<Chapter Label="Synchronization variables">
  <Heading>Synchronization variables</Heading>

<Section Label="section:Synchronization variables">
  <Heading>Synchronization variables</Heading>

Synchronization variables (also often called dataflow variables in the literature) are variables that can be written
only once; attempts to read the variable block until it has been written to.
<P/>
Synchronization variables are created with <Ref Func="CreateSyncVar"/>, written with <Ref Func="SyncWrite"/> and read
with <Ref Func="SyncRead"/>.

<Example><![CDATA[
gap> sv := CreateSyncVar();;
gap> RunAsyncTask(function()
>      Sleep(10);
>      SyncWrite(sv, MakeImmutable([1, 2, 3]));
>    end);;
gap> SyncRead(sv);
[ 1, 2, 3 ]
]]></Example>

<ManSection>
    <Func Name="CreateSyncVar"
      Arg=''/>

<Description>
The function <Ref Func="CreateSyncVar"/> takes no arguments. It returns a new synchronization variable. There is no need to
deallocate it; the garbage collector will free the memory and all related resources when it is no longer accessible.
</Description>
</ManSection>

<ManSection>
    <Func Name="SyncWrite"
      Arg='syncvar, obj'/>

<Description>
<Ref Func="SyncWrite"/> attempts to assign the value <A>obj</A> to <A>syncvar</A>. If <A>syncvar</A> has been previously
assigned a value, the call will fail with a runtime error; otherwise, <A>obj</A> will be assigned to <A>syncvar</A>.
<P/>
In order to make sure that the recipient can read the result, the <A>obj</A> argument should not be a thread-local
object; it should be public, read-only, or shared.
</Description>
</ManSection>

<ManSection>
    <Func Name="SyncRead"
      Arg='syncvar'/>

<Description>
<Ref Func="SyncRead"/> reads the value previously assigned to <A>syncvar</A> with <Ref Func="SyncWrite"/>. If no value has been
assigned yet, it blocks. It returns the assigned value.
</Description>
</ManSection>
  </Section>
</Chapter>
