Document revision date: 30 March 2001

## OpenVMS Programming Concepts Manual

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## 23.16 Obtaining Information About Physical Devices

The Get Device/Volume Information (SYS$GETDVI) system service returns information about devices. The information returned is specified by an item list created before the call to SYS$GETDVI.

When you call the SYS$GETDVI system service, you must provide the address of an item list that specifies the information to be returned. The format of the item list is described in the description of SYS$GETDVI in the OpenVMS System Services Reference Manual. The OpenVMS I/O User's Reference Manual contains details on the device-specific information these services return.

In cases where a generic (that is, nonspecific) device name is used in an I/O service, a program may need to find out what device has been used. To do this, the program should provide SYS$GETDVI with the number of the channel to the device and request the name of the device with the DVI$_DEVNAM item identifier.

The operating system also supports a device called the null device for program development. The mnemonic for the null device is NL. Its characteristics are as follows:

- A read from NL returns an end-of-file error (SS$_ENDOFFILE).
- A write to NL immediately returns a success message (SS$_NORMAL).

The null device functions as a virtual device to which you can direct output but from which the data does not return.

### 23.16.1 Checking the Terminal Device

You are restricted to a terminal device if you use any of the special functions described in this section. If the user of your program redirects SYS$INPUT or SYS$OUTPUT to a file or nonterminal device, an error occurs. You can use the SYS$GETDVIW system service to make sure the logical name is associated with a terminal, as shown in Example 23-7. SYS$GETDVIW returns a status of SS$_IVDEVNAM if the logical name is defined as a file or otherwise does not equate to a device name. The type of device is the response associated with the DVI$_DEVCLASS request code and should be DC$_TERM for a terminal.

Example 23-7 Using SYS$GETDVIW to Verify the Device Name

    RECORD /ITMLST/ DVI_LIST
    LOGICAL*4 STATUS
    ! GETDVI buffers
    INTEGER CLASS,             ! Response buffer
    2       CLASS_LEN          ! Response length
    ! GETDVI symbols
    INCLUDE '($DCDEF)'
    INCLUDE '($SSDEF)'
    INCLUDE '($DVIDEF)'
    ! Define subprograms
    INTEGER SYS$GETDVIW
    ! Find out the device class of SYS$INPUT
    DVI_LIST.BUFLEN = 4
    DVI_LIST.CODE = DVI$_DEVCLASS
    DVI_LIST.BUFADR = %LOC (CLASS)
    DVI_LIST.RETLENADR = %LOC (CLASS_LEN)
    STATUS = SYS$GETDVIW (,,'SYS$INPUT',
    2                     DVI_LIST,,,,,)
    IF ((.NOT. STATUS) .AND. (STATUS .NE. SS$_IVDEVNAM)) THEN
      CALL LIB$SIGNAL (%VAL (STATUS))
    END IF
    ! Make sure device is a terminal
    IF ((STATUS .NE. SS$_IVDEVNAM) .AND. (CLASS .EQ. DC$_TERM)) THEN
       .
       .
       .
    ELSE
      TYPE *, 'Input device not a terminal'
    END IF

### 23.16.2 Terminal Characteristics

The OpenVMS I/O User's Reference Manual describes device-specific characteristics associated with terminals. To examine a characteristic, issue a call to SYS$QIO or SYS$QIOW system service with the IO$_SENSEMODE function and examine the appropriate bit in the structure returned to the P1 argument. To change a characteristic:

1. Issue a call to SYS$QIO or SYS$QIOW system service with the IO$_SENSEMODE function.
2. Set or clear the appropriate bit in the structure returned to the P1 argument.
3. Issue a call to SYS$QIO or SYS$QIOW system service with the IO$_SETMODE function passing, as the P1 argument, to modify the structure you obtained from the sense mode operation.

Example 23-8 turns off the HOSTSYNC terminal characteristic. To check whether NOHOSTSYNC has been set, enter the SHOW TERMINAL command.

Example 23-8 Disabling the HOSTSYNC Terminal Characteristic

       .
       .
       .
    INTEGER*4 STATUS
    ! I/O channel
    INTEGER*2 INPUT_CHAN
    ! I/O status block
    STRUCTURE /IOSTAT_BLOCK/
      INTEGER*2 IOSTAT
      BYTE      TRANSMIT,
    2           RECEIVE,
    2           CRFILL,
    2           LFFILL,
    2           PARITY,
    2           ZERO
    END STRUCTURE
    RECORD /IOSTAT_BLOCK/ IOSB
    ! Characteristics buffer
    ! Note: basic characteristics are first three
    !       bytes of second longword -- length is
    !       last byte
    STRUCTURE /CHARACTERISTICS/
      BYTE      CLASS,
    2           TYPE
      INTEGER*2 WIDTH
      UNION
       MAP
        INTEGER*4 BASIC
       END MAP
       MAP
        BYTE LENGTH(4)
       END MAP
      END UNION
      INTEGER*4 EXTENDED
    END STRUCTURE
    RECORD /CHARACTERISTICS/ CHARBUF
    ! Define symbols used for I/O and terminal operations
    INCLUDE '($IODEF)'
    INCLUDE '($TTDEF)'
    ! Subroutines
    INTEGER*4 SYS$ASSIGN,
    2         SYS$QIOW
    ! Assign channel to terminal
    STATUS = SYS$ASSIGN ('SYS$INPUT',
    2                    INPUT_CHAN,,)
    IF (.NOT. STATUS) CALL LIB$SIGNAL (%VAL (STATUS))
    ! Get current characteristics
    STATUS = SYS$QIOW (,
    2                  %VAL (INPUT_CHAN),
    2                  %VAL (IO$_SENSEMODE),
    2                  IOSB,,,
    2                  CHARBUF,          ! Buffer
    2                  %VAL (12),,,,)    ! Buffer size
    IF (.NOT. STATUS) CALL LIB$SIGNAL (%VAL (STATUS))
    IF (.NOT. IOSB.IOSTAT) CALL LIB$SIGNAL (%VAL (IOSB.IOSTAT))
    ! Turn off hostsync
    CHARBUF.BASIC = IBCLR (CHARBUF.BASIC, TT$V_HOSTSYNC)
    ! Set new characteristics
    STATUS = SYS$QIOW (,
    2                  %VAL (INPUT_CHAN),
    2                  %VAL (IO$_SETMODE),
    2                  IOSB,,,
    2                  CHARBUF,
    2                  %VAL (12),,,,)
    IF (.NOT. STATUS) CALL LIB$SIGNAL (%VAL (STATUS))
    IF (.NOT. IOSB.IOSTAT) CALL LIB$SIGNAL (%VAL (IOSB.IOSTAT))

    END

If you modify terminal characteristics with set mode QIO operations, you should save the characteristics buffer that you obtain on the first sense mode operation, and restore those characteristics with a set mode operation before exiting. (Resetting is not necessary if you just use modifiers on each read operation.) To ensure that the restoration is performed if the program aborts (for example, if the user presses Ctrl/Y), you should restore the user's environment in an exit handler. See Chapter 9 for a description of exit handlers.

### 23.16.3 Record Terminators

A QIO read operation ends when the user enters a terminator or when the input buffer fills, whichever occurs first. The standard set of terminators applies unless you specify the 4 argument in the read QIO operation. You can examine the terminator that ended the read operation by examining the input buffer starting at the terminator offset (second word of the I/O status block). The length, in bytes, of the terminator is specified by the high-order word of the I/O status block. The third word of the I/O status block contains the value of the first character of the terminator.

Examining the terminator enables you to read escape sequences from the terminal, provided that you modify the QIO read operation with the IO$M_ESCAPE modifier (or the ESCAPE terminal characteristic is set). The first character of the terminator will be the ESC character (an ASCII value of 27). The remaining characters will contain the value of the escape sequence.

### 23.16.4 File Terminators

You must examine the terminator to detect end-of-file (Ctrl/Z) on the terminal. No error condition is generated at the QIO level. If the user presses Ctrl/Z, the terminator will be the SUB character (an ASCII value of 26).

## 23.17 Device Allocation

Many I/O devices are shareable; that is, more than one process at a time can access the device. By calling the Assign I/O Channel (SYS$ASSIGN) system service, a process is given a channel to the device for I/O operations.

In some cases, a process may need exclusive use of a device so that data is not affected by other processes. To reserve a device for exclusive use, you must allocate it.

Device allocation is normally accomplished with the DCL command ALLOCATE. A process can also allocate a device by calling the Allocate Device (SYS$ALLOC) system service. When a device has been allocated by a process, only the process that allocated the device and any subprocesses it creates can assign channels to the device.

When you call the SYS$ALLOC system service, you must provide a device name. The device name specified can be any of the following:

- A physical device name, for example, the tape drive MTB3:
- A logical name, for example, TAPE
- A generic device name, for example, MT:

If you specify a physical device name, SYS$ALLOC attempts to allocate the specified device.

If you specify a logical name, SYS$ALLOC translates the logical name and attempts to allocate the physical device name equated to the logical name.

If you specify a generic device name (that is, if you specify a device type but do not specify a controller or unit number, or both), SYS$ALLOC attempts to allocate any device available of the specified type. For more information about the allocation of devices by generic names, see Section 23.15.

When you specify generic device names, you must provide fields for the SYS$ALLOC system service to return the name and the length of the physical device that is actually allocated so that you can provide this name as input to the SYS$ASSIGN system service.

The following example illustrates the allocation of a tape device specified by the logical name TAPE:

    #include <descrip.h>
    #include <lib$routines.h>
    #include <ssdef.h>
    #include <starlet.h>
    #include <stdio.h>

    main() {
            unsigned int status;
            char devstr[64];
            unsigned short phylen, tapechan;

            $DESCRIPTOR(logdev,"TAPE");     /* Descriptor for logical name */
            $DESCRIPTOR(devdesc,devstr);    /* Descriptor for physical name */

    /* Allocate a device */
            status = SYS$ALLOC( &logdev,    /* devnam - device name */      (1)
                                &phylen,    /* phylen - length device name string */
                                &devdesc,   /* phybuf - buffer for devnam string */
                                0, 0);
            if (!$VMS_STATUS_SUCCESS( status ))
                    LIB$SIGNAL( status );

    /* Assign a channel to the device */
            status = SYS$ASSIGN( &devdesc,          /* devnam - device name */  (2)
                                 &tapechan,         /* chan - channel number */
                                 0, 0, 0);
            if (!$VMS_STATUS_SUCCESS( status ))
                    LIB$SIGNAL( status );

    /* Deassign the channel */
            status = SYS$DASSGN( tapechan );        /* chan - channel number */(3)
            if (!$VMS_STATUS_SUCCESS( status ))
                    LIB$SIGNAL( status );

    /* Deallocate the device */
            status = SYS$DALLOC( &devdesc,          /* devnam - device name */
                                 0 );               /* acmode - access mode */
            if (!$VMS_STATUS_SUCCESS( status ))
                    LIB$SIGNAL( status );

    }

1. The SYS$ALLOC system service call requests allocation of a device corresponding to the logical name TAPE, defined by the character string descriptor LOGDEV. The argument DEVDESC refers to the buffer provided to receive the physical device name of the device that is allocated and the length of the name string. The SYS$ALLOC service translates the logical name TAPE and returns the equivalence name string of the device actually allocated into the buffer at DEVDESC. It writes the length of the string in the first word of DEVDESC.
2. The SYS$ASSIGN command uses the character string returned by the SYS$ALLOC system service as the input device name argument, and requests that the channel number be written into TAPECHAN.
3. When I/O operations are completed, the SYS$DASSGN system service deassigns the channel, and the SYS$DALLOC system service deallocates the device. The channel must be deassigned before the device can be deallocated.

### 23.17.1 Implicit Allocation

Devices that cannot be shared by more than one process (for example, terminals and line printers) do not have to be explicitly allocated. Because they are nonshareable, they are implicitly allocated by the SYS$ASSIGN system service when SYS$ASSIGN is called to assign a channel to the device.

### 23.17.2 Deallocation

When the program has finished using an allocated device, it should release the device with the Deallocate Device (SYS$DALLOC) system service to make it available for other processes.

At image exit, the system automatically deallocates devices allocated by the image.

## 23.18 Mounting, Dismounting, and Initializing Volumes

This section introduces you to using system services to mount, dismount, and initialize disk and tape volumes.

### 23.18.1 Mounting a Volume

Mounting a volume establishes a link between a volume, a device, and a process. A volume, or volume set, must be mounted before I/O operations can be performed on the volume. You interactively mount or dismount a volume from the DCL command stream with the MOUNT or DISMOUNT command. A process can also mount or dismount a volume or volume set programmatically using the Mount Volume (SYS$MOUNT) or the Dismount Volume (SYS$DISMOU) system service, respectively.

Mounting a volume involves two operations:

1. Place the volume on the device and start the device (by pressing the START or LOAD button).
2. Mount the volume with the SYS$MOUNT system service.

#### 23.18.1.1 Calling the SYS$MOUNT System Service

The Mount Volume (SYS$MOUNT) system service allows a process to mount a single volume or a volume set. When you call the SYS$MOUNT system service, you must specify a device name.

The SYS$MOUNT system service has a single argument, which is the address of a list of item descriptors. The list is terminated by a longword of binary zeros. Figure 23-8 shows the format of an item descriptor.

Figure 23-8 SYS$MOUNT Item Descriptor

Most item descriptors do not have to be in any order. To mount volume sets, you must specify one item descriptor per device and one item descriptor per volume; you must specify the descriptors for the volumes in the same order as the descriptors for the devices on which the volumes are loaded.

For item descriptors other than device and volume names, if you specify the same item descriptor more than once, the last occurrence of the descriptor is used.

The following example illustrates a call to SYS$MOUNT. The call is equivalent to the DCL command that precedes the example.

    $ MOUNT/SYSTEM/NOQUOTA  DRA4:,DRA5:  USER01,USER02  USERD$

    #include <descrip.h>
    #include <lib$routines.h>
    #include <mntdef.h>
    #include <starlet.h>
    #include <stdio.h>
       .
       .
       .

    struct {
            unsigned short buflen, item_code;
            void *bufaddr;
            int *retlenaddr;
    }itm;

             struct itm itm[7];

    main() {
       .
       .
       .
            unsigned int status, flags;

            $DESCRIPTOR(dev1,"DRA4:");
            $DESCRIPTOR(vol1,"USER01");
            $DESCRIPTOR(dev2,"DRA5:");
            $DESCRIPTOR(vol2,"USER02");
            $DESCRIPTOR(log,"USERD$:");

     flags = MNT$M_SYSTEM | MNT$M_NODISKQ;

     i = 0;
     itm[i].buflen = sizeof( flags );
     itm[i].item_code = MNT$_FLAGS;
     itm[i].bufaddr = flags;
     itm[i++].retlenaddr = NULL;

     itm[i].buflen = dev1.dsc$w_length;
     itm[i].item_code = MNT$_DEVNAM;
     itm[i].bufaddr = dev1.dsc$a_pointer;
     itm[i++].retlenaddr = NULL;

     itm[i].buflen = vol1.dsc$w_length;
     itm[i].item_code = MNT$_VOLNAM;
     itm[i].bufaddr = vol1.dsc$a_pointer;
     itm[i++].retlenaddr = NULL;

     itm[i].buflen = dev2.dsc$w_length;
     itm[i].item_code = MNT$_DEVNAM;
     itm[i].bufaddr = dev2.dsc$a_pointer;
     itm[i++].retlenaddr = NULL;

     itm[i].buflen = vol2.dsc$w_length;
     itm[i].item_code = MNT$_VOLNAM;
     itm[i].bufaddr = vol2.dsc$a_pointer;
     itm[i++].retlenaddr = NULL;

     itm[i].buflen = log.dsc$w_length;
     itm[i].item_code = MNT$_LOGNAM;
     itm[i].bufaddr = log.dsc$a_pointer;
     itm[i++].retlenaddr = NULL;

     itm[i].buflen = 0;
     itm[i].item_code = 0;
     itm[i].bufaddr = NULL;
     itm[i++].retlenaddr = NULL;

       .
       .
       .
            status = SYS$MOUNT ( itm );
            if (!$VMS_STATUS_SUCCESS(status))
                    LIB$SIGNAL( status );
       .
       .
       .
    }

#### 23.18.1.2 Calling the SYS$DISMOU System Service

The SYS$DISMOU system service allows a process to dismount a volume or volume set. When you call SYS$DISMOU, you must specify a device name. If the volume mounted on the device is part of a fully mounted volume set, and you do not specify flags, the whole volume set is dismounted.

The following example illustrates a call to SYS$DISMOU. The call dismounts the volume set mounted in the previous example.

        $DESCRIPTOR(dev1_desc,"DRA4:");
       .
       .
       .
            status = SYS$DISMOU(&dev1_desc); /* devnam - device */
       .
       .
       .

### 23.18.2 Initializing Volumes

Initializing a volume writes a label on the volume, sets protection and ownership for the volume, formats the volume (depending on the device type), and overwrites data already on the volume.

You interactively initialize a volume from the DCL command stream using the INITIALIZE command. A process can programmatically initialize a volume using the Initialize Volume (SYS$INIT_VOL) system service.

#### 23.18.2.1 Calling the Initialize Volume System Service

You must specify a device name and a new volume name when you call the SYS$INIT_VOL system service. You can also use the itmlst argument of $INIT_VOL to specify options for the initialization. For example, you can specify that data compaction should be performed by specifying the INIT$_COMPACTION item code. See the OpenVMS System Services Reference Manual for more information on initialization options.

Before initializing the volume with SYS$INIT_VOL, be sure you have placed the volume on the device and started the device (by pressing the START or LOAD button).

The default format for files on disk volumes is called Files-11 On-Disk Structure Level 2. Files-11 On-Disk Structure Level 1 format, available on VAX systems, is used by other Compaq operating systems, including RSX-11M, RSX-11M-PLUS, RSX-11D, and IAS, but is not supported on Alpha systems. For more information, see the OpenVMS System Manager's Manual.

Here are two examples of calling SYS$INIT_VOL programmatically: one from a C program and one from a BASIC program.

The following example illustrates a call to SYS$INIT_VOL from Compaq C:

    #include <descrip.h>
    #include <initdef.h>
    #include <lib$routines.h>
    #include <starlet.h>
    #include <stsdef.h>

    struct item_descrip_3
    {
        unsigned short buffer_size;
        unsigned short item_code;
        void *buffer_address;
        unsigned short *return_length;
    };

    main ()
    {
        unsigned long
            density_code,
            status;
        $DESCRIPTOR(drive_dsc, "MUA0:");
        $DESCRIPTOR(label_dsc, "USER01");
        struct
        {
            struct item_descrip_3 density_item;
            long terminator;
        } init_itmlst;

        /*
        ** Initialize the input item list.
        */

        density_code = INIT$K_DENSITY_6250_BPI;
        init_itmlst.density_item.buffer_size = 4;
        init_itmlst.density_item.item_code = INIT$_DENSITY;
        init_itmlst.density_item.buffer_address = &density_code;

        init_itmlst.terminator = 0;

        /*
        ** Initialize the volume.
        */

        status = SYS$INIT_VOL (&drive_dsc, &label_dsc, &init_itmlst);

        /*
        ** Report an error if one occurred.
        */

        if (!$VMS_STATUS_SUCCESS (status ))
            LIB$STOP (status);
    }

The following example illustrates a call to SYS$INIT_VOL from VAX BASIC:

    OPTION TYPE = EXPLICIT

    %INCLUDE '$INITDEF' %FROM %LIBRARY

    EXTERNAL LONG FUNCTION SYS$INIT_VOL

    RECORD ITEM_DESC
            VARIANT
            CASE
                WORD BUFLEN
                WORD ITMCOD
                LONG BUFADR
                LONG LENADR
            CASE
                LONG TERMINATOR
            END VARIANT
    END RECORD

    DECLARE LONG RET_STATUS, &
        ITEM_DESC INIT_ITMLST(2)

    ! Initialize the input item list.

    INIT_ITMLST(0)::ITMCOD = INIT$_READCHECK
    INIT_ITMLST(1)::TERMINATOR = 0

    ! Initialize the volume.

    RET_STATUS = SYS$INIT_VOL ("DJA21:" BY DESC, "USERVOLUME" BY DESC,
    INIT_ITMLST() BY REF)

## 23.19 Formatting Output Strings

When you are preparing output strings for a program, you may need to insert variable information into a string prior to output, or you may need to convert a numeric value to an ASCII string. The Formatted ASCII Output (SYS$FAO) system service performs these functions.

Input to the SYS$FAO system service consists of the following:

- A control string that contains the fixed text portion of the output and formatting directives. The directives indicate the position within the string where substitutions are to be made, and describe the data type and length of the input values that are to be substituted or converted.
- An output buffer to contain the string after conversions and substitutions have been made.
- An optional argument indicating a word to receive the final length of the formatted output string.
- Parameters that provide arguments for the formatting directives.

The following example shows a call to the SYS$FAO system service to format an output string for a SYS$QIOW macro. Complete details on how to use SYS$FAO, with additional examples, are provided in the description of the SYS$FAO system service in the OpenVMS System Services Reference Manual.

    #include <descrip.h>
    #include <lib$routines.h>
    #include <ssdef.h>
    #include <starlet.h>
    #include <stdio.h>
    #include <stsdef.h>

    main() {

            unsigned int status, faolen;
            char faobuf[80];
            $DESCRIPTOR(faostr,"FILE !AS DOES NOT EXIST");    (1)
            $DESCRIPTOR(outbuf, faobuf);                      (2)
            $DESCRIPTOR(filespec,"DISK$USER:MYFILE.DAT");     (3)

            status = SYS$FAO( &faostr, &outlen, &outbuf, &filespec );  (4)
            if (!$VMS_STATUS_SUCCESS(status))
                    LIB$SIGNAL(status);
       .
       .
       .
            status = SYS$QIOW( ...faobuf, outlen, ... ); (5)
            if (!$VMS_STATUS_SUCCESS(status))
                    LIB$SIGNAL(status);
       .
       .
       .
    }

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