Screenshot code working perfectly in Simulator but not in iOS Device

Screenshot code working perfectly in Simulator but not in iOS Device

This is my code for screen shot to save in library or email. The problem
is, it is working perfectly in Simulator but when i run this code in
Device whatever on screen it only gets white image i have tried with iOS 5
and iOS 6 both . What should be the reason Where i am wrong
NSInteger myDataLength = 320 * 430 * 4;
GLubyte *buffer1 = (GLubyte *) malloc(myDataLength);
GLubyte *buffer2 = (GLubyte *) malloc(myDataLength);
//Read image memory form OpenGL
glReadPixels(0, 50, 320, 430, GL_RGBA, GL_UNSIGNED_BYTE, buffer1);
//Invert result image buffer into secondary buffer
for(int y = 0; y < 430; y++) {
for(int x = 0; x < 320 * 4; x++) {
buffer2[(429 - y) * 320 * 4 + x] = buffer1[y * 4 * 320 + x];
}
}
//Create bitmap context
CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
CGContextRef destContext = CGBitmapContextCreate(buffer2, 320, 430, 8,
320 * 4, colorSpace, kCGImageAlphaPremultipliedLast |
kCGBitmapByteOrder32Big);
//Get image from context
CGImageRef resultContext = CGBitmapContextCreateImage(destContext);
UIImage *resultImg = [UIImage imageWithCGImage: resultContext];
CGImageRelease(resultContext);
//Send to mail or Save to PhotoLibrary
if (sendMail) {
[self emailImage: resultImg];
} else {
UIImageWriteToSavedPhotosAlbum(resultImg, nil, nil, nil);
}
//Release allocated memory
// [resultImg release];
free(buffer2);
free(buffer1);
CGContextRelease(destContext);
CGColorSpaceRelease(colorSpace);