> You can't write to the framebuffer device while X11 is running because it will just get overwritten immediately.
You can't write to fbdev because that's not what X11 is using to display buffers to the screen, it uses the newer kernel mode-setting (KMS) API. It's only when you switch away to a VT that the "KMS master" drops its session, the kernel switches back to fbcon, and you can see the "raw fbdev" buffer again, which is usually a fake framebuffer. If you then write to the fbdev framebuffer, you'll see your changes, but fbcon will come in and overwrite them with the kernel console, unless you ioctl(KDSETMODE, KD_GRAPHICS); to turn off fbcon from drawing.
You can't write to fbdev because that's not what X11 is using to display buffers to the screen, it uses the newer kernel mode-setting (KMS) API. It's only when you switch away to a VT that the "KMS master" drops its session, the kernel switches back to fbcon, and you can see the "raw fbdev" buffer again, which is usually a fake framebuffer. If you then write to the fbdev framebuffer, you'll see your changes, but fbcon will come in and overwrite them with the kernel console, unless you ioctl(KDSETMODE, KD_GRAPHICS); to turn off fbcon from drawing.