Home > Articles > Programming > C/C++

This chapter is from the book

This chapter is from the book

13.8 Thread Termination

There's something unusual about the examples given so far, in particular writer defined on page 402 and fileWriter defined on the facing page: both functions contain an infinite loop. In fact, a closer look at the file copy example reveals that main and fileWriter understand each other well regarding copying things around but never discuss application termination; in other words, main does not ever tell fileWriter, "We're done; let's finish and go home."

Termination of multithreaded applications has always been tricky. Threads are easy to start, but once started they are difficult to finish; the application shutdown event is asynchronous and may catch a thread in the middle of an arbitrary operation. Low-level threading APIs do offer a means to forcefully terminate threads, but invariably with the cautionary note that such a function is a blunt tool that should be replaced with a higher-level shutdown protocol.

D offers a simple and robust thread termination protocol. Each thread has an owner thread; by default the owner is the thread that initiated the spawn. You can change the current thread's owner dynamically by calling setOwner(tid). Each thread has exactly one owner but a given thread may own multiple threads.

The most important manifestation of the owner/owned relationship is that when the owner thread terminates, the calls to receive in the owned thread will throw the OwnerTerminated exception. The exception is thrown only if receive has no more matching messages and must wait for a new message; as long as receive has something to fetch from the mailbox, it will not throw. In other words, when the owner thread terminates, the owned threads' calls to receive (or receiveOnly for that matter) will throw OwnerTerminated if and only if they would otherwise block waiting for a new message. The ownership relation is not necessarily unidirectional. In fact, two threads may even own each other; in that case, whichever thread finishes will notify the other.

With thread ownership in mind, let's take a fresh look at the file copy program on page 406. At any given moment, there are a number of messages in flight between the main thread and the secondary thread. The faster the reads are relative to writes, the more buffers will wait in the writer thread's mailbox waiting to be processed. When main returns, it will cause the call to receive to throw an exception, but not before all of the pending messages are handled. Right after the mailbox of the writer is cleared (and the last drop of data is written to the target file), the next call to receive throws. The writer thread exits with the OwnerTerminated exception, which is recognized by the runtime system, which simply ignores it. The operating system closes stdin and stdout as it always does, and the copy operation succeeds.

It may appear there is a race between the moment the last message is sent from main and the moment main returns (causing receive to throw). What if the exception "makes it" before the last message—or worse, before the last few messages? In fact there is no race because causality is always respected in the posting thread: the last message is posted onto the secondary thread's queue before the OwnerTerminated exception makes its way (in fact, propagating the exception is done via the same queue as regular messages). However, a race would exist if main exits while a different, third thread is posting messages onto fileWriter's queue.

A similar reasoning shows that our previous simple example that writes 200 messages in lockstep is also correct: main exits after mailing (in the nick of time) the last message to the secondary thread. The secondary thread first exhausts the queue and then ends with the OwnerTerminated exception.

If you find throwing an exception too harsh a mechanism for handling a thread's exit, you can always handle OwnerTerminated explicitly:

// Ends without an exception
void fileWriter() {
   // Write loop
   for (bool running = true; running; ) {
      receive(
         (immutable(ubyte)[] buffer) { tgt.write(buffer); },
         (OwnerTerminated) { running = false; }
      );
   }
   stderr.writeln("Normally terminated.");
}

In this case, fileWriter returns peacefully when main exits and everyone's happy. But what happens in the case when the secondary thread—the writer—throws an exception? The call to the write function may fail if there's a problem writing data to tgt. In that case, the call to send from the primary thread will fail by throwing an OwnedFailed exception, which is exactly what should happen. By the way, if an owned thread exits normally (as opposed to throwing an exception), subsequent calls to send to that thread also fail, just with a different exception type: OwnedTerminated.

The file copy program is more robust than its simplicity may suggest. However, it should be said that relying on the default termination protocol works smoothly when the relationships between threads are simple and well understood. When there are many participating threads and the ownership graph is complex, it is best to establish explicit "end-of-communication" protocols throughout. In the file copy example, a simple idea would be to send by convention a buffer of size zero to signal the writer that the reading thread has finished successfully. Then the writer acknowledges termination to the reader, which finally can exit. Such an explicit protocol scales well to cases when there are multiple threads processing the data stream between the reader and the writer.

  • + Share This
  • 🔖 Save To Your Account

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.

Overview


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information


To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.

Surveys

Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.

Newsletters

If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information


Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.

Security


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.

Children


This site is not directed to children under the age of 13.

Marketing


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information


If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.

Choice/Opt-out


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information


Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents


California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure


Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.

Links


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact


Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice


We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020