Call Go functions from C - c

Question

I am trying to create a static object written in Go to interface with a C program (say, a kernel module or something).
I have found documentation on calling C functions from Go, but I haven't found much on how to go the other way. What I've found is that it's possible, but complicated.
Here is what I found:
Blog post about callbacks between C and Go
Cgo documentation
Golang mailing list post
Does anyone have experience with this? In short, I'm trying to create a PAM module written entirely in Go.

Answer 1

You can call the Go code from C. It is a confusing proposition, though.
The process is outlined in the blog post you linked to. But I can see how that isn't very helpful. Here is a short snippet without any unnecessary bits. It should make things a little clearer.
package foo
// extern int goCallbackHandler(int, int);
//
// static int doAdd(int a, int b) {
// return goCallbackHandler(a, b);
// }
import "C"
//export goCallbackHandler
func goCallbackHandler(a, b C.int) C.int {
return a + b
}
// This is the public function, callable from outside this package.
// It forwards the parameters to C.doAdd(), which in turn forwards
// them back to goCallbackHandler(). This one performs the addition
// and yields the result.
func MyAdd(a, b int) int {
return int( C.doAdd( C.int(a), C.int(b)) )
}
The order in which everything is called is as follows:
foo.MyAdd(a, b) ->
C.doAdd(a, b) ->
C.goCallbackHandler(a, b) ->
foo.goCallbackHandler(a, b)
The key to remember here is that a callback function must be marked with the //export comment on the Go side and as extern on the C side. This means that any callback you wish to use, must be defined inside your package.
In order to allow a user of your package to supply a custom callback function, we use the exact same approach as above, but we supply the user's custom handler (which is just a regular Go function) as a parameter that is passed onto the C side as void*. It is then received by the callbackhandler in our package and called.
Let's use a more advanced example I am currently working with. In this case, we have a C function that performs a pretty heavy task: It reads a list of files from a USB device. This can take a while, so we want our app to be notified of its progress. We can do this by passing in a function pointer that we defined in our program. It simply displays some progress info to the user whenever it gets called. Since it has a well known signature, we can assign it its own type:
type ProgressHandler func(current, total uint64, userdata interface{}) int
This handler takes some progress info (current number of files received and total number of files) along with an interface{} value which can hold anything the user needs it to hold.
Now we need to write the C and Go plumbing to allow us to use this handler. Luckily the C function I wish to call from the library allows us to pass in a userdata struct of type void*. This means it can hold whatever we want it to hold, no questions asked and we will get it back into the Go world as-is. To make all this work, we do not call the library function from Go directly, but we create a C wrapper for it which we will name goGetFiles(). It is this wrapper that actually supplies our Go callback to the C library, along with a userdata object.
package foo
// #include <somelib.h>
// extern int goProgressCB(uint64_t current, uint64_t total, void* userdata);
//
// static int goGetFiles(some_t* handle, void* userdata) {
// return somelib_get_files(handle, goProgressCB, userdata);
// }
import "C"
import "unsafe"
Note that the goGetFiles() function does not take any function pointers for callbacks as parameters. Instead, the callback that our user has supplied is packed in a custom struct that holds both that handler and the user's own userdata value. We pass this into goGetFiles() as the userdata parameter.
// This defines the signature of our user's progress handler,
type ProgressHandler func(current, total uint64, userdata interface{}) int
// This is an internal type which will pack the users callback function and userdata.
// It is an instance of this type that we will actually be sending to the C code.
type progressRequest struct {
f ProgressHandler // The user's function pointer
d interface{} // The user's userdata.
}
//export goProgressCB
func goProgressCB(current, total C.uint64_t, userdata unsafe.Pointer) C.int {
// This is the function called from the C world by our expensive
// C.somelib_get_files() function. The userdata value contains an instance
// of *progressRequest, We unpack it and use it's values to call the
// actual function that our user supplied.
req := (*progressRequest)(userdata)
// Call req.f with our parameters and the user's own userdata value.
return C.int( req.f( uint64(current), uint64(total), req.d ) )
}
// This is our public function, which is called by the user and
// takes a handle to something our C lib needs, a function pointer
// and optionally some user defined data structure. Whatever it may be.
func GetFiles(h *Handle, pf ProgressFunc, userdata interface{}) int {
// Instead of calling the external C library directly, we call our C wrapper.
// We pass it the handle and an instance of progressRequest.
req := unsafe.Pointer(&progressequest{ pf, userdata })
return int(C.goGetFiles( (*C.some_t)(h), req ))
}
That's it for our C bindings. The user's code is now very straight forward:
package main
import (
"foo"
"fmt"
)
func main() {
handle := SomeInitStuff()
// We call GetFiles. Pass it our progress handler and some
// arbitrary userdata (could just as well be nil).
ret := foo.GetFiles( handle, myProgress, "Callbacks rock!" )
....
}
// This is our progress handler. Do something useful like display.
// progress percentage.
func myProgress(current, total uint64, userdata interface{}) int {
fc := float64(current)
ft := float64(total) * 0.01
// print how far along we are.
// eg: 500 / 1000 (50.00%)
// For good measure, prefix it with our userdata value, which
// we supplied as "Callbacks rock!".
fmt.Printf("%s: %d / %d (%3.2f%%)\n", userdata.(string), current, total, fc / ft)
return 0
}
This all looks a lot more complicated than it is. The call order has not changed as opposed to our previous example, but we get two extra calls at the end of the chain:
The order is as follows:
foo.GetFiles(....) ->
C.goGetFiles(...) ->
C.somelib_get_files(..) ->
C.goProgressCB(...) ->
foo.goProgressCB(...) ->
main.myProgress(...)

Answer 2

It is not a confusing proposition if you use gccgo. This works here:
foo.go
package main
func Add(a, b int) int {
return a + b
}
bar.c
#include <stdio.h>
extern int go_add(int, int) __asm__ ("example.main.Add");
int main() {
int x = go_add(2, 3);
printf("Result: %d\n", x);
}
Makefile
all: main
main: foo.o bar.c
gcc foo.o bar.c -o main
foo.o: foo.go
gccgo -c foo.go -o foo.o -fgo-prefix=example
clean:
rm -f main *.o

Answer 3

The answer has changed with the release of Go 1.5
This SO question that I asked some time ago addresses the issue again in light of the 1.5 added capabilities
Using Go code in an existing C project

Answer 4

As far as I am concerned it isn't possible:
Note: you can't define any C functions in preamble if you're using
exports.
source: https://github.com/golang/go/wiki/cgo