A crash happens if you try to access a memory location that you are not suppose to or if you have caused a memory corruption and it has caused a crash at some other instruction.
The questions are: can anyone suggest about the reason of this problem?
The back buffer. Also called the framebuffer.
What is the buffer that holds the RGB data of the image displayed on the screen?
Any function for screenshot ?
Use the GL documentation. http://www.opengl.org/sdk/docs/man3/