HP OpenVMS systems
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:
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, HP 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.)
You can also use symbols, but only between a parent and a spawned
subprocess that has inherited the parent's symbols.
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 34 for more information about logical name system services.
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 an
executive-mode logical name requires privilege.
22.214.171.124 Creating and Accessing Logical Names
Perform the following steps to create and access a logical name:
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 $LNMDEF.
|Example 3-1 Performing an Iterative Calculation with a Spawned Subprocess|
PROGRAM CALC ! Status variable and system routines INTEGER*4 STATUS, 2 SYS$CRELNM, 2 LIB$GET_EF, 2 LIB$SPAWN ! Define itmlst structure STRUCTURE /ITMLST/ UNION MAP INTEGER*2 BUFLEN INTEGER*2 CODE INTEGER*4 BUFADR INTEGER*4 RETLENADR END MAP MAP INTEGER*4 END_LIST END MAP END UNION END STRUCTURE ! Declare itmlst RECORD /ITMLST/ LNMLIST(2) ! Number to pass to REPEAT.FOR CHARACTER*3 REPETITIONS_STR INTEGER REPETITIONS ! Symbols for LIB$SPAWN and SYS$CRELNM ! Include FORSYSDEF symbol definitions: INCLUDE '($LNMDEF)' EXTERNAL CLI$M_NOLOGNAM, 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 LNMLIST(1).BUFLEN = 3 LNMLIST(1).CODE = %LOC (LNM$_STRING) LNMLIST(1).BUFADR = %LOC(REPETITIONS_STR) LNMLIST(1).RETLENADR = 0 LNMLIST(2).END_LIST = 0 STATUS = SYS$CRELNM (, 2 'LNM$JOB', ! Logical name table 2 'REP_NUMBER',, ! Logical name 2 LNMLIST) ! List specifying ! equivalence string IF (.NOT. STATUS) CALL LIB$SIGNAL (%VAL(STATUS)) ! Execute REPEAT.FOR in a subprocess MASK = %LOC (CLI$M_NOLOGNAM) .OR. 2 %LOC (CLI$M_NOCLISYM) .OR. 2 %LOC (CLI$M_NOKEYPAD) .OR. 2 %LOC (CLI$M_NOWAIT) STATUS = LIB$GET_EF (FLAG) IF (.NOT. STATUS) CALL LIB$SIGNAL (%VAL(STATUS)) STATUS = LIB$SPAWN ('RUN REPEAT',,,MASK,,,,FLAG) IF (.NOT. STATUS) CALL LIB$SIGNAL (%VAL(STATUS)) . . .
PROGRAM REPEAT ! Repeats a calculation REP_NUMBER of times, ! where REP_NUMBER is a logical name ! Status variables and system routines INTEGER STATUS, 2 SYS$TRNLNM, 2 SYS$DELLNM ! Number of times to repeat INTEGER*4 REITERATE, 2 REPEAT_STR_LEN CHARACTER*3 REPEAT_STR ! Item list for SYS$TRNLNM ! Define itmlst structure STRUCTURE /ITMLST/ UNION MAP INTEGER*2 BUFLEN INTEGER*2 CODE INTEGER*4 BUFADR INTEGER*4 RETLENADR END MAP MAP INTEGER*4 END_LIST END MAP END UNION END STRUCTURE ! Declare itmlst RECORD /ITMLST/ LNMLIST (2) ! Define item code EXTERNAL LNM$_STRING ! Set up and translate the logical name REP_NUMBER LNMLIST(1).BUFLEN = 3 LNMLIST(1).CODE = LNM$_STRING LNMLIST(1).BUFADR = %LOC(REPEAT_STR) LNMLIST(1).RETLENADR = %LOC(REPEAT_STR_LEN) LNMLIST(2).END_LIST = 0 STATUS = SYS$TRNLNM (, 2 'LNM$JOB', ! Logical name table 2 'REP_NUMBER',, ! Logical name 2 LNMLIST) ! List requesting ! equivalence string IF (.NOT. STATUS) CALL LIB$SIGNAL (%VAL(STATUS)) ! Convert equivalence string to integer ! BN causes spaces to be ignored READ (UNIT = REPEAT_STR (1:REPEAT_STR_LEN), 2 FMT = '(BN,I3)') REITERATE ! Calculations DO I = 1, REITERATE . . . END DO ! Delete logical name STATUS = SYS$DELLNM ('LNM$JOB', ! Logical name table 2 'REP_NUMBER',) ! Logical name IF (.NOT. STATUS) CALL LIB$SIGNAL (%VAL(STATUS)) END
The symbols you create and access for process communication are command
language interpreter (CLI) symbols. These symbols are stored in symbol
tables maintained for use within the context of DCL, the default
command language interpreter. They can store up to 255 bytes of
information. The use of these symbols is limited to processes using
DCL. If the process is not using DCL, an error status is returned by
the symbol routines.
126.96.36.199 Local and Global Symbols
The two kinds of CLI symbols and their definitions are as follows:
If you need to pass information from one program to another within a process, you can assign data to a global symbol when you create the symbol. Then, other programs can access the contents of the global symbol. You should use global symbols so the value within the symbol can be accessed by other programs.
See the HP OpenVMS RTL Library (LIB$) Manual for additional information.
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.
188.8.131.52 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.
184.108.40.206 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$RUN_PROGRAM.
220.127.116.11 Modifying or Deleting Data in the Common Block
You cannot modify or delete data in the process common area unless you
invoke LIB$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.
18.104.22.168 Specifying Other Types of Data
Although the descriptions of the LIB$PUT_COMMON and LIB$GET_COMMON routines in the HP OpenVMS RTL Library (LIB$) Manual specify a character string for the argument containing the data written to or read from the common area, you can specify other types of data. However, you must pass both noncharacter and character data by descriptor.
The following program segment reads statistics from the terminal and enters them into a binary file. After all of the statistics are entered into the file, the program places the name of the file into the per-process common area and exits.
. . . ! Enter statistics . . . ! Put the name of the stats file into common STATUS = LIB$PUT_COMMON (FILE_NAME (1:LEN)) . . .
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 STATUS = LIB$GET_COMMON (FILE_NAME, 2 LEN) ! Compile the report . . .
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 34.
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 3.2.2. 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 22.214.171.124.
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 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$SYSTEM (requires SYSNAM or SYSPRV privilege).
A mailbox is a virtual device used for communication among processes.
You must call OpenVMS RMS services, language I/O statements, or I/O
system services to perform actual data transfers.
126.96.36.199 Creating a Mailbox
The format for the SYS$CREMBX system service is as follows:
SYS$CREMBX ([prmflg] ,chan ,[maxmsg] ,[bufquo] ,[promsk] ,[acmode]
The SYS$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$CREMBX, refer to the
HP OpenVMS System Services Reference Manual.
188.8.131.52 Creating Temporary and Permanent Mailboxes
By default, a mailbox is deleted when no I/O channel is assigned to it. Such a mailbox is called a temporary mailbox. If you have PRMMBX privilege, you can create a permanent mailbox (specify the prmflg argument as 1 when you invoke SYS$CREMBX). A permanent mailbox is not deleted until it is marked for deletion with the SYS$DELMBX system service (requires PRMMBX). Once a permanent mailbox is marked for deletion, it is like a temporary mailbox; when the last I/O channel to the mailbox is deassigned, the mailbox is deleted.
The following statement creates a mailbox named MAIL_BOX. The I/O channel assigned to the mailbox is returned in MBX_CHAN.
! I/O channel INTEGER*2 MBX_CHAN ! Mailbox name CHARACTER*(*) MBX_NAME PARAMETER (MBX_NAME = 'MAIL_BOX') STATUS = SYS$CREMBX (, 2 MBX_CHAN, ! I/O channel 2 ,,,, 2 MBX_NAME) ! Mailbox name
If you use MAIL as the logical name for a mailbox, then the system will not execute the proper image in response to the DCL command MAIL.
The following program segment creates a permanent mailbox, then creates a subprocess that marks that mailbox for deletion:
INTEGER STATUS, 2 SYS$CREMBX INTEGER*2 MBX_CHAN ! Create permanent mailbox STATUS = SYS$CREMBX (%VAL(1), ! Permanence flag 2 MBX_CHAN, ! Channel 2 ,,,, 2 'MAIL_BOX') ! Logical name IF (.NOT. STATUS) CALL LIB$SIGNAL (%VAL(STATUS)) ! Create subprocess to delete it STATUS = LIB$SPAWN ('RUN DELETE_MBX') IF (.NOT. STATUS) CALL LIB$SIGNAL (%VAL(STATUS)) END
The following program segment executes in the subprocess. Notice that the subprocess must assign a channel to the mailbox and then use that channel to delete the mailbox. Any process that deletes a permanent mailbox, unless it is the creating process, must use this technique. (Use SYS$ASSIGN to assign the channel to the mailbox to ensure that the mailbox already exists. SYS$CREMBX system service assigns a channel to a mailbox; however, SYS$CREMBX also creates the mailbox if it does not already exist.)
INTEGER STATUS, 2 SYS$DELMBX, 2 SYS$ASSIGN INTEGER*2 MBX_CHAN ! Assign channel to mailbox STATUS = SYS$ASSIGN ('MAIL_BOX', 2 MBX_CHAN,,) IF (.NOT. STATUS) CALL LIB$SIGNAL (%VAL(STATUS)) ! Delete the mailbox STATUS = SYS$DELMBX (%VAL(MBX_CHAN)) IF (.NOT. STATUS) CALL LIB$SIGNAL (%VAL(STATUS)) END