When do I need to call `glUseProgram`?

Question

Looking at examples at http://learnopengl.com/#!Getting-started/Hello-Triangle and many different places, I've found that the draw sequence usually goes like this:

// setup VBO, VAO, load shaders


while (true) { // game loop
  glUseProgram(shader);
  glBindVertexArray(someVAO);
  drawStuff(...);

  // ...
}

Now my question is, is there any reason why I'd want to repeatedly do glUseProgram(...) on each frame, unless I'm using multiple shaders (such as OpenGL clearing currently used shader after rendering?)?

Also, if I am using multiple shaders, do I need to glUseProgram before switching to a different VAO? I'm unclear on whether the VAO just gets set as active, or if it actually modifies the state of a currently used shader. I've tried looking this up in the OpenGL spec, but couldn't quite find it.

edit: Yes I did mean glUseProgram, sorry.

Answer 1

Maybe you mean glUseProgram()? I can't find something related to glUseShader, so if i'm wrong then sorry. But if this is the case then basically you use it when you want to use the shader itself.

This is an example from my current opengl project (EDIT: Oh, by the way, this example code is on the main rendering loop):

GL30.glBindVertexArray(vao); // Binding vao.
majorShader.glUseProgram(); // Don't mind the "majorShader" it's an util class i made.
GL11.glBindTexture(GL11.GL_TEXTURE_2D, 0); //Don't mind these, some framebuffer stuffs.
GL30.glBindFramebuffer(GL30.GL_FRAMEBUFFER, fb);
display.renderTriangle(); // Render the scene.

GL30.glBindVertexArray(screenVao); // Bind a different vao.
screenShader.glUseProgram(); // Switching shader because it does a different thing
GL11.glBindTexture(GL11.GL_TEXTURE_2D, fbTex);
GL30.glBindFramebuffer(GL30.GL_FRAMEBUFFER, 0);
display.renderScreen(); // Render the frame buffer, which used different vao, vbo and shader.

But, if you're planning on using only one shader, then you can just call glUseProgram() once on initialization function or something like that before drawing.

EDIT2 (Summary): So, 1. I don't see why you need to call glUseProgram() repeatedly if you're only using one shader (Unless maybe you call glUseProgram(0) for some reason and want to use shader again..) 2. You can see in my case, i use glUseProgram() after calling glBindVertexArray() when i want to do a something with a different shader