How to write directly to linux framebuffer?

look at FBIOPUT_VSCREENINFO, ioctl and mmap

(I have the code but not at this pc, sorry)

edit: this should get you started

  //open file descriptor and get info
inf fdScreen = open( "devicename", O_RDWR );
fb_var_screeninfo varInfo;
ioctl( fdScreen, FBIOGET_VSCREENINFO, &varInfo );

  //set resolution/dpi/color depth/.. in varInfo, then write it back
ioctl( fdScreen, FBIOPUT_VSCREENINFO, &varInfo );

  //get writable screen memory; unsigned short here for 16bit color
unsigned short* display = mmap( 0, nScreenSize,
                                PROT_READ | PROT_WRITE, MAP_SHARED, fdScreen, 0 );

Basically you open /dev/fb0, do some ioctls on it, then mmap it. Then you just write to the mmap'd area in your process.

Are you looking to write a device driver? If so check out this HowTo guide