Using strfmon with cgo

Question

I'm trying to use the C function strfmon using cgo.

The example C code that works is:

#include <stdio.h>
#include <monetary.h>

int main(void)
{
    char str[100];
    double money = 1234.56;
    strfmon(str, 100, "%i", money);
    printf("%s\n", string);
}

The Go code I've written so far is:

package main

// #cgo CFLAGS: -g -Wall
// #include <stdlib.h>
// #include <monetary.h>
import "C"
import (
    "fmt"
)

func main() {
    str := [100]C.char{}
    var money C.double = 1234.56
    C.strfmon(str, 100, "%i", money)
    fmt.Printf("%+v\n", str)
}

When I go run main.go I get the following error:

./main.go:14:2: unexpected type: ...

I believe the ... refers to the variadic argument in strfmon but I'm not sure how to work around that from Go.

Answer 1

According to the cgo command documentation:

Calling variadic C functions is not supported. It is possible to circumvent this by using a C function wrapper.

And strfmon(3p) is indeed a variadic function as indicated by the ... characters in the signature:

ssize_t strfmon(char *restrict s, size_t maxsize,
   const char *restrict format, ...);

As such, you can create a wrapper function in C which has a fixed number of arguments and calls strfmon(...) as needed, for example:

package main

// #cgo CFLAGS: -g -Wall
//
// #include <locale.h>
// #include <monetary.h>
// #include <stdlib.h>
//
// size_t format_amount(char * s, size_t maxsize, char * format, double amount)
// {
//   setlocale(LC_ALL, "en_US");
//   return strfmon(s, maxsize, format, amount);
// }
//
import "C"
import "fmt"
import "unsafe"

const SIZE = 100

func main() {
  str := C.CString(string(make([]byte, SIZE)))
  money := C.double(1234.56)
  format := C.CString("[%n]")

  C.format_amount(str, SIZE-1, format, money) // Call our wrapper here.
  fmt.Printf("OK: %s\n", C.GoString(str))
  // OK: [$1,234.56]

  C.free(unsafe.Pointer(str))
  C.free(unsafe.Pointer(format))
}