Call Go function with string parameter from C?

Question

I can call a Go function without parameters from C, per below. This compiles via go build and prints

Hello from Golang main function!
CFunction says: Hello World from CFunction!
Hello from GoFunction!

main.go

package main

//extern int CFunction();
import "C"
import "fmt"

func main() {
  fmt.Println("Hello from Golang main function!")
  //Calling a CFunction in order to have C call the GoFunction
  C.CFunction();
}

//export GoFunction
func GoFunction() {
  fmt.Println("Hello from GoFunction!")
}

file1.c

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

int CFunction() {
  char message[] = "Hello World from CFunction!";
  printf("CFunction says: %s\n", message);
  GoFunction();
  return 0;
}

Now, I want to pass a string/char array from C to GoFunction.

According to "C references to Go" in the cgo documentation this is possible, so I add a string parameter to GoFunction and pass the char array message to GoFunction:

main.go

package main

//extern int CFunction();
import "C"
import "fmt"

func main() {
  fmt.Println("Hello from Golang main function!")
  //Calling a CFunction in order to have C call the GoFunction
  C.CFunction();
}

//export GoFunction
func GoFunction(str string) {
  fmt.Println("Hello from GoFunction!")
}

file1.c

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

int CFunction() {
  char message[] = "Hello World from CFunction!";
  printf("CFunction says: %s\n", message);
  GoFunction(message);
  return 0;
}

Upon go build I receive this error:

./file1.c:7:14: error: passing 'char [28]' to parameter of incompatible type 'GoString'
./main.go:50:33: note: passing argument to parameter 'p0' here

According to the "strings and things" section of the above "C? Go? Cgo!" blog post:

Conversion between Go and C strings is done with the C.CString, C.GoString, and C.GoStringN functions.

But these are for use in Go, and not helpful if I want to pass string data into Go.

If you read the documentation under that, there will be a `_cgo_export.h ` generated with the type `GoString` which you can use. It looks like: `typedef struct { const char *p; GoInt n; } GoString` — JimB, Sep 26 '16 at 17:44

score 6 · Accepted Answer · answered Sep 26 '16 at 17:33

6

A string in C is a *C.char, not a Go string. Have your exported function accept the correct C type, and convert it as necessary in Go:

//export GoFunction
func GoFunction(str *C.char) {
    fmt.Println("Hello from GoFunction!")
    fmt.Println(C.GoString(str))
}

answered Sep 26 '16 at 17:33

JimB

104,193
13
262
255

I think OP was asking, how can one call a Go function that only accepts Go strings. But I guess creating a wrapper function is one way to do it. – Ainar-G Sep 26 '16 at 17:34
@Ainar-G: good point, I should have have just waited for the comment reply ;) – JimB Sep 26 '16 at 17:36
This works! When I tried this while troubleshooting I incorrectly used `str *C.Char`, resulting in the error `could not determine kind of name for C.Char` – Joe Perks Sep 26 '16 at 17:38
@JoePrvacy See my answer if you actually need `str` to be `string`. – Ainar-G Sep 26 '16 at 17:46

score 4 · Answer 2 · answered Sep 26 '16 at 17:43

If you want to pass a C string to a function that accepts only Go strings, you can use GoString type on the C side:

char message[] = "Hello World from CFunction!";
printf("CFunction says: %s\n", message);
GoString go_str = {p: message, n: sizeof(message)}; // N.B. sizeof(message) will
                                                    // only work for arrays, not
                                                    // pointers.
GoFunction(go_str);
return 0;

Thanks much! I upvoted but it won't show on the counter since I am low rep. — Joe Perks, Sep 26 '16 at 18:00

Call Go function with string parameter from C?

2 Answers2

Linked