Question

On GLFW FAQ, item 2.9 it is stated:

[…] It is strongly recommended that all OpenGL and GLFW calls
(except for thread management and synchronization calls) are made from
the main thread, which should not be a big problem since only a single
window is supported. This method is also compatible with the future
direction of GLFW.

The emphasis is mine.

So, what is the difference between the main thread and other threads?

Answer 1

The statement

“Is … thread safe? No. However, neither is OpenGL.”

is wrong. OpenGL is of course thread safe.

Here’s the deal: For each thread either one or no OpenGL context can be bound to a drawable (made current). OpenGL calls operate on the context that is active in the thread the calls are made from. It is perfectly possible to transfer a OpenGL context between threads. For this the context to be transfered first must be unbound, then it can be rebound in another thread.

Each OpenGL context manages its own set of state variable and objects (textures, buffers). However context can be “entangled”, i.e. share their object space. State is still individual though.

A single drawable (window, PBuffer) can have multiple contexts from different threads being bound to. If contexts from different threads draw to the same drawable a race condition occours and the results are undefined. However in the case of depth tested drawing the outcome should be reasonable. However simultanous drawing to a single drawable will strongly impair performance, so it better is avoided.

The main use for multiple OpenGL contexts in multiple threads is to share their objects so that one thread can load and update data for the other context. It makes sense to bind the helper contexts to off-screen or hidden drawables to prevent race conditions to happen.

There’s no technical difference between the threads. From a programming point of view each thread will have a slightly different semantic, which is imposed by the programm running, not by the system architecture. In the case of most OpenGL applications the conventional semantics are, that the main thread will create the window, draw all elements visible to the user (including OpenGL operations) and collect user input. The threads launched from the main thread are worker threads without direct user interaction. However this task distribution is purely by choice and because it turned out to work well. But it’s perfectly possible, and sometimes advisable, to use a different scheme. And like already said, there is no technical difference about the threads within a program. All threads are equal rights citizens within a process.