Home > Articles > Programming > General Programming/Other Languages

An Overview of PROMELA

  • Print
  • + Share This
This chapter is from the book

Message Channels

Message channels are used to model the exchange of data between processes. They are declared either locally or globally. In the declaration

chan qname = [16] of { short, byte, bool }

the typename chan introduces a channel declaration. In this case, the channel is named qname, and it is declared to be capable of storing up to sixteen messages. There can be any finite number of fields per message. In the example, each message is said to consist of three fields: the first is declared to be of type short, the second is of type byte, and the last is of type bool. Each field must be either a user-defined type, such as Field from the last section, or a predefined type from Table 3.1. In particular, it is not possible to use an array as a type declarator in a message field. An indirect way of achieving this effect is again to embed the array into a user-defined type, and to use the type name as the type declarator for the message field. Note also that since the type chan appears in Table 3.1, it is always valid to use chan itself as a field declarator. We can make good use of this capability to pass channel identifiers from one process to another.

The statement

qname!expr1,expr2,expr3

sends a message with the values of the three expressions listed to the channel that we just created. The value of each expression is cast to the type of the message field that corresponds with its relative position in the list of message parameters. By default 2 the send statement is only executable if the target channel is not yet full, and otherwise it blocks.

The statement

qname?var1,var2,var3

retrieves a message from the head of the same buffer and stores the values from the three fields into the corresponding variables.

The receive statement is executable only if the source channel is non-empty.

It is an error to send or receive either more or fewer message fields than were declared for the message channel that is addressed.

An alternative, and equivalent, notation for the send and receive operations is to use the first message field as a message type indication, and to enclose the remaining fields in parentheses, for instance, as follows:

qname!expr1(expr2,expr3)
qname?var1(var2,var3)

Some or all of the parameters to a receive operation can be given as constants (e.g., mtype symbolic constants) instead of variables:

qname?cons1,var2,cons2

In this case, an extra condition on the executability of the receive operation is that the value of all message fields specified as constants match the value of the corresponding fields in the message that is to be received. If we want to use the current value of a variable for this purpose, that is, to constrain the receive operation to messages that have a matching field, we can use the predefined function eval, for instance, as follows:

qname?eval(var1),var2,var3

In this case, the variable var1 is evaluated, and its value is used as a constraint on incoming messages, just like a constant. The receive operation is now executable only if a message is available that has a first field with a value that matches the current value of var1. If so, the values of var2 and var3 are set to the values of the corresponding fields in that message, and the message is removed from channel qname.

A simple example of the mechanisms discussed so far is as follows:

mtype = { msg0, msg1, ack0, ack1 };

chan    to_sndr = [2] of { mtype };
chan    to_rcvr = [2] of { mtype };

active proctype Sender()
{
again:  to_rcvr!msg1;
        to_sndr?ack1;
        to_rcvr!msg0;
        to_sndr?ack0;
        goto again
}

active proctype Receiver()
{
again:  to_rcvr?msg1;
        to_sndr!ack1;
        to_rcvr?msg0;
        to_sndr!ack0;
        goto again
}

The model shown here is a simplified version of the alternating bit protocol as defined by Bartlett, Scantlebury, and Wilkinson [1969]. We will extend it into a more complete version shortly, after we have covered a little bit more of the language.

The declaration

mtype = { msg0, msg1, ack0, ack1 };

introduces the four types of messages we will consider as symbolic constants.

We have used a label, named again in each proctype and a goto statement, with the usual semantics. We talk in more detail about control-flow constructs towards the end of this chapter. The first ten steps of a simulation run with the model above generate the following output.

$ spin -c -u10 alternatingbit.pml
proc 0 = Sender
proc 1 = Receiver
q   0   1
 1   to_rcvr!msg1
 1   .   to_rcvr?msg1
 2   .   to_sndr!ack1
 2   to_sndr?ack1
 1   to_rcvr!msg0
 1   .   to_rcvr?msg0
 2   .   to_sndr!ack0
 2   to_sndr?ack0
------------
depth-limit (-u10 steps) reached
------------
...

We have used the SPIN option -c to generate a columnated display of just the send and receive operations, which in many cases gives us just the right type of information about process interaction patterns. Every channel and every process is assigned an identifying instantiation number. Each column in the display above corresponds to a process number as before. Each row (line) of output also contains the instantiation number of the channel that is addressed in the left margin.

We have also used the SPIN option -u10 to limit the maximum number of steps that will be executed in the simulation to ten.

There are many more operations in PROMELA that may be performed on message channels. We will review the most important operations here.

The predefined function len(qname) returns the number of messages that is currently stored in channel qname. Some shorthands for the most common uses of this function are: empty(qname), nempty(qname), full(qname), and nfull(qname) with the obvious connotations.

In some cases we may want to test whether a send or receive operation would be executable, without actually executing the operation. To do so, we can transform each of the channel operations into a side effect free expression. It is, for instance, not valid to say:

(a > b && qname?msg0)            /* not valid */

or

(len(qname) == 0 && qname!msg0)    /* not valid */

because these expressions cannot be evaluated without side effects, or more to the point, because send and receive operations do not qualify as expressions (they are i/o statements).

To state a condition that should evaluate to true when both (a > b) and the first message in channel qname is of type msg0, we can, however, write in

PROMELA:

(a > b && qname?[msg0])       /* valid */

The expression qname?[msg0] is true precisely when the receive statement qname?msg0 would be executable at the same point in the execution, but the actual receive is not executed, only its precondition is evaluated. Any receive statement can be turned into a side effect free expression in a similar way, by placing square brackets around the list of message parameters. The channel contents remain undisturbed by the evaluation of such expressions.

  • + 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