go-fbdev

go-fbdev

Package fbdev provides tools for working with the Frame Buffer Device (fbdev) (that is common on Linux based on operating systems), for the Go programming language.

The Frame Buffer Device is an easy way to do pixel oriented graphics programming.

It is easy because you can treat the Frame Buffer Device as a file (that you can read from, and write to), or as memory (that you can also read from, and write to).

Documention

Online documentation, which includes examples, can be found at: http://godoc.org/github.com/reiver/go-fbdev

Introduction

Documentation on The Frame Buffer Device introduces it as:

The frame buffer device provides an abstraction for the graphics hardware. It represents the frame buffer of some video hardware and allows application software to access the graphics hardware through a well-defined interface, so the software doesn't need to know anything about the low-level (hardware register) stuff.

The device is accessed through special device nodes, usually located in the /dev directory, i.e. /dev/fb*.

Examples of /dev/fb* device nodes are:

• /dev/fb0
• /dev/fb1
• /dev/fb2
• etc.

Treating a Frame Buffer Device as a File

A Frame Buffer Device can be accessed as a file, such as the file /dev/fb0, /dev/fb1, /dev/fb2, /dev/fb3, etc.

For instance, in Go, you might do something such as the following (if you wanted to open the Frame Buffer Device at /dev/fb0):

fbdev, err := os.Open("/dev/fb0")

... or:

fbdev, err := os.OpenFile("/dev/fb0", os.O_WRONLY)

... or even:

fbdev, err := os.OpenFile("/dev/fb0", os.O_RDWR)

(If you wanted to open a different Frame Buffer Device, you would just replace the name in the os.Open() or os.OpenFile() call, from /dev/fb0 to /dev/fb1, /dev/fb2, /dev/fb3, etc.)

Once you have done something such as that, you can then read from one of these types of files, to determine what color specific pixel values are.

For example, in Go, you might do something such as:

var buffer [307200]byte

var p []byte = buffer[:]

// ...

n, err := fbdev.Read(p)

Or:

var offset int64
var whence int

// ...

ret, err := fbdev.Seek(offset, whence)

// ...

n, err := fbdev.Read(p)

Or alternatively:

var offset int64

// ...

n, err := fbdev.ReadAt(p, offset)

For example, you might use Read or ReadAt in that way if you wanted to create the ability for a program to take a screenshot.

(Although, the astute reader might have noticed that we don't know the width or height of the resulting image, and don't know how many bytes make up a single pixel, and don't know what color format each pixel is using.)

(But we can figure those things out using syscall.Syscall(syscall.SYS_IOCTL). More on that later.)

To write to (i.e., to draw to) the Frame Buffer Device, we can use Write and WriteAt, as in the following examples:

n, err := fbdev.Write(b)

Or:

var offset int64
var whence int

// ...

ret, err := fbdev.Seek(offset, whence)

// ...

n, err := fbdev.Write(b)

Or alternatively:

var offset int64

// ...

n, err := fbdev.WriteAt(b, offset)

(But again, the astute reader might have noticed that we don't know the width or height of the resulting image, and don't know how many bytes make up a single pixel, and don't know what color format each pixel is using.)

(And again we can figure those things out using syscall.Syscall(syscall.SYS_IOCTL). More on that later.)

Memory

The frame buffer can also be accessed as memory,

Troubleshooting #1: Testing The Framebuffer Device

One way to see if the Frame Buffer Device is working on your computer is to try to run the following command:

cat /dev/urandom >/dev/fb0

If you see this error message:

-bash: /dev/fb0: Permission denied

... then you need to run that command as root.

But anyways...

If this worked, then you should see some colored pixels. At least in the top-left corner, if not filling the screen.

But, if that didn't work, keep on reading....

Troubleshooting #2: Virtual Consoles

Note that, if you are in X11, and this test (i.e., running cat /dev/urandom >/dev/fb0 from a terminal in X11) didn't work for you, then you may need to switch to a different virtual console to get this to work.

If you are in X11, then you are probably in the virtual console that can be reached by pressing [CTRL]+[ALT]+[F7].

So, to try the test again, first switch to a different virtual console.

Ex: press [CTRL]+[ALT]+[F6].

(After pressing that, you should have been switched to a different virtual console, and should see a terminal screen, that takes up the whole screen. It will probably be a a lot of black, with some white text in the top-left corner.)

At this point, you will probably see a prompt that is asking you to login. So... login.

(Once you are logged in....)

Next try running the same command from before. I.e.,:

cat /dev/urandom >/dev/fb0

And again, if you see this error message:

-bash: /dev/fb0: Permission denied

... then you need to need to run the program as root.

And just like before, if this worked, then you should see some colored pixels. At least in the top-left corner, if not filling the screen.

(And then, once you are done, logout of that virtual console, and switch back to your original virtual console. Probably by pressing [CTRL]+[ALT]+[F7].)

Troubleshooting #3: X11

Note that, if you tried that command (i.e,. cat /dev/urandom >/dev/fb0) from X11, but it didn't work for you, there is a way to make it work from X11.

First you install the Frame Buffer Device driver for X11, with something like this:

sudo apt-get install xserver-xorg-video-fbdev

And then you create the configuration file:

/usr/share/X11/xorg.conf.d/99-fbdev.conf

... with contents such as:

Section "Device"
  Identifier "myfb"
  Driver "fbdev"
  Option "fbdev" "/dev/fb1"
EndSection

And then restart X11.

The easiest way (to restart X11) if you are running a typical Linux based operating system (such as elementary OS, Fedora, openSUSE, Mint, [Ubuntu](https://www.ubuntu.com/], etc, or even Linux From Scratch), (on a laptop, desptop, or tower computer) that automatically starts X11 for you, is to just to first (completely) shut down you computer (i.e., don't just suspend it, it needs to be completely shut down), and then turn it back on.

(Alternatively, if you are running a system that doesn't automatically start into X11 for you, you can start X11 with the command: startx.)

(Also, if you are running a system that doesn't automatically start into X11 for you, and you don't want to create that configuration file, then you can make X11 use the Frame Buffer Device by starting X11 with the command: FRAMEBUFFER=/dev/fb1 startx.)

See Also

• The Frame Buffer Device
• The Frame Buffer Device API
• The Frame Buffer Device Documentation
• Framebuffer use
• Framebuffer HOWTO