HP OpenVMS Systems Documentation

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OpenVMS Programming Concepts Manual

Previous Contents Index Per-Kernel-Thread Data Cells

Several pages in P1 space contain process state in the form of data cells. A number of these cells must have a per-kernel-thread equivalent. These data cells do not all reside on pages with the same protection. Because of this, the per-kernel-thread area reserves two pages for these cells. Each page has a different page protection; one page protection is user read, user write (URUW); the other is user read, executive write (UREW). Summary of Process Data Structures

Process creation results in a PCB/KTB, a PHD/FRED, and a set of stacks. All processes have a single kernel thread, the initial thread.

A multithreaded process always begins as a single threaded process. A multithreaded process contains a PCB/KTB pair and a PHD/FRED pair for the initial thread; for its other threads, it contains additional KTBs, additional FREDs, and additional sets of stacks. When the multithreaded application exits, the process returns to its single threaded state, and all additional KTBs, FREDs, and stacks are deleted. Kernel Thread Priorities

The SYS$SETPRI system service and the SET PROCESS/PRIORITY DCL command both take a process identification value (PID) as an input and therefore affect only a single kernel thread at a time. If you want to change the base priorities of all kernel threads in a process, you must either make a separate call to SYS$SETPRI or invoke the SET PROCESS/PRIORITY command for each thread.

In OpenVMS Alpha Version 7.2, a new value for the 'policy' parameter to the SYS$SETPRI system service has been added. If JPI$K_ALL_THREADS is specified, the call to SYS$SETPRI changes the base priorities of all kernel threads in the target process.

The same support is provided by the ALL_THREADS qualifier to the SET PROCESS/PRIORITY DCL command.

Chapter 3
Process Communication

This chapter describes communication mechanisms used within a process and between processes. It also describes programming with intra-cluster communication (ICC). It contains the following sections:

Section 3.1 describes communication within a process.

Section 3.2 describes communication between processes.

Section 3.3 describes intra-cluster communication.

The operating system allows your process to communicate within itself and with other processes. Processes can be either wholly independent or cooperative. This chapter presents considerations for developing applications that require the concurrent execution of many programs, and how you can use process communication to perform the following functions:

  • Synchronize events
  • Share data
  • Obtain information about events important to the program you are executing

3.1 Communication Within a Process

Communicating within a process, from one program component to another, can be performed using the following methods:

  • Local event flags
  • Logical names (in supervisor mode)
  • Global symbols (command language interpreter symbols)
  • Common area

For passing information among chained images, you can use all four methods because the image reading the information executes immediately after the image that deposited it. Only the common area allows you to pass data reliably from one image to another in the event that another image's execution intervenes the two communicating images.

For communicating within a single image, you can use event flags, logical names, and symbols. For synchronizing events within a single image, use event flags. See Chapter 6 for more information about synchronizing events.

Because permanent mailboxes and permanent global sections are not deleted when the creating image exits, they also can be used to pass information from the current image to a later executing image. However, Compaq recommends that you use the common area because it uses fewer system resources than the permanent structures and does not require privilege. (You need the PRMMBX privilege to create a permanent mailbox and the PRMGBL privilege to create a permanent global section.)

3.1.1 Using Local Event Flags

Event flags are status-posting bits maintained by the operating system for general programming use. Programs can set, clear, and read event flags. By setting and clearing event flags at specific points, one program component can signal when an event has occurred. Other program components can then check the event flag to determine when the event has been completed. For more information about using local and common event flags for synchronizing events, refer to Chapter 6.

3.1.2 Using Logical Names

Logical names can store up to 255 bytes of data. When you need to pass information from one program to another within a process, you can assign data to a logical name when you create the logical name; then, other programs can access the contents of the logical name. See Chapter 32 for more information about logical name system services. Using Logical Name Tables

If both processes are part of the same job, you can place the logical name in the process logical name table (LNM$PROCESS) or in the job logical name table (LNM$JOB). If a subprocess is prevented from inheriting the process logical name table, you must communicate using the job logical name table. If the processes are in the same group, place the logical name in the group logical name table LNM$GROUP (requires GRPNAM or SYSPRV privilege). If the processes are not in the same group, place the logical name in the system logical name table LNM$SYSTEM (requires SYSNAM or SYSPRV privilege). You can also use symbols, but only between a parent and a spawned subprocess that has inherited the parent's symbols. Using Access Modes

You can create a logical name under three access modes---user, supervisor, or executive. If you create a process logical name in user mode, it is deleted after the image exits. If you create a logical name in supervisor or executive mode, it is retained after the image exits. Therefore, to share data within the process from one image to the next, use supervisor-mode or executive-mode logical names. Creating and Accessing Logical Names

Perform the following steps to create and access a logical name:

  1. Create the logical name and store data in it. Use LIB$SET_LOGICAL to create a supervisor logical name. No special privileges are required. You can also use the system service SYS$CRELNM. SYS$CRELNM also allows you to create a logical name for the system or group table and to create a logical name in any other mode, assuming you have appropriate privileges.
  2. Access the logical name. Use the system service SYS$TRNLNM. SYS$TRNLNM searches for the logical name and returns information about it.
  3. Once you have finished using the logical name, delete it. Use the routine LIB$DELETE_LOGICAL or SYS$DELLNM. LIB$DELETE_LOGICAL deletes the supervisor logical name without requiring any special privileges. SYS$DELLNM requires special privileges to delete logical names for privileged modes. However, you can also use this routine to delete either logical name tables or a logical name within a system or group table.

Example 3-1 creates a spawned subprocess to perform an iterative calculation. The logical name REP_NUMBER specifies the number of times that REPEAT, the program executing in the subprocess, should perform the calculation. Because both the parent process and the subprocess are part of the same job, REP_NUMBER is placed in the job logical name table LNM$JOB. (Note that logical names are case sensitive; specifically, LNM$JOB is a system-defined logical name that refers to the job logical name table, whereas lnm$job is not.) To satisfy the references to LNM$_STRING, the example includes the file $LNMDEF.

Example 3-1 Performing an Iterative Calculation with a Spawned Subprocess


! Status variable and system routines
2         SYS$CRELNM,
2         LIB$GET_EF,
2         LIB$SPAWN
! Define itmlst structure
! Declare itmlst
! Number to pass to REPEAT.FOR
! Symbols for LIB$SPAWN and SYS$CRELNM
! Include FORSYSDEF symbol definitions:
INCLUDE        '($LNMDEF)'
2        CLI$M_NOCLISYM,
2        CLI$M_NOKEYPAD,
2        CLI$M_NOWAIT,
2        LNM$_STRING
                 . ! Set REPETITIONS_STR
! Set up and create logical name REP_NUMBER in job table
2                    'LNM$JOB',     ! Logical name table
2                    'REP_NUMBER',, ! Logical name
2                    LNMLIST)       ! List specifying
                                    ! equivalence string
! Execute REPEAT.FOR in a subprocess
2      %LOC (CLI$M_NOWAIT)


! Repeats a calculation REP_NUMBER of times,
! where REP_NUMBER is a logical name

! Status variables and system routines
2       SYS$TRNLNM,
2       SYS$DELLNM

! Number of times to repeat
2           REPEAT_STR_LEN
! Item list for SYS$TRNLNM
! Define itmlst structure
! Declare itmlst
! Define item code
! Set up and translate the logical name REP_NUMBER
2                    'LNM$JOB',     ! Logical name table
2                    'REP_NUMBER',, ! Logical name
2                    LNMLIST)       ! List requesting
                                    ! equivalence string
! Convert equivalence string to integer
! BN causes spaces to be ignored
2     FMT = '(BN,I3)') REITERATE
! Calculations
! Delete logical name
STATUS = SYS$DELLNM ('LNM$JOB',     ! Logical name table
2                    'REP_NUMBER',) ! Logical name


3.1.3 Using Command Language Interpreter Symbols

The symbols you create and access for process communication are command language interpreter (CLI) symbols. These symbols are stored in symbol tables maintained for use within the context of DCL, the default command language interpreter. They can store up to 255 bytes of information. The use of these symbols is limited to processes using DCL. If the process is not using DCL, an error status is returned by the symbol routines. Local and Global Symbols

The two kinds of CLI symbols and their definitions are as follows:

  • Local---A local symbol is available to the command level that defined it, any command procedure executed from that command level, and lower command levels.
  • Global---A global symbol can be accessed from any command level, regardless of the level at which it was defined. Creating and Using Global Symbols

If you need to pass information from one program to another within a process, you can assign data to a global symbol when you create the symbol. Then, other programs can access the contents of the global symbol. You should use global symbols so the value within the symbol can be accessed by other programs.

To use DCL global symbols, follow this procedure:

  1. Create the symbol and assign data to it using the routine LIB$SET_SYMBOL. Make sure you specify that the symbol will be placed in the global symbol table in the tbl-ind argument. If you do not specify the global symbol table, the symbol will be a local symbol.
  2. Access the symbol with the LIB$GET_SYMBOL routine. This routine uses DCL to return the value of the symbol as a string.
  3. Once you have finished using the symbol, delete it with the LIB$DELETE_SYMBOL routine. If you created a global symbol, make sure you specify the global symbol table in the tbl-ind argument. By default, the system searches the local symbol table.

3.1.4 Using the Common Area

Use the common area to store data from one image to the next. Such data is unlikely to be corrupted between the time one image deposits it in a common area and another image reads it from the area. The common area can store 252 bytes of data. The LIB$PUT_COMMON routine writes information to this common area; the LIB$GET_COMMON routine reads information from this common area. Creating the Process Common Area

The common area for your process is automatically created for you; no special declaration is necessary. To pass more than 255 bytes of data, put the data into a file instead of in the common area and use the common area to pass the specification. Common I/O Routines

The LIB$PUT_COMMON routine allows a program to copy a string into the process's common storage area. This area remains defined during multiple image activations. LIB$GET_COMMON allows a program to copy a string from the common area into a destination string. The programs reading and writing the data in the common area must agree upon its amount and format. The maximum length of the destination string is defined as follows:

[min(256, the length of the data in the common storage area) - 4]

This maximum length is normally 252.

In BASIC and Fortran, you can use these routines to allow a USEROPEN routine to pass information back to the routine that called it. A USEROPEN routine cannot write arguments. However, it can call LIB$PUT_COMMON to put information into the common area. The calling program can then use LIB$GET_COMMON to retrieve it.

You can also use these routines to pass information between images run successively, such as chained images run by LIB$RUN_PROGRAM. Modifying or Deleting Data in the Common Block

You cannot modify or delete data in the process common area unless you invoke LIB$PUT_COMMON. Therefore, you can execute any number of images between one image and another, provided that you have not invoked LIB$PUT_COMMON. Each subsequent image reads the correct data. Invoking LIB$GET_COMMON to read the common block does not modify the data. Specifying Other Types of Data

Although the descriptions of the LIB$PUT_COMMON and LIB$GET_COMMON routines in the OpenVMS RTL Library (LIB$) Manual specify a character string for the argument containing the data written to or read from the common area, you can specify other types of data. However, you must pass both noncharacter and character data by descriptor.

The following program segment reads statistics from the terminal and enters them into a binary file. After all of the statistics are entered into the file, the program places the name of the file into the per-process common area and exits.

! Enter statistics
! Put the name of the stats file into common

The following program segment reads the file name from the per-process common block and compiles a report using the statistics from that file.

! Read the name of the stats file from common
2                        LEN)

! Compile the report

3.2 Communication Between Processes

Communication between processes, or interprocess communication, can be performed in the following ways:

  • Shared files
  • Common event flags
  • Logical names
  • Mailboxes
  • Global sections
  • Lock management system services

Each approach offers different possibilities in terms of the speed at which it communicates information and the amount of information it can communicate. For example, shared files offer the possibility of sharing an unlimited amount of information; however, this approach is the slowest because the disk must be accessed to share information.

Like shared files, global sections offer the possibility of sharing large amounts of information. Because sharing information through global sections requires only memory access, it is the fastest communication method.

Logical names and mailboxes can communicate moderate amounts of information. Because each method operates through a relatively complex system service, each is faster than files, but slower than the other communication methods.

The lock management services and common event flag cluster methods can communicate relatively small amounts of information. With the exception of global sections, they are the fastest of the interprocess communication methods.

Common event flags: Processes executing within the same group can use common event flags to signal the occurrence or completion of particular activities. For details about event flags, and an example of how cooperating processes in the same group use a common event flag, see Chapter 6.

Logical name tables: Processes executing in the same job can use the job logical name table to provide member processes with equivalence names for logical names. Processes executing in the same group can use the group logical name table. A process must have the GRPNAM or SYSPRV privilege to place names in the group logical name table. All processes in the system can use the system logical name table. A process must have the SYSNAM or SYSPRV privilege to place names in the system logical name table. Processes can also create and use user-defined logical name tables. For details about logical names and logical name tables, see Chapter 32.

Mailboxes: You can use mailboxes as virtual input/output devices to pass information, messages, or data among processes. For additional information on how to create and use mailboxes, see Section 3.2.1. Mailboxes may also be used to provide a creating process with a way to determine when and under what conditions a created subprocess was deleted. For an example of a termination mailbox, see Section

Global sections: Global sections can be either disk files or page-file sections that contain shareable code or data. Through the use of memory management services, these files can be mapped to the virtual address space of more than one process. In the case of a data file on disk, cooperating processes can synchronize reading and writing the data in physical memory; as data is updated, system paging results in the updated data being written directly back into the disk file. Global page-file sections are useful for temporary storage of common data; they are not mapped to a disk file. Instead, they page only to the system default page file. Global sections are described in more detail in Chapter 13 and Chapter 12.

Lock management system services: Processes can use the lock management system services to control access to resources (any entity on the system that the process can read, write, or execute). In addition to controlling access, the lock management services provide a mechanism for passing information among processes that have access to a resource (lock value blocks). Blocking ASTs can be used to notify a process that other processes are waiting for a resource. Using lock value blocks is a practical technique for communicating in cluster environments. With lock value blocks, communication between two processes from node to node in a distributed environment is an effective way of implementing cluster communication. For more information about the lock management system services, see Chapter 7.

While common event flags and lock management services establish communication, they are most useful for synchronizing events and are discussed in Chapter 6. Global sections and shared files are best used for sharing data and are discussed in Chapter 26.

3.2.1 Mailboxes

A mailbox is a virtual device used for communication among processes. You must call OpenVMS RMS services, language I/O statements, or I/O system services to perform actual data transfers. Creating a Mailbox

To create a mailbox, use the SYS$CREMBX system service. SYS$CREMBX creates the mailbox and returns the number of the I/O channel assigned to the mailbox.

The format for the SYS$CREMBX system service is as follows:

SYS$CREMBX ([prmflg] ,chan ,[maxmsg] ,[bufquo] ,[promsk] ,[acmode] ,[lognam]
,[flags] ,[nullarg])

When you invoke SYS$CREMBX, you usually specify the following two arguments:

  • Specify a variable to receive the I/O channel number using the chan argument. This argument is required.
  • Specify the logical name to be associated with the mailbox using the lognam argument. The logical name identifies the mailbox for other processes and for input/output statements.

The SYS$CREMBX system service also allows you to specify the message size, buffer size, mailbox protection code, and access mode of the mailbox; however, the default values for these arguments are usually sufficient. For more information on SYS$CREMBX, refer to the OpenVMS System Services Reference Manual.

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