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fix go 1.6 build by changing pointer passing
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In Go 1.6, we're no longer allowed to pass a pointer to
a Go struct with Go fields to Cairo functions that want a callback.

Work around this by instead making a map keyed by integers and
passing integer keys through to C.  This is a bit cumbersome but
it's the recommended fix for this problem.
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@evmar
evmar committed Feb 20, 2016
1 parent 2b68506 commit 6a65339
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Showing 3 changed files with 60 additions and 16 deletions.
16 changes: 10 additions & 6 deletions cairo/cairo.go
View file
Original file line number Diff line number Diff line change
Expand Up @@ -29,14 +29,14 @@ import (
#include <cairo-xlib.h>
#include <stdlib.h>
int gocairoWriteFunc(void* closure, const unsigned char* data, unsigned int length);
int gocairoReadFunc(void* closure, const unsigned char* data, unsigned int length);
int gocairoWriteFunc(int key, const unsigned char* data, unsigned int length);
int gocairoReadFunc(int key, const unsigned char* data, unsigned int length);
// A cairo_write_func_t for use in cairo_surface_write_to_png.
cairo_status_t gocairo_write_func(void *closure,
const unsigned char *data,
unsigned int length) {
return gocairoWriteFunc(closure, data, length)
return gocairoWriteFunc(*(int*)closure, data, length)
? CAIRO_STATUS_SUCCESS
: CAIRO_STATUS_WRITE_ERROR;
}
Expand All @@ -45,7 +45,7 @@ cairo_status_t gocairo_write_func(void *closure,
cairo_status_t gocairo_read_func(void *closure,
const unsigned char *data,
unsigned int length) {
return gocairoReadFunc(closure, data, length)
return gocairoReadFunc(*(int*)closure, data, length)
? CAIRO_STATUS_SUCCESS
: CAIRO_STATUS_WRITE_ERROR;
}
Expand All @@ -60,9 +60,11 @@ func (s Status) Error() string {
// WriteToPNG encodes a Surface to an io.Writer as a PNG file.
func (surface *Surface) WriteToPNG(w io.Writer) error {
data := writeClosure{w: w}
key := goPointers.put(data)
status := C.cairo_surface_write_to_png_stream((*C.cairo_surface_t)(surface.Ptr),
(C.cairo_write_func_t)(unsafe.Pointer(C.gocairo_write_func)),
unsafe.Pointer(&data))
unsafe.Pointer(&key))
goPointers.clear(key)
// TODO: which should we prefer between writeClosure.err and status?
// Perhaps test against CAIRO_STATUS_WRITE_ERROR? Needs a test case.
return Status(status).toError()
Expand All @@ -72,9 +74,11 @@ func (surface *Surface) WriteToPNG(w io.Writer) error {
// PNG data.
func ImageSurfaceCreateFromPNGStream(r io.Reader) (*ImageSurface, error) {
data := readClosure{r: r}
key := goPointers.put(data)
surf := &ImageSurface{wrapSurface(C.cairo_image_surface_create_from_png_stream(
(C.cairo_read_func_t)(unsafe.Pointer(C.gocairo_read_func)),
unsafe.Pointer(&data)))}
unsafe.Pointer(&key)))}
goPointers.clear(key)
// TODO: which should we prefer between readClosure.err and status?
// Perhaps test against CAIRO_STATUS_WRITE_ERROR? Needs a test case.
return surf, surf.status()
Expand Down
44 changes: 40 additions & 4 deletions cairo/util.go
View file
Original file line number Diff line number Diff line change
Expand Up @@ -17,6 +17,7 @@ package cairo
import (
"io"
"reflect"
"sync"
"unsafe"
)

Expand All @@ -36,14 +37,49 @@ func (s Status) toError() error {
return s
}

// In Go 1.6, you're not allowed to pass Go pointers through C.
// To work around this, use a map keyed by integers for stashing
// arbitrary Go data.
type goPointerStash struct {
sync.Mutex
data map[C.int]interface{}
nextKey C.int
}

var goPointers = &goPointerStash{}

func (gp *goPointerStash) put(data interface{}) C.int {
gp.Lock()
defer gp.Unlock()
if gp.data == nil {
gp.data = make(map[C.int]interface{})
}
key := gp.nextKey
gp.nextKey++
gp.data[key] = data
return key
}

func (gp *goPointerStash) get(key C.int) interface{} {
gp.Lock()
defer gp.Unlock()
return gp.data[key]
}

func (gp *goPointerStash) clear(key C.int) {
gp.Lock()
defer gp.Unlock()
delete(gp.data, key)
}

type writeClosure struct {
w io.Writer
err error
}

//export gocairoWriteFunc
func gocairoWriteFunc(closure unsafe.Pointer, data unsafe.Pointer, clength C.uint) bool {
writeClosure := (*writeClosure)(closure)
func gocairoWriteFunc(key C.int, data unsafe.Pointer, clength C.uint) bool {
writeClosure := goPointers.get(key).(writeClosure)
length := uint(clength)
slice := ((*[1 << 30]byte)(data))[:length:length]
_, writeClosure.err = writeClosure.w.Write(slice)
Expand All @@ -56,8 +92,8 @@ type readClosure struct {
}

//export gocairoReadFunc
func gocairoReadFunc(closure unsafe.Pointer, data unsafe.Pointer, clength C.uint) bool {
readClosure := (*readClosure)(closure)
func gocairoReadFunc(key C.int, data unsafe.Pointer, clength C.uint) bool {
readClosure := goPointers.get(key).(readClosure)
length := uint(clength)
buf := ((*[1 << 30]byte)(data))[:length:length]
_, readClosure.err = io.ReadFull(readClosure.r, buf)
Expand Down
16 changes: 10 additions & 6 deletions gen.go
View file
Original file line number Diff line number Diff line change
Expand Up @@ -678,14 +678,14 @@ import (
#include <cairo-xlib.h>
#include <stdlib.h>
int gocairoWriteFunc(void* closure, const unsigned char* data, unsigned int length);
int gocairoReadFunc(void* closure, const unsigned char* data, unsigned int length);
int gocairoWriteFunc(int key, const unsigned char* data, unsigned int length);
int gocairoReadFunc(int key, const unsigned char* data, unsigned int length);
// A cairo_write_func_t for use in cairo_surface_write_to_png.
cairo_status_t gocairo_write_func(void *closure,
const unsigned char *data,
unsigned int length) {
return gocairoWriteFunc(closure, data, length)
return gocairoWriteFunc(*(int*)closure, data, length)
? CAIRO_STATUS_SUCCESS
: CAIRO_STATUS_WRITE_ERROR;
}
Expand All @@ -694,7 +694,7 @@ cairo_status_t gocairo_write_func(void *closure,
cairo_status_t gocairo_read_func(void *closure,
const unsigned char *data,
unsigned int length) {
return gocairoReadFunc(closure, data, length)
return gocairoReadFunc(*(int*)closure, data, length)
? CAIRO_STATUS_SUCCESS
: CAIRO_STATUS_WRITE_ERROR;
}
Expand All @@ -709,9 +709,11 @@ func (s Status) Error() string {
// WriteToPNG encodes a Surface to an io.Writer as a PNG file.
func (surface *Surface) WriteToPNG(w io.Writer) error {
data := writeClosure{w: w}
key := goPointers.put(data)
status := C.cairo_surface_write_to_png_stream((*C.cairo_surface_t)(surface.Ptr),
(C.cairo_write_func_t)(unsafe.Pointer(C.gocairo_write_func)),
unsafe.Pointer(&data))
unsafe.Pointer(&key))
goPointers.clear(key)
// TODO: which should we prefer between writeClosure.err and status?
// Perhaps test against CAIRO_STATUS_WRITE_ERROR? Needs a test case.
return Status(status).toError()
Expand All @@ -721,9 +723,11 @@ func (surface *Surface) WriteToPNG(w io.Writer) error {
// PNG data.
func ImageSurfaceCreateFromPNGStream(r io.Reader) (*ImageSurface, error) {
data := readClosure{r: r}
key := goPointers.put(data)
surf := &ImageSurface{wrapSurface(C.cairo_image_surface_create_from_png_stream(
(C.cairo_read_func_t)(unsafe.Pointer(C.gocairo_read_func)),
unsafe.Pointer(&data)))}
unsafe.Pointer(&key)))}
goPointers.clear(key)
// TODO: which should we prefer between readClosure.err and status?
// Perhaps test against CAIRO_STATUS_WRITE_ERROR? Needs a test case.
return surf, surf.status()
Expand Down

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