Home > Articles

This chapter is from the book

Item 10: Enable the Efficient Reproduction of the Problem

A key to effective debugging is a problem that you can reliably and easily reproduce. You need this for a number of reasons. First, if you can always reproduce the issue with a single hit of a button, you can focus on tracking down the cause rather than wasting time randomly fumbling to make the problem appear. In addition, if you can provide an easy way to reproduce the problem, you can easily take the description and ask for outside help (see Item 2: “Use Focused Queries to Search the Web for Insights into Your Problem”). Finally, once you fix the fault, you can easily demonstrate that your fix works by running the sequence that demonstrated the problem again and witnessing that the failure no longer occurs.

Creating a short example or a test case that reproduces the problem can go a long way in increasing your efficiency. The golden standard is a minimal example: the shortest possible that reproduces the problem. The platinum standard, which goes under the name SSCCE (see Item 1: “Handle All Problems through an Issue-Tracking System”), has the example be not only short, but also self-contained and correct (compilable and runable). With a minimal example at hand, you won’t waste time exploring code paths that could have been eliminated. Also, any logs and traces you create and must examine won’t be longer than what’s actually needed. And, a short example will also execute more quickly than a longer one, especially when executed in a debugging mode that imposes a significant performance overhead.

To shorten your example, you can proceed top-down or bottom-up (see Item 4: “Drill Up from the Problem to the Bug or Down from the Program’s Start to the Bug”). Select the most expedient method. If the code has many dependencies, starting bottom-up from a clean slate may be preferable. If you don’t really understand the problem’s likely cause, creating a test case in a top-down fashion may help you narrow down the possibilities.

In the bottom-up fashion, you theorize the cause of the problem, for example, a call to a specific API, and you build up a test case that demonstrates the problem. In one case, I was trying to find out why a 27,000-line program was extremely slow in the complex code it used for processing its input files. By looking at the program’s invoked system calls, I hypothesized that the problem had something to do with calling tellg— a function returning the file stream’s offset—while reading the file. Indeed, running the following short snippet confirmed my suspicion (see Item 58: “Trace the Code’s Execution”) and was also useful to test the workaround (a wrapper class).

ifstream in(fname.c_str(), ios::binary);
do {
 (void)in.tellg();
} while ((val = in.get()) != EOF);

In the top-down fashion, you remove elements from the scenario that demonstrates the problem, until there’s nothing left to remove. A binary search technique is often quite useful. Say you have an HTML file that makes the browser behave in an erratic way. First eliminate the file’s head elements. If the problem persists, eliminate the body elements. If that cures the problem, restore the body elements, and then remove half of them. Repeat the process until you’ve nailed down the elements that cause the problem. Keeping your editor open and using its undo function to backtrack when you follow a wrong path will mightily increase your efficiency.

With a short example at hand, it’s also easy to make it self-contained. This means that you can take the example and replicate the problem somewhere else without external dependencies, such as libraries, headers, CSS files, and web services. If your test case requires some external elements, you can bundle them with it. Use a portable notation for referring to them, avoiding things such as absolute file paths and hard-coded IP addresses. For instance use ../resources/file.css rather than /home/susan/resources/file.css, and http://localhost:8081/myService rather than http://193.92.66.100:8081/myService. A self-contained example will make it easier for you to try it on the customer’s premises, examine it on another platform (say, on Windows instead of Linux), publish it on a Q&A forum (see Item 2: “Use Focused Queries to Search the Web for Insights into Your Problem”), and ship it to a vendor for further help.

In addition, you want to work on a replicable execution environment. If you don’t nail down the code you’re working on and the system it executes in, then you might end up searching for a bug that simply isn’t there. Consider the case of debugging a software installer. Every time you install it, it messes up your operating system configuration, which is exactly what you want to avoid when you’re trying to debug it. In this case, a useful technique is to create a virtual machine image with a pristine system in a state ready for the software installation. After every failed installation, you can simply start afresh with that image. You can also often achieve a similar result using operating-system-level virtualization or containment with a tool such as Docker. Even better, consider adopting a system configuration management tool, such as Ansible, CFEngine, Chef, Puppet, or Salt. These tools allow you to reliably create a specified system configuration from your high-level instructions. This makes it easy to maintain compatible production, testing, and development environments, and to control their evolution in the same way as you control your software.

You also want to be able to reliably replicate the failing version of your software. To do this, first put your software under configuration management with a tool, such as Git. Then, make your build process embed into the software an identifier of the source code version used for the build. The following shell command will print a variable initialization with the abbreviated Git hash of the last commit, which you can embed into your source code.

git log -n 1 --format='const string version = "%h";'

Here is an example of its output.

const string version = "035cd45";

Add to your software a way to display this version string; a command-line option or a line in the About dialog are all that’s needed. With this version identifier at hand you can then obtain a copy of the failing source code with a command such as the following:

git checkout 035cd45

If you want to increase the fidelity of builds you run on old code, don’t forget to put under version control all elements that affect what ends up in your distribution, such as the compiler, system and third-party libraries and header files, as well as the build specification (the Makefiles or IDE project configuration). As a final step, you may need to remove the variability introduced by your tools and your runtime environment (see Item 52: “Configure Deterministic Builds and Executions”).

Things to Remember

  • Reproducible runs simplify your debugging process.

  • Create a short self-contained example that reproduces the problem.

  • Have mechanisms to create a replicable execution environment.

  • Use a revision control system to label and retrieve your software’s versions.

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