Updated: 11 December 1998

OpenVMS Programming Concepts Manual

The gsdnam argument specifies a descriptor that points to a character string.

Translation of the gsdnam argument proceeds in the following manner:

1. The current name string is prefixed with GBL$ and the result is subject to logical name translation.
2. If the result is a logical name, step 1 is repeated until translation does not succeed or until the number of translations performed exceeds the number specified by the system parameter LNM$C_MAXDEPTH.
3. The GBL$ prefix is stripped from the current name string that could not be translated. This current string is the name of the global section.

For example, assume that you have made the following logical name assignment:

$ DEFINE GBL$GSDATA GSDATA_001

Your program contains the following statements:

#include <descrip.h> . . . $DESCRIPTOR(gsdnam,"GSDATA"); . . . status = sys$crmpsc(&gsdnam, ...);

The following logical name translation takes place:

1. GBL$ is prefixed to GDSDATA.
2. GBL$GSDATA is translated to GSDATA_001. (Further translation is not successful. When logical name translation fails, the string is passed to the service.)

There are three exceptions to the logical name translation method discussed in this section:

• If the name string starts with an underscore (_), the operating system strips the underscore and considers the resultant string to be the actual name (that is, further translation is not performed).
• If the name string is the result of a logical name translation, then the name string is checked to see whether it has the terminal attribute. If the name string is marked with the terminal attribute, the operating system considers the resultant string to be the actual name (that is, further translation is not performed).
• If the global section has a name in the format name_nnn, the operating system first strips the underscore and the digits (nnn), then translates the resultant name according to the sequence discussed in this section, and finally reappends the underscore and digits. The system uses this method in conjunction with known images and shared files installed by the system manager.

When you call SYS$CRMPSC to create or map a section, or both, you must provide the service with a range of virtual addresses (inadr argument) into which the section is to be mapped.

If you know specifically which pages the section should be mapped into, you provide these addresses in a 2-longword array. For example, to map a private section of 10 pages into virtual pages 10 through 19 of the program region, specify the input address array as follows:

unsigned int maprange[1]; maprange[0]= 0x1400; /* Address (hex) of page 10 */ maprange[1]= 0x2300; /* Address (hex) of page 19 */

You do not need to know the explicit addresses to provide an input address range. If you want the section mapped into the first available virtual address range in the program region (P0) or the control region (P1), you can specify the SEC$M_EXPREG flag bit in the flags argument. In this case, the addresses specified by the inadr argument control whether the service finds the first available space in P0 or P1. The value specified or defaulted for the pagcnt argument determines the number of pages mapped. The following example shows part of a program used to map a section at the current end of the program region:

unsigned int status, inadr[1], retadr[1], flags; inadr[0]= 0x200; /* Any program (P0) region address */ inadr[1]= 0x200; /* Any P0 address (can be same) */ . . . /* Address range returned in retadr */ flags = SEC$M_EXPREG; status = sys$crmpsc(&inadr, &retadr, flags, ...);

The addresses specified do not have to be currently in the virtual address space of the process. SYS$CRMPSC creates the required virtual address space during the mapping of the section. If you specify the retadr argument, the service returns the range of addresses actually mapped.

After a section is mapped successfully, the image can refer to the pages using one of the following:

• A base register or pointer and predefined symbolic offset names
• Labels defining offsets of an absolute program section or structure

The following example shows part of a program used to create and map a process section:

#include <rms.h> #include <rmsdef.h> #include <string.h> #include <secdef.h> struct FAB secfab; main() { unsigned short chan; unsigned int status, inadr[1], retadr[1], pagcnt=1, flags; char *fn = "SECTION.TST"; /* Initialize FAB fields */ secfab = cc$rms_fab; secfab.fab$b_fac = FAB$M_PUT; secfab.fab$b_shr = FAB$M_SHRGET || FAB$V_SHRPUT || FAB$V_UPI; secfab.fab$l_fna = fn; secfab.fab$b_fns = strlen(fn); secfab.fab$l_fop = FAB$V_CIF; secfab.fab$b_rtv = -1; /* Create a file if none exists */ status = SYS$CREATE( &secfab, 0, 0 ); if ((status & 1) != 1) LIB$SIGNAL( status ); inadr[0] = X1400; inadr[1] = X2300; flags = SEC$M_WRT; chan = secfab.fab$l_stv; status = SYS$CRMPSC(&inadr, &retadr, 0, 0, 0, 0, flags, chan, pagcnt, 0, 0, 0); if ((status & 1) != 1) LIB$SIGNAL( status ); }

Notes on Example

1. The OPEN macro opens the section file defined in the file access block SECFAB. (The FOP parameter to the $FAB macro must specify the UFO option.)
2. SYS$CRMPSC uses the addresses specified at MAPRANGE to specify an input range of addresses into which the section will be mapped. The pagcnt argument requests that only 4 pages of the file be mapped.
3. The flags argument requests that the pages in the section have read/write access. The symbolic flag definitions for this argument are defined in the $SECDEF macro. Note that the file access field (FAC parameter) in the FAB also indicates that the file is to be opened for writing.
4. When SYS$CRMPSC completes, the addresses of the 4 pages that were mapped are returned in the output address array at RETRANGE. The address of the beginning of the section is placed in general register 6, which serves as a pointer to the section.

A process that creates a global section can map that global section. Then other processes can map it by calling the Map Global Section (SYS$MGBLSC) system service.

When a process maps a global section, it must specify the global section name assigned to the section when it was created, whether it is a group or system global section, and whether it wants read-only or read/write access. The process may also specify the following:

• A version identification (ident argument), indicating the version number of the global section (when multiple versions exist) and whether more recent versions are acceptable to the process.
• A relative page number (relpag argument) that specifies the page number relative to the beginning of the section to begin mapping the section. In this way, processes can use only portions of a section. Additionally, a process can map a piece of a section into a particular address range and subsequently map a different piece of the section into the same virtual address range.

To specify that the global section being mapped is located in physical memory that is being shared by multiple processors, you can include the shared memory name in the gsdnam argument character string (see Section 21.5.6.6). A demand-zero global section in memory shared by multiple processors must be mapped when it is created.

Cooperating processes can issue a call to SYS$CRMPSC to create and map the same global section. The first process to call the service actually creates the global section; subsequent attempts to create and map the section result only in mapping the section for the caller. The successful return status code SS$_CREATED indicates that the section did not already exist when the SYS$CRMPSC system service was called. If the section did exist, the status code SS$_NORMAL is returned.

The example in Section 21.5.6.10 shows one process (ORION) creating a global section and a second process (CYGNUS) mapping the section.

21.5.6.9 Global Page-File Sections

Global page-file sections are used to store temporary data in a global section. A global page-file section is a section of virtual memory that is not mapped to a file. The section can be deleted when processes have finished with it. (Contrast this to demand-zero pages, where initialization is not necessary but the pages are saved in a file.) The system parameter GBLPAGFIL controls the total number of global page-file pages in the system.

To create a global page-file section, you must set the flag bits SEC$M_GBL and SEC$M_PAGFIL in the flags argument to the Create and Map Section (SYS$CRMPSC) system service. The channel (chan argument) must be 0.

You cannot specify the flag bit SEC$M_CRF with the flag bit SEC$M_PAGFIL.

21.5.6.10 Section Paging

The first time an image executing in a process refers to a page that was created during the mapping of a disk file section, the page is copied into physical memory. The address of the page in the virtual address space of a process is mapped to the physical page. During the execution of the image, normal paging can occur; however, pages in sections are not written into the page file when they are paged out, as is the normal case. Rather, if they have been modified, they are written back into the section file on disk. The next time a page fault occurs for the page, the page is brought back from the section file.

If the pages in a section were defined as demand-zero pages or copy-on-reference pages when the section was created, the pages are treated differently, as follows:

• If the call to SYS$CRMPSC requested that pages in the section be treated as demand-zero pages, these pages are initialized to zero when they are created in physical memory. If the file is either a new file being created as a section or a file being completely rewritten, demand-zero pages provide a convenient way of initializing the pages. The pages are paged back into the section file.
• When the virtual address space is deleted, all unreferenced pages are written back to the file as zeros. This causes the file to be initialized, no matter how few pages were modified.
• If the call to SYS$CRMPSC requested that pages in the section be copy-on-reference pages, each process that maps to the section receives its own copy of the section, on a page-by-page basis from the file, as it refers to them. These pages are never written back into the section file but are paged to the paging file as needed.

In the case of global sections, more than one process can be mapped to the same physical pages. If these pages need to be paged out or written back to the disk file defined as the section, these operations are done only when the pages are not in the working set of any process.

In the following example, process ORION creates a global section, and process CYGNUS maps to that section:

/* Process ORION */ #include <rms.h> #include <rmsdef.h> #include <string.h> #include <secdef.h> #include <descrip.h> struct FAB gblfab; main() { unsigned short chan; unsigned int status, flags, efn=65; char *fn = "SECTION.TST"; $DESCRIPTOR(name, "FLAG_CLUSTER"); /* Common event flag cluster name */ $DESCRIPTOR(gsdnam, "GLOBAL_SECTION"); /* Global section name */ (1)status = SYS$ASCEFC(efn, &name, 0); if ((status & 1) != 1) LIB$SIGNAL( status ); /* Initialize FAB fields */ gblfab = cc$rms_fab; gblfab.fab$l_alq = 4; gblfab.fab$b_fac = FAB$M_PUT; gblfab.fab$l_fnm = fn; gblfab.fab$l_fop = FAB$M_CIF | FABM$_CBT; . . . /* Create a file if none exists */ (2)status = SYS$CREATE( &gblfab, 0, 0 ); if ((status & 1) != 1) LIB$SIGNAL( status ); flags = SEC$M_GBL || SEC$M_WRT; status = SYS$CRMPSC(0, 0, 0, flags, &gsdnam, ...); if ((status & 1) != 1) LIB$SIGNAL( status ); status = SYS$SETEF(efn); if ((status & 1) != 1) LIB$SIGNAL( status ); . . . } /* Process CYGNUS */ unsigned int status, efn=65; $DESCRIPTOR(cluster,"FLAG_CLUSTER"); $DESCRIPTOR(section,"GLOBAL_SECTION"); . . . (3)status = SYS$ASCEFC(efn, &cluster, 0); if ((status & 1) != 1) LIB$SIGNAL( status ); status = SYS$WAITFR(efn); if ((status & 1) != 1) LIB$SIGNAL( status ); status = SYS$MGBLSC(&inadr, &retadr, 0, flags, &section, 0, 0); if ((status & 1) != 1) LIB$SIGNAL( status ); }

1. The processes ORION and CYGNUS are in the same group. Each process first associates with a common event flag cluster named FLAG_CLUSTER to use common event flags to synchronize its use of the section.
2. The process ORION creates the global section named GLOBAL_SECTION, specifying section flags that indicate that it is a global section (SEC$M_GBL) and has read/write access. Input and output address arrays, the page count parameter, and the channel number arguments are not shown; procedures for specifying them are the same, as shown in this example.
3. The process CYGNUS associates with the common event flag cluster and waits for the flag defined as FLGSET; ORION sets this flag when it has finished creating the section. To map the section, CYGNUS specifies the input and output address arrays, the flag indicating that it is a global section, and the global section name. The number of pages mapped is the same as that specified by the creator of the section.

Read/write sections provide a way for a process or cooperating processes to share data files in virtual memory.

The sharing of global sections may involve application-dependent synchronization techniques. For example, one process can create and map to a global section in read/write fashion; other processes can map to it in read-only fashion and interpret data written by the first process. Alternatively, two or more processes can write to the section concurrently. (In this case, the application must provide the necessary synchronization and protection.)

After a file is updated, the process or processes can release (or unmap) the section. The modified pages are then written back into the disk file defined as a section.

When this is done, the revision number of the file is incremented, and the version number of the file remains unchanged. A full directory listing indicates the revision number of the file and the date and time that the file was last updated.

21.5.6.12 Releasing and Deleting Sections

A process unmaps a section by deleting the virtual addresses in its own virtual address space to which it has mapped the section. If a return address range was specified to receive the virtual addresses of the mapped pages, this address range can be used as input to the Delete Virtual Address Space (SYS$DELTVA) system service, as follows:

$DELTVA_S INADR=RETRANGE

When a process unmaps a private section, the section is deleted; that is, all control information maintained by the system is deleted. A temporary global section is deleted when all processes that have mapped to it have unmapped it. Permanent global sections are not deleted until they are specifically marked for deletion with the Delete Global Section (SYS$DGBLSC) system service; they are then deleted when no more processes are mapped.

Note that deleting the pages occupied by a section does not delete the section file but rather cancels the process's association with the file. Moreover, when a process deletes pages mapped to a read/write section and no other processes are mapped to it, all modified pages are written back into the section file.

After a section is deleted, the channel assigned to it can be deassigned. The process that created the section can deassign the channel with the Deassign I/O Channel (SYS$DASSGN) system service, as follows:

$DASSGN_S CHAN=GBLFAB+FAB$L_STV

21.5.6.13 Writing Back Sections

Because read/write sections are not normally updated on disk until the physical pages they occupy are paged out or until all processes referring to the section have unmapped it, a process should ensure that all modified pages are successfully written back into the section file at regular intervals.

The Update Section File on Disk (SYS$UPDSEC) system service writes the modified pages in a section into the disk file. SYS$UPDSEC is described in the OpenVMS System Services Reference Manual.

21.5.6.14 Image Sections

Global sections can contain shareable code. The operating system uses global sections to implement shareable code, as follows:

1. The object module containing code to be shared is linked to produce a shareable image. The shareable image is not, in itself, executable. It contains a series of sections called image sections.
2. You link private object modules with the shareable image to produce an executable image. No code or data from the shareable image is put into the executable image.
3. The system manager uses the INSTALL command to create a permanent global section from the shareable image file, making the image sections available for sharing.
4. When you run the executable image, the operating system automatically maps the global sections created by the INSTALL command into the virtual address space of your process.

For details about how to create and identify shareable images and how to link them with private object modules, see the OpenVMS Linker Utility Manual. For information about how to install shareable images and make them available for sharing as global sections, see the OpenVMS System Manager's Manual.

21.5.6.15 Page Frame Sections

A page frame section is one or more contiguous pages of physical memory or I/O space that have been mapped as a section. One use of page frame sections is to map to an I/O page, thus allowing a process to read device registers. A process mapped to an I/O page can also connect to a device interrupt vector.

A page frame section differs from a disk file section in that it is not associated with a particular disk file and is not paged. However, it is similar to a disk file section in most other respects: you create, map, and define the extent and characteristics of a page frame section in essentially the same manner as you do for a disk file section.

To create a page frame section, you must specify page frame number (PFN) mapping by setting the SEC$M_PFNMAP flag bit in the flags argument to the Create and Map Section (SYS$CRMPSC) system service. The vbn argument is now used to specify that the first page frame is to be mapped instead of the first virtual block. You must have the user privilege PFNMAP to create or delete a page frame section but not to map to an existing one.

Because a page frame section is not associated with a disk file, you do not use the relpag, chan, and pfc arguments to the SYS$CRMPSC service to create or map this type of section. For the same reason, the SEC$M_CRF (copy-on-reference) and SEC$M_DZRO (demand-zero) bit settings in the flags argument do not apply. Pages in page frame sections are not written back to any disk file (including the paging file).

21.5.7 Example of Using Memory Management System Services

In the following example, two programs are communicating through a global section. The first program creates and maps a global section (by using SYS$CRMPSC) and then writes a device name to the section. This program also defines the device terminal and process names and sets the event flags that synchronize the processes.

The second program maps the section (by using SYS$MGBLSC) and then reads the device name and the process that allocated the device and any terminal allocated to that process. This program also writes the process named to the terminal global section where the process name can be read by the first program.

The common event cluster is used to synchronize access to the global section. The first program sets REQ_FLAG to indicate that the device name is in the section. The second program sets INFO_FLAG to indicate that the process and terminal names are available.

Data in a section must be page aligned. The following is the option file used at link time that causes the data in the common area named DATA to be page aligned:

PSECT_ATTR = DATA, PAGE

For high-level language usage, use the solitary attribute of the linker. See the OpenVMS Linker Utility Manual for an explanation of how to use the solitary attribute.

Before executing the first program, you need to write a user-open routine that sets the user open bit (FAB$V_UFO) of the FAB options longword (FAB$L_FOP). The user-open routine would then read the channel number that the file is opened on from the status longword (FAB$L_STV) and return that channel number to the main program by using a common block (CHANNEL in this example).

!This is the program that creates the global section. 
 
! Define global section flags 
INCLUDE '($SECDEF)' 
! Mask for section flags 
INTEGER SEC_MASK 
 
! Logical unit number for section file 
INTEGER INFO_LUN 
! Channel number for section file 
! (returned from useropen routine) 
INTEGER SEC_CHAN 
COMMON /CHANNEL/ SEC_CHAN 
! Length for the section file 
INTEGER SEC_LEN 
! Data for the section file 
CHARACTER*12 DEVICE, 
2            PROCESS 
CHARACTER*6 TERMINAL 
COMMON /DATA/ DEVICE, 
2             PROCESS, 
2             TERMINAL 
! Location of data 
INTEGER PASS_ADDR (2), 
2       RET_ADDR (2) 
 
! Two common event flags 
INTEGER REQUEST_FLAG, 
2       INFO_FLAG 
DATA REQUEST_FLAG /70/ 
DATA INFO_FLAG /71/ 
 
! User-open routines 
INTEGER UFO_CREATE 
EXTERNAL UFO_CREATE 
. 
. 
. 
 
! Open the section file 
STATUS = LIB$GET_LUN (INFO_LUN) 
IF (.NOT. STATUS) CALL LIB$SIGNAL(%VAL(STATUS)) 
SEC_MASK = SEC$M_WRT .OR. SEC$M_DZRO .OR. SEC$M_GBL 
! (Last element - first element + size of last element + 511)/512 
SEC_LEN = ( (%LOC(TERMINAL) - %LOC(DEVICE) + 6 + 511)/512 ) 
OPEN (UNIT=INFO_LUN, 
2     FILE='INFO.TMP', 
2     STATUS='NEW', 
2     INITIALSIZE = SEC_LEN, 
2     USEROPEN = UFO_CREATE) 
! Free logical unit number and map section 
CLOSE (INFO_LUN) 
 
! Get location of data 
PASS_ADDR (1) = %LOC (DEVICE) 
PASS_ADDR (2) = %LOC (TERMINAL) 
 
STATUS = SYS$CRMPSC (PASS_ADDR,   ! Address of section 
2                    RET_ADDR,    ! Addresses mapped 
2                    , 
2                    %VAL(SEC_MASK), ! Section mask 
2                    'GLOBAL_SEC',   ! Section name 
2                    ,, 
2                    %VAL(SEC_CHAN), ! I/O channel 
2                    ,,,) 
IF (.NOT. STATUS) CALL LIB$SIGNAL(%VAL(STATUS)) 
 
! Create the subprocess 
STATUS = SYS$CREPRC (, 
2                    'GETDEVINF',  ! Image 
2                    ,,,,, 
2                    'GET_DEVICE', ! Process name 
2                    %VAL(4),,,)   ! Priority 
IF (.NOT. STATUS) CALL LIB$SIGNAL(%VAL(STATUS)) 
 
! Write data to section 
DEVICE = '$FLOPPY1' 
 
! Get common event flag cluster and set flag 
STATUS = SYS$ASCEFC (%VAL(REQUEST_FLAG), 
2                    'CLUSTER',,) 
IF (.NOT. STATUS) CALL LIB$SIGNAL(%VAL(STATUS)) 
STATUS = SYS$SETEF (%VAL(REQUEST_FLAG)) 
IF (.NOT. STATUS) CALL LIB$SIGNAL(%VAL(STATUS)) 
 
! When GETDEVINF has the information, INFO_FLAG is set 
STATUS = SYS$WAITFR (%VAL(INFO_FLAG)) 
IF (.NOT. STATUS) CALL LIB$SIGNAL(%VAL(STATUS)) 
. 
. 
. 
 
! This is the program that maps to the global section 
! created by the previous program. 
 
! Define section flags 
INCLUDE '($SECDEF)' 
! Mask for section flags 
INTEGER SEC_MASK 
! Data for the section file 
CHARACTER*12 DEVICE, 
2            PROCESS 
CHARACTER*6  TERMINAL 
COMMON /DATA/ DEVICE, 
2             PROCESS, 
2             TERMINAL 
 
! Location of data 
INTEGER PASS_ADDR (2), 
2       RET_ADDR (2) 
 
! Two common event flags 
INTEGER REQUEST_FLAG, 
2       INFO_FLAG 
DATA REQUEST_FLAG /70/ 
DATA INFO_FLAG /71/ 
. 
. 
. 
! Get common event flag cluster and wait 
! for GBL1.FOR to set REQUEST_FLAG 
STATUS = SYS$ASCEFC (%VAL(REQUEST_FLAG), 
2                    'CLUSTER',,) 
IF (.NOT. STATUS) CALL LIB$SIGNAL (%VAL(STATUS)) 
STATUS = SYS$WAITFR (%VAL(REQUEST_FLAG)) 
IF (.NOT. STATUS) CALL LIB$SIGNAL (%VAL(STATUS)) 
 
! Get location of data 
PASS_ADDR (1) = %LOC (DEVICE) 
PASS_ADDR (2) = %LOC (TERMINAL) 
 
! Set write flag 
SEC_MASK = SEC$M_WRT 
 
! Map the section 
STATUS = SYS$MGBLSC (PASS_ADDR, ! Address of section 
2                    RET_ADDR,  ! Address mapped 
2                    , 
2                    %VAL(SEC_MASK), ! Section mask 
2                    'GLOBAL_SEC',,) ! Section name 
IF (.NOT. STATUS) CALL LIB$SIGNAL (%VAL(STATUS)) 
 
! Call GETDVI to get the process ID of the 
! process that allocated the device, then 
! call GETJPI to get the process name and terminal 
! name associated with that process ID. 
! Set PROCESS equal to the process name and 
! set TERMINAL equal to the terminal name. 
. 
. 
. 
! After information is in GLOBAL_SEC 
STATUS = SYS$SETEF (%VAL(INFO_FLAG)) 
IF (.NOT. STATUS) CALL LIB$SIGNAL (%VAL(STATUS)) 
 
END 
      

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