HP OpenVMS Systems Documentation

The LIB$DO_COMMAND routine stops program execution and directs the CLI to execute a command. The routine's argument is the text of the command line that you want to execute.

This routine is especially useful when you want to execute a CLI command after your program has finished executing. For example, you could set up a series of conditions, each associated with a different command. You could also use the routine to execute a SUBMIT or PRINT command to handle a file that your program creates.

Because of the following restrictions on LIB$DO_COMMAND, you should be careful when you incorporate it in your program:

• After the call to LIB$DO_COMMAND, the current image exits, and control cannot return to it.
• The text of the command is passed to the current CLI. Because you can define your own CLI in addition to DCL and MCR, you must make sure that the command is handled by the intended CLI.
• If the routine is called from a subprocess and a CLI is not associated with that subprocess, the routine executes correctly.

You can also use LIB$DO_COMMAND to execute a DCL command file. To do this, include the at sign (@) along with a command file specification as the input argument to the routine.

Some DCL CLI$ routines perform the functions of LIB$DO_COMMAND. See the OpenVMS DCL Dictionary for more information.

Example

The following PL/I example prompts the user for a DCL command to execute after the program exits:

EXECUTE: ROUTINE OPTIONS (MAIN) RETURNS (FIXED BINARY (31));

DECLARE LIB$DO_COMMAND ENTRY (CHARACTER (*))  /* Pass DCL command  */
                                              /*  by descriptor    */
         RETURNS (FIXED BINARY (31));         /* Return status     */
%INCLUDE $STSDEF;    /* Include definition of return status values */

DECLARE COMMAND CHARACTER (80);

        GET LIST (COMMAND) OPTIONS (PROMPT('DCL command to execute: '));
        STS$VALUE = LIB$DO_COMMAND (COMMAND);
/*
   If the call to LIB$DO_COMMAND is successful, the program will terminate
   here.  Otherwise, it will return the error status to command level.
*/

        RETURN (STS$VALUE);

END EXECUTE;

This example displays the following prompt:

DCL command to execute:

What you type after this prompt determines the action of LIB$DO_COMMAND. LIB$DO_COMMAND executes any command that is entered as a valid string according to the syntax of PL/I. If the command you enter is incomplete, you are prompted for the rest of the command. For example, if you enter the SHOW command, you receive the following prompt:

$_Show what?:

The RTL provides a number of routines that give you access to the CLI callback facility. These routines allow a program to "call back" to the CLI to perform functions that normally are performed by CLI commands. These routines perform the following functions:

For information about using logical names, see Chapter 32.

24.2.5 Disabling and Enabling Control Characters

Two run-time library routines, LIB$ENABLE_CTRL and LIB$DISABLE_CTRL, allow you to call the CLI to enable or disable control characters. These routines take a longword bit mask argument that specifies the control characters to be disabled or enabled. Acceptable values for this argument are LIB$M_CLI_CTRLY and LIB$M_CLI_CTRLT.

You can use LIB$SPAWN and LIB$ATTACH together to spawn a subprocess and attach the terminal to that subprocess. These routines execute correctly only if the current CLI is DCL. For more information on the SPAWN and ATTACH commands, see the OpenVMS DCL Dictionary. For more information on creating processes, see Chapter 2.

The VAX instruction set was designed for efficient use by high-level languages and, therefore, contains many functions that are directly useful in your programs. However, some of these functions cannot be used directly by high-level languages.

The run-time library provides routines that allow your high-level language program to use most VAX machine instructions that are otherwise unavailable. On Alpha machines, these routines execute a series of Alpha instructions that emulate the operation of the VAX instructions. In most cases, these routines simply execute the instruction, using the arguments you provide. Some routines that accept string arguments, however, provide some additional functions that make them easier to use.

These routines fall into the following categories:

• Variable-length bit field instruction routines ( Section 24.3.1)
• Integer and floating-point instructions ( Section 24.3.2)
• Queue instructions ( Section 24.3.3)
• Character string instructions ( Section 24.3.4)

• Routine call instructions ( Section 24.3.5)
• Cyclic redundancy check (CRC) instruction ( Section 24.3.5)

The VAX Architecture Reference Manual describes the VAX instruction set in detail.

24.3.1 Variable-Length Bit Field Instruction Routines

The variable-length bit field is a VAX data type used to store small integers packed together in a larger data structure. It is often used to store single flag bits.

The run-time library contains five routines for performing operations on variable-length bit fields. These routines give higher-level languages that do not have the inherent ability to manipulate bit fields direct access to the bit field instructions in the VAX instruction set. Further, if a program calls a routine written in a different language to perform some function that also involves bit manipulation, the called routine can include a call to the run-time library to perform the bit manipulation.

Table 24-3 lists the run-time library variable-length bit field routines.

Three scalar attributes define a variable bit field:

• Base address---The address of the byte in memory that serves as a reference point for locating the bit field.
• Bit position---The signed longword containing the displacement of the least significant bit of the field with respect to bit 0 of the base address.
• Size---A byte integer indicating the size of the bit field in bits (in the range 0 <= size <= 32). That is, a bit field can be no more than one longword in length.

Figure 24-1 shows the format of a variable-length bit field. The shaded area indicates the field.

Figure 24-1 Format of a Variable-Length Bit Field

Bit fields are zero-origin, which means that the routine regards the first bit in the field as being the zero position. For more detailed information about VAX bit numbering and data formats, see the VAX Architecture Reference Manual.

The attributes of the bit field are passed to an RTL routine in the form of three arguments in the following order:

pos

Operating system usage: longword_signed
type: longword integer (signed)
access: read only
mechanism: by reference

Bit position relative to the base address. The pos argument is the address of a signed longword integer that contains this bit position.

size

Operating system usage: byte_unsigned
type: byte (unsigned)
access: read only
mechanism: by reference

Size of the bit field. The size argument is the address of an unsigned byte that contains this size.

base

Operating system usage: longword_unsigned
type: longword (unsigned)
access: read only
mechanism: by reference

Base address. The base argument contains the address of the base address.

Example

The following BASIC example illustrates three RTL routines. It opens the terminal as a file and specifies HEX> as the prompt. This prompt allows you to obtain input from the terminal without the question mark that VAX BASIC normally adds to the prompt in an INPUT statement. The program calls OTS$CVT_TZ_L to convert the character string input to a longword. It then calls LIB$EXTZV once for each position in the longword to extract the bit in that position. Because LIB$EXTVZ is called with a function reference within the PRINT statement, the bits are displayed.

10      EXTERNAL LONG FUNCTION
                OTS$CVT_TZ_L,           ! Convert hex text to LONG
                LIB$EXTZV               ! Extract zero-ended bit field

20      OPEN "TT:" FOR INPUT AS FILE #1%     ! Open terminal as a file
        INPUT #1%, "HEX>"; HEXIN$            ! Prompt for input
        STAT%=OTS$CVT_TZ_L(HEXIN$, BINARY%)   ! Convert to longword
        IF (STAT% AND 1%) <> 1%               ! Failed?
        THEN
                PRINT "Conversion failed, decimal status ";STAT%
                GO TO 20                      ! Try again
        ELSE
                PRINT HEXIN$,
                PRINT STR$(LIB$EXTZV(N%, 1%, BINARY%));
                        FOR N%=31% to 0% STEP -1%

Integer and floating-point routines give a high-level language program access to the corresponding machine instructions. For a complete description of these instructions, see the VAX Architecture Reference Manual. Table 24-4 lists the integer and floating-point routines once up front.

A queue is a doubly linked list. A run-time library routine specifies a queue entry by its address. Two longwords, a forward link and a backward link, define the location of the entry in relation to the preceding and succeeding entries. A self-relative queue is a queue in which the links between entries are displacements; the two longwords represent the displacements of the current entry's predecessor and successor. The VAX instructions INSQHI, INSQTI, REMQHI, and REMQTI allow you to insert and remove an entry at the head or tail of a self-relative queue. Each queue instruction has a corresponding RTL routine.

The self-relative queue instructions are interlocked and cannot be interrupted, so that other processes cannot insert or remove queue entries while the current program is doing so. Because the operation requires changing two pointers at the same time, a high-level language cannot perform this operation without calling the RTL queue access routines.

When you use these routines, cooperating processes can communicate without further synchronization and without danger of being interrupted, either on a single processor or in a multiprocessor environment. The queue access routines are also useful in an AST environment; they allow you to add or remove an entry from a queue without being interrupted by an asynchronous system trap.

The remove queue instructions (REMQHI or REMQTI) return the address of the removed entry. Some languages, such as BASIC, COBOL, and Fortran, do not provide a mechanism for accessing an address returned from a routine. Further, BASIC and COBOL do not allow routines to be arguments.

Table 24-5 lists the queue access routines.

Examples

LIB$INSQHI

In BASIC and Fortran, queues can be quadword aligned in a named COMMON block by using a linker option file to specify alignment of program sections. The LIB$GET_VM routine returns memory that is quadword aligned. Therefore, you should use LIB$GET_VM to allocate the virtual memory for a queue. For instance, to create a COMMON block called QUEUES, use the LINK command with the FILE/OPTIONS qualifier, where FILE.OPT is a linker option file containing the line:

PSECT = QUEUES, QUAD

A Fortran application using processor-shared memory follows:

INTEGER*4 FUNCTION INSERT_Q (QENTRY)
COMMON/QUEUES/QHEADER
INTEGER*4 QENTRY(10), QHEADER(2)
INSERT_Q = LIB$INSQHI (QENTRY, QHEADER)
RETURN
END

A BASIC application using processor-shared memory follows:

     COM (QUEUES) QENTRY%(9), QHEADER%(1)
     EXTERNAL INTEGER FUNCTION LIB$INSQHI
     IF LIB$INSQHI (QENTRY%() BY REF, QHEADER%() BY REF) AND 1%
          THEN GOTO 1000
             .
             .
             .
1000 REM  INSERTED OK

LIB$REMQHI

In Fortran, the address of the removed queue entry can be passed to another routine as an array using the %VAL built-in function.

In the following example, queue entries are 10 longwords, including the two longword pointers at the beginning of each entry:

COMMON/QUEUES/QHEADER
INTEGER*4 QHEADER(2), ISTAT
ISTAT = LIB$REMQHI (QHEADER, ADDR)
IF (ISTAT) THEN
        CALL PROC (%VAL (ADDR)) ! Process removed entry
        GO TO ...
ELSE IF (ISTAT .EQ. %LOC(LIB$_QUEWASEMP)) THEN
                GO TO ...       ! Queue was empty
                ELSE IF
                        ...     ! Secondary interlock failed
END IF
   .
   .
   .
END
SUBROUTINE PROC (QENTRY)
INTEGER*4 QENTRY(10)
   .
   .
   .
RETURN
END

The character string routines listed in Table 24-6 give a high-level language program access to the corresponding VAX machine instructions. For a complete description of these instructions, see the VAX Architecture Reference Manual. For each instruction, the VAX Architecture Reference Manual specifies the contents of all the registers after the instruction executes. The corresponding RTL routines do not make the contents of all the registers available to the calling program.

Table 24-6 lists the LIB$ character string routines and their functions.

The OpenVMS RTL String Manipulation (STR$) Manual describes STR$ string manipulation routines.

Example

This COBOL program uses LIB$LOCC to return the position of a given letter of the alphabet.

IDENTIFICATION DIVISION.
PROGRAM-ID.        LIBLOC.

ENVIRONMENT DIVISION.

DATA DIVISION.

WORKING-STORAGE SECTION.

01   SEARCH-STRING  PIC X(26)
                    VALUE "ABCDEFGHIJKLMNOPQRSTUVWXYZ".
01   SEARCH-CHAR    PIC X.
01   IND-POS        PIC 9(9) USAGE IS COMP.
01   DISP-IND       PIC 9(9).

ROUTINE DIVISION.

001-MAIN.
        MOVE SPACE TO SEARCH-CHAR.
        DISPLAY " ".
        DISPLAY "ENTER SEARCH CHARACTER: " WITH NO ADVANCING.
        ACCEPT SEARCH-CHAR.
        CALL "LIB$LOCC"
            USING BY DESCRIPTOR SEARCH-CHAR, SEARCH-STRING
            GIVING IND-POS.
        IF IND-POS = ZERO
            DISPLAY
                "CHAR ENTERED (" SEARCH-CHAR ") NOT A VALID SEARCH CHAR"
            STOP RUN.
        MOVE IND-POS TO DISP-IND.
        DISPLAY
             "SEARCH CHAR (" SEARCH-CHAR ") WAS FOUND IN POSITION "
             DISP-IND.
        GO TO 001-MAIN.