What does the suspend function mean in a Kotlin Coroutine?

Question

I'm reading Kotlin Coroutine and know that it is based on suspend function. But what does suspend mean?

Coroutine or function gets suspended?

From https://kotlinlang.org/docs/reference/coroutines.html

Basically, coroutines are computations that can be suspended without blocking a thread

I heard people often say "suspend function". But I think it is the coroutine who gets suspended because it is waiting for the function to finished? "suspend" usually means "cease operation", in this case the coroutine is idle.

Should we say the coroutine is suspended ?

Which coroutine gets suspended?

From https://kotlinlang.org/docs/reference/coroutines.html

To continue the analogy, await() can be a suspending function (hence also callable from within an async {} block) that suspends a coroutine until some computation is done and returns its result:

async { // Here I call it the outer async coroutine
    ...
    // Here I call computation the inner coroutine
    val result = computation.await()
    ...
}

It says "that suspends a coroutine until some computation is done", but coroutine is like a lightweight thread. So if the coroutine is suspended, how can the computation is done ?

We see await is called on computation, so it might be async that returns Deferred, which means it can start another coroutine

fun computation(): Deferred<Boolean> {
    return async {
        true
    }
}

The quote say that suspends a coroutine. Does it mean suspend the outer async coroutine, or suspend the inner computation coroutine?

Does suspend mean that while outer async coroutine is waiting (await) for the inner computation coroutine to finish, it (the outer async coroutine) idles (hence the name suspend) and returns thread to the thread pool, and when the child computation coroutine finishes, it (the outer async coroutine) wakes up, takes another thread from the pool and continues?

The reason I mention the thread is because of https://kotlinlang.org/docs/tutorials/coroutines-basic-jvm.html

The thread is returned to the pool while the coroutine is waiting, and when the waiting is done, the coroutine resumes on a free thread in the pool

Answer 1

Suspending functions are at the center of everything coroutines. A suspending function is simply a function that can be paused and resumed at a later time. They can execute a long running operation and wait for it to complete without blocking.

The syntax of a suspending function is similar to that of a regular function except for the addition of the suspend keyword. It can take a parameter and have a return type. However, suspending functions can only be invoked by another suspending function or within a coroutine.

suspend fun backgroundTask(param: Int): Int {
     // long running operation
}

Under the hood, suspend functions are converted by the compiler to another function without the suspend keyword, that takes an addition parameter of type Continuation<T>. The function above for example, will be converted by the compiler to this:

fun backgroundTask(param: Int, callback: Continuation<Int>): Int {
   // long running operation
}

Continuation<T> is an interface that contains two functions that are invoked to resume the coroutine with a return value or with an exception if an error had occurred while the function was suspended.

interface Continuation<in T> {
   val context: CoroutineContext
   fun resume(value: T)
   fun resumeWithException(exception: Throwable)
}