android get screen size via C

Yes, you can get the screen resolution from /dev/graphics/fb0, but (at least on my phone), read access is restricted to root and users in the 'graphics' group. At any rate, you can do something like this (error checking removed for clarity):

// ... other standard includes ...
#include <sys/ioctl.h>
#include <linux/fb.h>

//...

struct fb_var_screeninfo fb_var;
int fd = open("/dev/graphics/fb0", O_RDONLY);
ioctl(fd, FBIOGET_VSCREENINFO, &fb_var);
close(fd);
// screen size will be in fb_var.xres and fb_var.yres

I'm not sure if these are considered "public" NDK interfaces, since I don't know if Google considers "Android runs on Linux and uses the Linux Framebuffer interface" to be a part of the public API, but it does appear to work.

If you do want to make sure it's portable, you should probably have a JNI fallback that calls into the Java WindowManager API.