glXCopyContext copies selected groups of state variables from src to dst. mask indicates which groups of state variables are to be copied. mask contains the bitwise OR of the same symbolic names that are passed to the GL command glPushAttrib. The single symbolic constant GL_ALL_ATTRIB_BITS can be used to copy the maximum possible portion of rendering state.The copy can be done only if the renderers named by src and dst share an address space. Two rendering contexts share an address space if both are nondirect using the same server, or if both are direct and owned by a single process. Note that in the nondirect case it is not necessary for the calling threads to share an address space, only for their related rendering contexts to share an address space.
Not all values for GL state can be copied. For example, pixel pack and unpack state, render mode state, and select and feedback state are not copied. The state that can be copied is exactly the state that is manipulated by the GL command
glPushAttrib.
An implicit glFlush is done by glXCopyContext if src is the current context for the calling thread.