| The most basic operations on threads include creating a thread, passing information in and out, stopping a thread, and so on. We can also obtain lists of threads, check the state of a thread, and check various other information. We present an overview of these basic operations here. Creating Threads Creating a thread is easy. We call the new method and attach a block that will be the body of the thread.
thread = Thread.new do # Statements comprising # the thread... end
The value returned is obviously an object of type Thread, which is used by the main thread to control the thread it has created. What if we want to pass parameters into a thread? We can do this by passing parameters into Thread.new, which in turn will pass them into the block.
a = 4 b = 5 c = 6 thread2 = Thread.new(a,b,c) do |a, x, y| # Manipulate a, x, and y as needed. end # Note that if a is changed in the new thread, it will # change suddenly and without warning in the main # thread.
Similar to any other block parameters, any of these that correspond to existing variables will be effectively identical to those variables. The variable a in the previous fragment is a "dangerous" variable in this sense, as the comment points out. Threads might also access variables from the scope in which they were created. Obviously, without synchronization, this can be problematic. The main thread and one or more other threads might modify the variable independently of each other, and the results can be unpredictable.
x = 1 y = 2 thread3 = Thread.new do # This thread can manipulate x and y from the outer scope, # but this is not always safe. sleep(rand(0)) # Sleep a random fraction of a second. x = 3 end sleep(rand(0)) puts x # Running this code repeatedly, x may be 1 or 3 when it # is printed here!
The method fork is an alias for new; this name is derived from the well-known Unix system call of the same name. Accessing Thread-local Variables We know that it can be dangerous for a thread to use variables from outside its scope; we know also that a thread can have local data of its own. But what if a thread wants to "make public" some of the data that it owns? A special mechanism exists for this purpose. If a thread object is treated as a hash, thread-local data can be accessed from anywhere within the scope of that thread object. We don't mean that actual local variables can be accessed in this way, but only that we have access to named data on a per-thread basis. There is also a method called key? that will tell us whether a given name is in use for this thread. Within the thread, we must refer to the data in the same hash-like way. Using Thread.current will make this a little less unwieldy.
thread = Thread.new do t = Thread.current t[:var1] = "This is a string" t[:var2] = 365 end # Access the thread-local data from outside... x = thread[:var1] # "This is a string" y = thread[:var2] # 365 has_var2 = thread.key?("var2") # true has_var3 = thread.key?("var3") # false
Note that these data are accessible from other threads even after the thread that owned them is dead (as in this case). Besides a symbol (as you just saw), we can also use a string to identify the thread-local variable.
thread = Thread.new do t = Thread.current t["var3"] = 25 t[:var4] = "foobar" end a = thread[:var3] = 25 b = thread["var4"] = "foobar"
Don't confuse these special names with actual local variables.
thread = Thread.new do t = Thread.current t["var3"] = 25 t[:var4] = "foobar" var3 = 99 # True local variables (not var4 = "zorch" # accessible from outside) end a = thread[:var3] # 25 b = thread["var4"] # "foobar"
Finally, note that an object reference to a true local variable can be used as a sort of shorthand within the thread. This is true as long as you carefully preserve the same object reference rather than creating a new one.
thread = Thread.new do t = Thread.current x = "nXxeQPdMdxiBAxh" t[:my_message] = x x.reverse! x.delete! "x" x.gsub!(/[A-Z]/,"") # On the other hand, assignment would create a new # object and make this shorthand useless... end a = thread[:my_message] # "hidden"
Also, this shortcut obviously won't work when you are dealing with values such as Fixnums, which are stored as immediate values rather than object references. Querying and Changing Thread Status The Thread class has several class methods that serve various purposes. The list method returns an array of all living threads, the main method returns a reference to the main thread that spawns the others, and the current method that allows a thread to find its own identity.
t1 = Thread.new { sleep 100 } t2 = Thread.new do if Thread.current == Thread.main puts "This is the main thread." # Does NOT print end 1.upto(1000) sleep 0.1 end end count = Thread.list.size # 3 if Thread.list.include?(Thread.main) puts "Main thread is alive." # Always prints! end if Thread.current == Thread.main puts "I'm the main thread." # Prints here... end
The exit, pass, start, stop, and kill methods are used to control the execution of threads (often from inside or outside).
# In the main thread... Thread.kill(t1) # Kill this thread now Thread.pass(t2) # Pass execution to t2 now t3 = Thread.new do sleep 20 Thread.exit # Exit the thread puts "Can't happen!" # Never reached end Thread.kill(t2) # Now kill t2 # Now exit the main thread (killing any others) Thread.exit
Note that there is no instance method stop, so a thread can stop itself but not another thread. Various methods exist for checking the state of a thread. The instance method alive? will tell whether the thread is still "living" (not exited), and stop? will tell whether the thread is in a stopped state.
count = 0 t1 = Thread.new { loop { count += 1 } } t2 = Thread.new { Thread.stop } sleep 1 flags = [t1.alive?, # true t1.stop?, # false t2.alive?, # true t2.stop?] # true
The status of a thread can be determined using the status method. The value returned will be "run" if the thread is currently running; sleep if it is stopped, sleeping, or waiting on I/O; false if it terminated normally, and nil if it terminated with an exception.
t1 = Thread.new { loop {} } t2 = Thread.new { sleep 5 } t3 = Thread.new { Thread.stop } t4 = Thread.new { Thread.exit } t5 = Thread.new { raise "exception" } s1 = t1.status # "run" s2 = t2.status # "sleep" s3 = t3.status # "sleep" s4 = t4.status # false s5 = t5.status # nil
The global variable $SAFE can be set differently in different threads. In this sense, it isn't truly a global variable at all; but we shouldn't complain because this allows us to have threads run with different levels of safety. The safe_level method will tell us at what level a thread is running.
t1 = Thread.new { $SAFE = 1; sleep 5 } t2 = Thread.new { $SAFE = 3; sleep 5 } sleep 1 lev0 = Thread.main.safe_level # 0 lev1 = t1.safe_level # 1 lev2 = t2.safe_level # 3
The priority of a thread can be examined and changed using the priority accessor.
t1 = Thread.new { loop { sleep 1 } } t2 = Thread.new { loop { sleep 1 } } t2.priority = 3 # Set t2 at priority 3 p1 = t1.priority # 0 p2 = t2.priority # 3
A thread with higher (numerically greater) priority will be scheduled more often. The special method pass is used when a thread wants to yield control to the scheduler. The thread merely yields its current timeslice; it doesn't actually stop or go to sleep.
t1 = Thread.new do puts "alpha" Thread.pass puts "beta" end t2 = Thread.new do puts "gamma" puts "delta" end t1.join t2.join
In this contrived example, we get output in the order alpha gamma delta beta when Thread.pass is called as shown. Without it, we get alpha beta gamma delta as the order. Of course, this feature shouldn't be used for synchronization, but only for the "thrifty" allocation of timeslices. A thread that is stopped can be awakened by using the run or wakeup methods:
t1 = Thread.new do Thread.stop puts "There is an emerald here." end t2 = Thread.new do Thread.stop puts "You're at Y2." end sleep 1 t1.wakeup t2.run
The difference in these is somewhat subtle. The wakeup call changes the state of the thread so that it is runnable, but won't schedule it to be run; on the other hand, run wakes up the thread and schedules it for immediate running. In this particular case, the result is that t1 wakes up before t2, but t2 gets scheduled first, producing the following output:
You're at Y2. There is an emerald here.
Of course, it would be unwise to attempt true synchronization by depending on this subtlety. The raise instance method raises an exception in the thread specified as the receiver. (The call doesn't have to originate within the thread.)
factorial1000 = Thread.new do begin prod = 1 1.upto(1000) { |n| prod *= n } puts "1000! = #{ prod} " rescue # Do nothing... end end sleep 0.01 # Your mileage may vary. if factorial1000.alive? factorial1000.raise("Stop!") puts "Calculation was interrupted!" else puts "Calculation was successful." end
The thread spawned previously tries to calculate the factorial of 1000; if it doesn't succeed within a hundredth of a second, the main thread will kill it. Thus, on a relatively slow machine, this code fragment will print the message Calculation was interrupted!. Concerning the rescue clause inside the thread, obviously we could have put any code there that we wanted, as with any other such clause. Achieving a Rendezvous (and Capturing a Return Value) Sometimes the main thread wants to wait for another thread to finish. The instance method join will accomplish this.
t1 = Thread.new { do_something_long() } do_something_brief() t1.join # Wait for t1
Note that a join is necessary if the main thread is to wait on another thread. Otherwise, when the main thread exits, a thread is killed. For example, this code fragment would never give us its final answer without the join at the end:
meaning_of_life = Thread.new do puts "The answer is..." sleep 10 puts 42 end sleep 9 meaning_of_life.join
Here is a useful little idiom. It will call join on every living thread except the main one. (It is an error for any thread, even the main thread, to call join on itself.)
Thread.list.each { |t| t.join if t != Thread.main }
It is, of course, possible for one thread to do a join on another when neither is the main thread. If the main thread and another attempt to join each other, a deadlock results; the interpreter will detect this case and exit the program.
thr = Thread.new { sleep 1; Thread.main.join } thr.join # Deadlock results!
A thread has an associated block, and a block can have a return value. This implies that a thread can return a value. The value method will implicitly do a join operation and wait for the thread to complete; then it will return the value of the last evaluated expression in the thread.
max = 10000 thr = Thread.new do sum = 0 1.upto(max) { |i| sum += i } sum end guess = (max*(max+1))/2 print "Formula is " if guess == thr.value puts "right." else puts "right." end
Dealing with Exceptions What happens if an exception occurs within a thread? As it turns out, the behavior is configurable. A flag called abort_on_exception operates both at the class and instance levels. This is implemented as an accessor at both levels (that is, it is readable and writable). In short, if abort_on_exception is true for a thread, an exception in that thread will terminate all the other threads also.
Thread.abort_on_exception = true t1 = Thread.new do puts "Hello" sleep 2 raise "some exception" puts "Goodbye" end t2 = Thread.new { sleep 100 } sleep 2 puts "The End"
In the preceding code, the systemwide abort_on_exception is set to true (overriding the default). Thus, when t1 gets an exception, t1 and the main thread are also killed. The word Hello is the only output generated. In this next example, the effect is the same:
t1 = Thread.new do puts "Hello" sleep 2 raise "some exception" puts "Goodbye" end t1.abort_on_exception = true t2 = Thread.new { sleep 100 } sleep 2 puts "The End"
In the final example, the default of false is assumed, and we finally get to see the output The End from the main thread. (We never see Goodbye because t1 is always terminated when the exception is raised.)
t1 = Thread.new do puts "Hello" sleep 2 raise "some exception" puts "Goodbye" end t2 = Thread.new { sleep 100 } sleep 2 puts "The End"
Output:
Hello The End
Using a Thread Group A thread group is a way of managing threads that are logically related to each other. Normally, all threads belong to the Default thread group (which is a class constant). But if a new thread group is created, new threads can be added to it. A thread can only be in one thread group at a time. When a thread is added to a thread group, it is automatically removed from whatever group it was in previously. The ThreadGroup.new class method creates a new thread group, and the add instance method adds a thread to the group:
f1thread = Thread.new("file1") { |file| waitfor(file) } f2thread = Thread.new("file2") { |file| waitfor(file) } file_threads = ThreadGroup.new file_threads.add f1 file_threads.add f2
The instance method list returns an array of all the threads in the thread group:
# Count living threads in this_group count = 0 this_group.list.each { |x| count += 1 if x.alive? } if count < this_group.list.size puts "Some threads in this_group are not living." else puts "All threads in this_group are alive." end
There is plenty of room for useful methods to be added to ThreadGroup. Here we show methods to wake up every thread in a group, to wait for all threads to catch up (via join), and to kill all threads in a group:
class ThreadGroup def wakeup list.each { |t| t.wakeup } end def join list.each { |t| t.join if t != Thread.current } end def kill list.each { |t| t.kill } end end
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