An Introduction to EGL - Praise for OpenGL ES 30 Programming Guide - page 63

To associate a particular EGLContext with an EGLSurface , use the call
EGLBoolean eglMakeCurrent (EGLDisplay display ,
EGLSurface draw ,
EGLSurface read ,
EGLContext context )
display specifies the EGL display connection
draw specifies the EGL draw surface
read specifies the EGL read surface
context specifies the EGL rendering context to be attached to the
surfaces
This function returns EGL_TRUE if the call succeeded. On failure, it returns
EGL_FALSE .
You probably noticed that this call takes two EGLSurface s. Although
this approach allows flexibility that we will exploit in our discussion of
advanced EGL usage, we set both read and draw to the same value, the
window that we created previously.
Note: Because the EGL specification requires a flush for eglMakeCurrent
implementation, this call is expensive for tile-based architectures.
Putting All Our EGL Knowledge Together
This chapter concludes with a complete example showing the entire
process starting with the initialization of the EGL through binding an
EGLContext to an EGLSurface . We will assume that a native window has
already been created, and that if any errors occur, the application will
terminate.
In fact, Example 3-7 is similar to what is done in esCreateWindow , our
homegrown function that wraps the required EGL window creation code,
as shown in Chapter 2, “Hello Triangle: An OpenGL ES 3.0 Example,”
except for those routines that separate the creation of the window and the
context (for reasons that we discuss later).