cgo.md

# Introduction

First, http://golang.org/cmd/cgo is the primary cgo documentation.

There is also a good introduction article at http://golang.org/doc/articles/c_go_cgo.html.

## The basics

If a Go source file imports ` "C" `, it is using cgo. The Go file will have access to anything appearing in the comment immediately preceding the line ` import "C" `, and will be linked against all other cgo comments in other Go files, and all C files included in the build process.

Note that there must be no blank lines in between the cgo comment and the import statement.

To access a symbol originating from the C side, use the package name ` C `. That is, if you want to call the C function ` printf() ` from Go code, you write ` C.printf() `.  Since variable argument methods like printf aren't supported yet (issue [975](https://github.com/golang/go/issues/975)), we will wrap it in the C method "myprint":

```go
package cgoexample

/*
#include <stdio.h>
#include <stdlib.h>

void myprint(char* s) {
	printf("%s\n", s);
}
*/
import "C"

import "unsafe"

func Example() {
	cs := C.CString("Hello from stdio\n")
	C.myprint(cs)
	C.free(unsafe.Pointer(cs))
}
```

## Calling Go functions from C

It is possible to call both top-level Go functions and function variables from C code invoked from Go code using cgo.

### Global functions

Go makes its functions available to C code through use of a special ` //export ` comment.
Note: you can't define any C functions in preamble if you're using exports.

For example, there are two files, foo.c and foo.go:
foo.go contains:

```go
package gocallback

import "fmt"

/*
#include <stdio.h>
extern void ACFunction();
*/
import "C"

//export AGoFunction
func AGoFunction() {
	fmt.Println("AGoFunction()")
}

func Example() {
	C.ACFunction()
}
```

foo.c contains:

```go
#include "_cgo_export.h"
void ACFunction() {
	printf("ACFunction()\n");
	AGoFunction();
} 
```

### Function variables

The following code shows an example of invoking a Go callback from C code. Because of the [pointer passing rules](https://golang.org/cmd/cgo/#hdr-Passing_pointers) Go code can not pass a function value directly to C.  Instead it is necessary to use an indirection. This example uses a registry with a mutex, but there are many other ways to map from a value that can be passed to C to a Go function.

```go
package gocallback

import (
	"fmt"
	"sync"
)

/*
extern void go_callback_int(int foo, int p1);

// normally you will have to define function or variables
// in another separate C file to avoid the multiple definition
// errors, however, using "static inline" is a nice workaround
// for simple functions like this one.
static inline void CallMyFunction(int foo) {
	go_callback_int(foo, 5);
}
*/
import "C"

//export go_callback_int
func go_callback_int(foo C.int, p1 C.int) {
	fn := lookup(int(foo))
	fn(p1)
}

func MyCallback(x C.int) {
	fmt.Println("callback with", x)
}

func Example() {
	i := register(MyCallback)
	C.CallMyFunction(C.int(i))
	unregister(i)
}

var mu sync.Mutex
var index int
var fns = make(map[int]func(C.int))

func register(fn func(C.int)) int {
	mu.Lock()
	defer mu.Unlock()
	index++
	for fns[index] != nil {
		index++
	}
	fns[index] = fn
	return index
}

func lookup(i int) func(C.int) {
	mu.Lock()
	defer mu.Unlock()
	return fns[i]
}

func unregister(i int) {
	mu.Lock()
	defer mu.Unlock()
	delete(fns, i)
}
```

### Function pointer callbacks

C code can call exported Go functions with their explicit name. But if a C-program wants a function pointer, a gateway function has to be written. This is because we can't take the address of a Go function and give that to C-code since the cgo tool will generate a stub in C that should be called. The following example shows how to integrate with C code wanting a function pointer of a give type.

Place these source files under _$GOPATH/src/ccallbacks/_. Compile and run with:

```console
$ gcc -c clibrary.c
$ ar cru libclibrary.a clibrary.o
$ go build
$ ./ccallbacks
Go.main(): calling C function with callback to us
C.some_c_func(): calling callback with arg = 2
C.callOnMeGo_cgo(): called with arg = 2
Go.callOnMeGo(): called with arg = 2
C.some_c_func(): callback responded with 3
```

**goprog.go**

```go
package main

/*
#cgo CFLAGS: -I .
#cgo LDFLAGS: -L . -lclibrary

#include "clibrary.h"

int callOnMeGo_cgo(int in); // Forward declaration.
*/
import "C"

import (
	"fmt"
	"unsafe"
)

//export callOnMeGo
func callOnMeGo(in int) int {
	fmt.Printf("Go.callOnMeGo(): called with arg = %d\n", in)
	return in + 1
}

func main() {
	fmt.Printf("Go.main(): calling C function with callback to us\n")
	C.some_c_func((C.callback_fcn)(unsafe.Pointer(C.callOnMeGo_cgo)))
}
```

**cfuncs.go**

```go
package main

/*

#include <stdio.h>

// The gateway function
int callOnMeGo_cgo(int in)
{
	printf("C.callOnMeGo_cgo(): called with arg = %d\n", in);
	int callOnMeGo(int);
	return callOnMeGo(in);
}
*/
import "C"
```

**clibrary.h**
```
#ifndef CLIBRARY_H
#define CLIBRARY_H
typedef int (*callback_fcn)(int);
void some_c_func(callback_fcn);
#endif
```

**clibrary.c**

```go
#include <stdio.h>

#include "clibrary.h"

void some_c_func(callback_fcn callback)
{
	int arg = 2;
	printf("C.some_c_func(): calling callback with arg = %d\n", arg);
	int response = callback(2);
	printf("C.some_c_func(): callback responded with %d\n", response);
}
```

## Go strings and C strings

Go strings and C strings are different. Go strings are the combination of a length and a pointer to the first character in the string. C strings are just the pointer to the first character, and are terminated by the first instance of the null character, ` '\0' `.

Go provides means to go from one to another in the form of the following three functions:
  * ` func C.CString(goString string) *C.char `
  * ` func C.GoString(cString *C.char) string `
  * ` func C.GoStringN(cString *C.char, length C.int) string `

One important thing to remember is that ` C.CString() ` will allocate a new string of the appropriate length, and return it. That means the C string is not going to be garbage collected and it is up to **you** to free it. A standard way to do this follows.

```go
// #include <stdlib.h>
import "C"
import "unsafe"
...
	var cmsg *C.char = C.CString("hi")
	defer C.free(unsafe.Pointer(cmsg))
	// do something with the C string
```

Of course, you aren't required to use ` defer ` to call ` C.free() `. You can free the C string whenever you like, but it is your responsibility to make sure it happens.

## Turning C arrays into Go slices

C arrays are typically either null-terminated or have a length kept elsewhere.

Go provides the following function to make a new Go byte slice from a C array:
  * ` func C.GoBytes(cArray unsafe.Pointer, length C.int) []byte `

To create a Go slice backed by a C array (without copying the original data), one needs to acquire this length at runtime and use a type conversion to a pointer to a very big array and then slice it to the length that you want (also remember to set the cap if you're using Go 1.2 or later), for example (see http://play.golang.org/p/XuC0xqtAIC for a runnable example):

```go
import "C"
import "unsafe"
...
        var theCArray *C.YourType = C.getTheArray()
        length := C.getTheArrayLength()
        slice := (*[1 << 30]C.YourType)(unsafe.Pointer(theCArray))[:length:length]
```

It is important to keep in mind that the Go garbage collector will not interact with this data, and that if it is freed from the C side of things, the behavior of any Go code using the slice is nondeterministic.

## Common Pitfalls
### Struct Alignment Issues
As Go doesn't support packed struct (e.g., structs where maximum alignment is 1 byte), you can't
use packed C struct in Go. Even if you program passes compilation, it won't do what you want.
To use it, you have to read/write the struct as byte array/slice.

Another problem is that some types has lower alignment requirement than their counterpart in Go,
and if that type happens to be aligned in C but not in Go rules, that struct simply can't be represented
in Go. An example is this ([issue 7560](https://github.com/golang/go/issues/7560)):

```go
struct T {
    uint32_t pad;
    complex float x;
};
```
Go's complex64 has an alignment of 8-byte, where as C has only 4-byte (because C treats the
complex float internally as a ` struct { float real; float imag; } `, not a basic type), this T struct simply
doesn't have a Go representation. For this case, if you control the layout of the struct, move the
complex float so that it is also aligned to 8-byte is better, and if you're not willing to move it,
use this form will force it to align to 8-byte (and waste 4-byte):

```go
struct T {
   uint32_t pad;
   __attribute__((align(8))) complex float x;
};
```

However, if you don't control the struct layout, you will have to define accessor C functions for
that struct because cgo won't be able to translate that struct into equivalent Go struct.

### ` //export ` and definition in preamble
If a Go source file uses any ` //export ` directives, then the C code in the comment may only include declarations (` extern int f(); `), not definitions (` int f() { return 1; }  ` or ` int n; `).
Note: you can use ` static inline ` trick to work around this restriction for tiny functions defined
in the preamble (see above for a complete example).

### Windows

In order to use cgo on Windows, you'll also need to first install a gcc compiler (for instance, mingw-w64) and have gcc.exe (etc.) in your PATH environment variable before compiling with cgo will work.