Hello Triangle: An OpenGL ES 3.0 Example - Praise for OpenGL ES 30 Programming Guide - page 39

char* infoLog = malloc(sizeof(char) * infoLen);
glGetProgramInfoLog(programObject, infoLen, NULL,infoLog);
esLogMessage("Error linking program:\n%s\n", infoLog);
free(infoLog) ;
}
glDeleteProgram(programObject) ;
return FALSE;
}
// Store the program object
userData->programObject = programObject;
After all of these steps, we have finally compiled the shaders, checked for
compile errors, created the program object, attached the shaders, linked
the program, and checked for link errors. After successful linking of the
program object, we can now finally use the program object for rendering!
To use the program object for rendering, we bind it using glUseProgram .
// Use the program object
glUseProgram(userData->programObject);
After calling glUseProgram with the program object handle, all
subsequent rendering will occur using the vertex and fragment shaders
attached to the program object.
Setting the Viewport and Clearing the Color Buffer
Now that we have created a rendering surface with EGL and initialized
and loaded shaders, we are ready to actually draw something. The Draw
callback function draws the frame. The first command that we execute in
Draw is glViewport , which informs OpenGL ES of the origin, width, and
height of the 2D rendering surface that will be drawn to. In OpenGL ES,
the viewport defines the 2D rectangle in which all OpenGL ES rendering
operations will ultimately be displayed.
// Set the viewport
glviewport(0, 0, esContext->width, esContext->height);
The viewport is defined by an origin ( x , y ) and a width and height. We
cover glViewport in more detail in Chapter 7, “Primitive Assembly and
Rasterization,” when we discuss coordinate systems and clipping.