XGetImage takes a lot of time to run - c

Question

XGetImage takes 3-4 seconds to execute and completely freezes X11
Display *display;
display = XOpenDisplay(NULL);
if (!display) {fprintf(stderr, "unable to connect to display");return 7;}
Window w;
int x,y,i;
unsigned m;
Window root = XDefaultRootWindow(display);
if (!root) {fprintf(stderr, "unable to open rootwindow");return 8;}
//sleep(1);
if(!XQueryPointer(display,root,&root,&w,&x,&y,&i,&i,&m))
{ printf("unable to query pointer\n"); return 9;}
XImage *image;
XWindowAttributes attr;
XGetWindowAttributes(display, root, &attr);
image = XGetImage(display,root,0,0,attr.width,attr.height,AllPlanes,XYPixmap);
XCloseDisplay(display);
if (!image) {printf("unable to get image\n"); return 10;}
In the Xorg log:
[ 13234.693] AUDIT: Thu Jan 7 20:12:13 2016: 3856: client 45 connected from local host ( uid=500 gid=500 pid=12993 )
Auth name: MIT-MAGIC-COOKIE-1 ID: 153
[ 13238.774] AUDIT: Thu Jan 7 20:12:18 2016: 3856: client 45 disconnected
time:
real 0m4.080s
user 0m0.002s
sys 0m0.007s
Ideally I want this function to run in less than 0.1 seconds

Answer 1

XYPixmap is a very specialized format that doesn't have many uses. You should use ZPixmap nearly always.
XYPixmap works plane by plane. What does it mean? Take bit 0 of every pixel, and tightly pack all these bits in an array of unsigned int. That's youir plane 0. Then take bit 1 of every pixel, and pack all these bits in an array. That's your plane 1. Then take bit 2 of every pixel...
Framebuffer
__________________________________________________________________
/
Pixel 0 Pixel 1 Pixel 2
[0][1][2][3][4][5][6][7] [0][1][2][3][4][5][6][7] [0][1][2]....
| | |
| +------------------------+ |
| | |
| | +--------------------------------------------------+
| | |
v v v
[0][0][0]..... \
(Plane 0) |
|
[1][1][1].... | Result
(Plane 1) |
.... |
[7][7][7].... |
(Plane 7) |
/
If your framebuffer is stored like this, which is the case for most modern hardware, that's a lot of bit manipulation!
The picture shows 8 bit pixels, but it's the same for any other depth.
ZPixmap on the other hand takes entire pixels and stuffs them into an array:
Framebuffer
__________________________________________________________________
/
Pixel 0 Pixel 1 Pixel 2
[0][1][2][3][4][5][6][7] [0][1][2][3][4][5][6][7] [0][1][2]....
| | | | | | | | | | | | | | | | | | |
v v v v v v v v v v v v v v v v v v v
[0][1][2][3][4][5][6][7] [0][1][2][3][4][5][6][7] [0][1][2]....
\_____________________________________________________________________
Result
This is simple direct copying, which should be very fast.