Const NULL As Any Ptr = 0

Type Nodo
    As String valor
    As Nodo Ptr sgte
End Type

Type QueueSeguro
    As Nodo Ptr head, tail
    As Any Ptr mutex
    Declare Constructor
    Declare Destructor
    Declare Function TryDequeue(Byref resultado As String) As Boolean
    Declare Sub Enqueue(Byval valor As String)
End Type

Constructor QueueSeguro
    head = NULL
    tail = NULL
    mutex = Mutexcreate()
End Constructor

Destructor QueueSeguro
    Mutexdestroy(mutex)
End Destructor

Function QueueSeguro.TryDequeue(Byref resultado As String) As Boolean
    Mutexlock(mutex)
    If head = NULL Then
        Mutexunlock(mutex)
        Return False
    End If

    resultado = head->valor
    Dim temp As Nodo Ptr = head
    head = head->sgte
    If head = NULL Then tail = NULL
    Delete temp
    Mutexunlock(mutex)

    Return True
End Function

Sub QueueSeguro.Enqueue(Byval valor As String)
    Dim newNode As Nodo Ptr = New Nodo
    newNode->valor = valor
    newNode->sgte = NULL
    Mutexlock(mutex)
    If tail = NULL Then
        head = newNode
        tail = newNode
    Else
        tail->sgte = newNode
        tail = newNode
    End If
    Mutexunlock(mutex)
End Sub

Dim Shared m_EscribirQueue As QueueSeguro
Dim Shared m_LeerQueue As QueueSeguro

Sub Leer(Byval userdata As Any Ptr)
    Dim linea As String
    Dim f As Integer = Freefile
    Open "input.txt" For Input As #f
    While Not Eof(f)
        Line Input #f, linea
        m_EscribirQueue.Enqueue(linea)
    Wend
    Close #f
    m_EscribirQueue.Enqueue("")

    Dim resultado As String
    Do Until m_LeerQueue.TryDequeue(resultado)
        Sleep(10)
    Loop

    Print resultado
End Sub

Sub Escribir(Byval userdata As Any Ptr)
    Dim cnt As Integer = 0
    Dim linea As String
    Do
        Do Until m_EscribirQueue.TryDequeue(linea)
            Sleep(10)
        Loop
        If linea <> "" Then
            Print linea
            cnt += 1
        Else
            m_LeerQueue.Enqueue(Str(cnt))
            Exit Do
        End If
    Loop
End Sub

Dim t1 As Any Ptr = Threadcreate(@Leer, 0)
Dim t2 As Any Ptr = Threadcreate(@Escribir, 0)
Threadwait(t1)
Threadwait(t2)

Sleep