C.GoString(p_go.b)

    fmt.Println("Call C function by passing GO structure\n")
    C.cmain((*C.test_c)(unsafe.Pointer(&p_c)))
}

cmain.c

#include <stdio.h>
#include "cmain.h"
#include "_cgo_export.h"

void cmain(test_c *value) {
    printf("Inside C code\n");
    printf("C code structure Member:a=%s\n", value->a);
    printf("C code structure Member:b=%s\n", value->b);
}

cmain.h

typedef struct {
    unsigned char a[10];
    unsigned char b[10];
}test_c;
void cmain(test_c* value);
6

It’s saying a *char is not a [10]char. C.GoString returns a char pointer, and that’s presumably what you should have in your C structure.

7

This topic was automatically closed 90 days after the last reply. New replies are no longer allowed.