This captures the screen of Macs and saves it to a file.

// g++ -o SaveScreen SaveScreen.cpp -framework Cocoa

#include </System/Library/Frameworks/ApplicationServices.framework/Frameworks/CoreGraphics.framework/Headers/CoreGraphics.h>
#include <objc/objc-runtime.h>

CGImageRef captureScreenImage(int x,int y,int width,int height) {
 CGRect captureRect;
 captureRect.origin.x = x;
 captureRect.origin.y = y;
 captureRect.size.width = width;
 captureRect.size.height = height;
 CGImageRef img = CGWindowListCreateImage(captureRect, kCGWindowListOptionOnScreenOnly, kCGNullWindowID, kCGWindowImageDefault);
 return img;
}

objc_object* charsToNSString(char *chars) {
 objc_object *nsStringAlloc = objc_msgSend(objc_getClass("NSString"),sel_getUid("alloc"));
 return objc_msgSend(nsStringAlloc,sel_getUid("initWithUTF8String:"),chars);
}

void saveImage(CGImageRef img,char* filename) {
 objc_object *ciImageObject = objc_msgSend(objc_getClass("CIImage"),sel_getUid("imageWithCGImage:"),img);
 objc_object *imageRepObject = objc_msgSend(objc_getClass("NSBitmapImageRep"),sel_getUid("alloc"));
 objc_object *bitmapRep = objc_msgSend(imageRepObject,sel_getUid("initWithCIImage:"),ciImageObject);
 objc_object *properties = nil;
 objc_object *jpegType = nil;
 objc_object *blob = objc_msgSend(bitmapRep,sel_getUid("representationUsingType:properties:"),jpegType,properties);
 objc_msgSend(blob,sel_getUid("writeToFile:atomically:"),charsToNSString(filename),NO);
 printf("%s was written successfully.\n",filename);
}

int main() {
 saveImage(captureScreenImage(0,0,800,600),"desktop.png");
}