Goal:

• Send an Array of a given struct from C to Golang using cgo.

Working Code (no arrays)

Disclaimer: This is my first functional C code, things may be wrong.

GetPixel.c

#include "GetPixel.h"
#include <stdio.h>
#include <stdlib.h>
#include <X11/Xlib.h>
#include <X11/Xutil.h>

Display *display;
XImage *im;

void open_display()
{
    // xlib: must be called before any other X* methods, enables multithreading for this client
    XInitThreads();

    // save display in a global variable so we don't allocate it per `get_pixel`
    // TODO: figure out classes later (or are these C++ only? Can I use them in Golang?)
    display = XOpenDisplay((char *) NULL);
}

void close_display()
{
    // xlib: use XCloseDisplay instead of XFree or free for Display objects
    XCloseDisplay(display);
}

void create_image(int x, int y, int w, int h)
{
    // save image in a global variable so we don't allocate it per `get_pixel`
    im = XGetImage(display, XRootWindow(display, XDefaultScreen(display)), x, y, w, h, AllPlanes, XYPixmap);
}

void destroy_image()
{
    // xlib: use XDestroyImage instead of XFree or free for XImage objects
    XDestroyImage(im);
}

void get_pixel(struct Colour3 *colour, int x, int y)
{
    // TODO: could I return `c` without converting it to my struct?
    XColor c;
    c.pixel = XGetPixel(im, x, y);
    XQueryColor(display, XDefaultColormap(display, XDefaultScreen(display)), &c);

    // xlib: stored as values 0-65536
    colour->r = c.red / 256;
    colour->g = c.green / 256;
    colour->b = c.blue / 256;
}

GetPixel.h

# Trial and Error:
# - Golang needs me to define crap in an H file
# - C needs me to define my struct in an H file
# - C needs me to use `typedef` and name my struct twice (???)

#ifndef __GETPIXEL_
#define __GETPIXEL_

typedef struct Colour3 {
  int r, g, b ;
} Colour3 ;              # redundant?

void open_display();
void close_display();

void create_image(int x, int y, int w, int h);
void destroy_image();

void get_pixel(struct Colour3 *colour, int x, int y);

#endif

GetPixel.go

package x11util

func Screenshot(sx, sy, w, h int, filename string) {
    img := image.NewRGBA(image.Rectangle{
        image.Point{sx, sy}, image.Point{w, h},
    })

    defer trace(sx, sy, w, h, filename)(img)

    C.open_display()
    C.create_image(C.int(sx), C.int(sy), C.int(w), C.int(h))
    defer func() {
        # does this work?
        C.destroy_image()
        C.close_display()
    }()

    # Trial and Error
    # - C needs me to pass a pointer to a struct
    p := C.Colour3{}
    for x := sx; x < w; x++ {
        for y := sy; y < h; y++ {
            C.get_pixel(&p, C.int(x), C.int(y))
            img.Set(x, y, color.RGBA{uint8(p.r), uint8(p.g), uint8(p.b), 255})
        }
    }

    f, err := os.Create(filename)
    if err != nil {
        log.Error("unable to save screenshot", "filename", filename, "error", err)
    }
    defer f.Close()
    png.Encode(f, img)
}

Sure, it works -- but takes between 30 and 55 seconds for a 1080p screen.

Attempt 2

All the existing code, but instead of get_pixel, let's try to get patches of (3x3) 9 pixels at a time, as an optimization.

GetPixel.c

# ... existing code ...

struct Colour3* get_pixel_3x3(int sx, int sy)
{
    XColor c;

    # the internet collectively defines this as "a way to define C arrays where the data remains after the function returns"
    struct Colour3* pixels = (struct Colour3 *)malloc(9 * sizeof(Colour3));

    for(int x=sx; x<sx+3; ++x)
    {
        for(int y=sy; y<sy+3; ++y)
        {
            # ... existing code from Attempt 1 ...

            // Is this even the correct way to into C arrays?
            pixels++;
        }
    }

    return pixels;
}

GetPixel.h

# ... existing code ...
struct Colour3* get_pixel_3x3(int sx, int sy);

GetPixel.go

package x11util

func ScreenshotB(sx, sy, w, h int, filename string) {
    # ... existing code ...

    var i int
    for x := sx; x < w; x += 3 {
        for y := sy; y < h; y += 3 {
            // returns an array of exactly 9 pixels
            p := C.get_pixel_3x3(C.int(x), C.int(y))

            // unsafely convert to an array we can use -- at least, this was supposed to work, but never did
            // pb := (*[9]C.Colour3)(unsafe.Pointer(&p))

            // convert to a slice we can use... with reflect -- this code is magic, have no idea how it works.
            var pa []C.Colour3
            sliceHeader := (*reflect.SliceHeader)((unsafe.Pointer(&pa)))
            sliceHeader.Cap = 9
            sliceHeader.Len = 9
            sliceHeader.Data = uintptr(unsafe.Pointer(&p))

            // assign pixels from base (adding an offset 0-2) to print 3x3 blocks
            for xo := 0; xo < 3; xo++ {
                for yo := 0; yo < 3; yo++ {
                    img.Set(x+xo, y+yo, color.RGBA{uint8(pa[i].r), uint8(pa[i].g), uint8(pa[i].b), 255})
                    i++
                }
            }

            i = 0
        }
    }

    # ... existing code ...
}

This takes 20% less time to execute, the optimization is definitely there-- HOWEVER: the resulting image is a mess of either pink or yellow pixels, instead of the expected result. Which leads me to believe that I am reading random memory instead of my intention.

Since I know that reading a single pixel works and that the C loop works, I can only think that I totally misunderstand arrays in C, or how to pass them to Golang, or how to read/iterate them in Golang.

At this point, I have no idea what else to try, four pages of Stack Overflow and 20-ish pages of Googling has given me lots of different answers for the other direction (Go->C) -- but not a whole lot here. I could not find an example of C.GoBytes that worked either.

Attempt 3

Letting Golang handle allocation has simplified accessing the array. This code now works for 3x3, but fails when attempting to get "the whole screen at once" (see Attempt 4)

GetPixel.c

# ... existing code from Attempt 1 ...

# out-param instead of a return variable, let Golang allocate
void get_pixel_3x3(struct Colour3 *pixels, int sx, int sy)
{
    XColor c;
    for(int x=sx; x<sx+3; ++x)
    {
        for(int y=sy; y<sy+3; ++y)
        {
            # ... existing code from Attempt 2 ...
        }
    }
}

GetPixel.h

# ... existing code from Attempt 1 ...
void get_pixel_3x3(struct Colour3* pixels, int sx, int sy);

GetPixel.go

package x11util

func ScreenshotB(sx, sy, w, h int, filename string) {
    # ... existing code from Attempt 1 ...

    var i int
    var p C.Colour3 // why does this even work?
    for x := sx; x < w; x += 3 {
        for y := sy; y < h; y += 3 {
            // returns an array of 9 pixels
            C.get_pixel_3x3(&p, C.int(x), C.int(y))

            // unsafely convert to an array we can use
            pb := (*[9]C.Colour3)(unsafe.Pointer(&p))
            pa := pb[:] // seems to be required?

            // assign pixels from base (adding an offset 0-2) to print 3x3 blocks
            for xo := 0; xo < 3; xo++ {
                for yo := 0; yo < 3; yo++ {
                    # ... existing code from Attempt 1 ...
                }
            }

            i = 0
        }
    }

    # ... existing code from Attempt 1 ...
}

Attempt 4

Results in Segmentation Violation (SEGFAULT)

GetPixel.c

# ... existing code from Attempt 1 ...
void get_pixel_arbitrary(struct Colour3 *pixels, int sx, int sy, int w, int h)
{
    XColor c;
    for(int x=sx; x<sx+w; ++x)
    {
        for(int y=sy; y<sy+h; ++y)
        {
            # ... existing code from Attempt 3 ...
        }
    }
}

GetPixel.h

# ... existing code from Attempt 1 ...
void get_pixel_arbitrary(struct Colour3 *pixels, int sx, int sy, int w, int h);

GetPixel.go

package x11util

func ScreenshotC(sx, sy, w, h int, filename string) {
    # ... existing code from Attempt 1 ...

    var p C.Colour3 // I'm sure this is the culprit

    // returns an array of "all the screens"
    // 240x135x3x8 = 777600 (<768KB)
    C.get_pixel_arbitrary(&p, 0, 0, C.int(w), C.int(h)) // segfault here

    // unsafely convert to an array we can use
    pb := (*[1 << 30]C.Colour3)(unsafe.Pointer(&p)) // internet showed this magic 1<<30 is required because Golang won't make an array with an unknown length
    pa := pb[:w*h] // not sure if this is correct, but it doesn't matter yet, we don't get this far (segfault happens above.)

    // assign pixels from base (adding an offset 0-2) to print 3x3 blocks
    for x := 0; x < w; x++ {
        for y := 0; y < h; y++ {
            # ... existing code from Attempt 1 ...
        }
    }

    # ... existing code from Attempt 1 ...
}

Attempt 5

GetPixel.c

struct Colour3* get_pixel_arbitrary(int sx, int sy, int w, int h)
{
    XColor c;

    struct Colour3* pixels = (struct Colour3 *)malloc(w*h * sizeof(Colour3));
    struct Colour3* start = pixels;

    for(int x=sx; x<sx+w; ++x)
    {
        for(int y=sy; y<sy+h; ++y)
        {
            c.pixel = XGetPixel(im, x, y);
            XQueryColor(display, XDefaultColormap(display, XDefaultScreen(display)), &c);

            pixels->r = c.red / 256;
            pixels->g = c.green / 256;
            pixels->b = c.blue / 256;
            pixels++;
        }
    }

    return start;
}

GetPixel.go

    p := C.get_pixel_arbitrary(0, 0, C.int(w), C.int(h))

    // unsafely convert to an array we can use
    pb := (*[1 << 30]C.Colour3)(unsafe.Pointer(&p))
    pa := pb[: w*h : w*h] // magic :len:len notation shown in docs but not explained? (if [start:end] then [?:?:?])

    for x := 0; x < w; x++ {
        for y := 0; y < h; y++ {
            img.Set(x, y, color.RGBA{uint8(pa[i].r), uint8(pa[i].g), uint8(pa[i].b), 255})
            i++
        }
    }

    // assume I should be freeing in C instead of here?
    C.free(unsafe.Pointer(p))

Which produces a "stretched mess" which I then overflow on (now I have to assume I've done something wrong on the C side again, unless this is completely the wrong idea when it was mentioned I should allocate back in C?)

I am very willing to accept a guide with examples as the answer at this point, as there are things I'm missing surely -- however, when Googling (and even on GitHub) looking for examples of this, I couldn't find any. I would love to take the results of this thread and do just that :).

https://i.sstatic.net/Ht6Xq.jpg