Joe Privacy Joe Privacy - 2 months ago 11
C Question

Call Go function with string parameter from C?

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


Other references:
https://blog.golang.org/c-go-cgo
(not enough reputation to post 3 links)
According to the "strings and things" section of the above 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.

Answer

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))
}