HP OpenVMS Systems Documentation
OpenVMS Programming Concepts Manual
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.
24.2.6 Creating and Connecting to a Subprocess
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.
24.3 Access to VAX Machine Instructions
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:
The VAX Architecture Reference Manual describes the VAX instruction set in detail.
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.
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:
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.
24.3.2 Integer and Floating-Point Routines
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.
24.3.3 Queue Access Routines
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.
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$GET_VM to allocate the virtual memory for a queue. For instance, to create a COMMON block called QUEUES, use the LINK command with the FILE/OPTIONS qualifier, where FILE.OPT is a linker option file containing the line:
A Fortran application using processor-shared memory follows:
A BASIC application using processor-shared memory follows:
In Fortran, the address of the removed queue entry can be passed to another routine as an array using the %VAL built-in function.
In the following example, queue entries are 10 longwords, including the two longword pointers at the beginning of each entry:
24.3.4 Character String Routines
The character string routines listed in Table 24-6 give a high-level language program access to the corresponding VAX machine instructions. For a complete description of these instructions, see the VAX Architecture Reference Manual. For each instruction, the VAX Architecture Reference Manual specifies the contents of all the registers after the instruction executes. The corresponding RTL routines do not make the contents of all the registers available to the calling program.
Table 24-6 lists the LIB$ character string routines and their functions.
The OpenVMS RTL String Manipulation (STR$) Manual describes STR$ string manipulation routines.
This COBOL program uses LIB$LOCC to return the position of a given letter of the alphabet.
24.3.5 Miscellaneous Instruction Routines
Table 24-7 lists additional routines that you can use.
The LIB$CALLG routine gives your program access to the CALLG instruction. This instruction calls a routine using an argument list stored as an array in memory, as opposed to the CALLS instruction, in which the argument list is pushed on the stack.
The LIB$CRC routine allows your high-level language program to use the CRC instruction, which calculates the cyclic redundancy check. This instruction checks the integrity of a data stream by comparing its state at the sending point and the receiving point. Each character in the data stream is used to generate a value based on a polynomial. The values for each character are then added together. This operation is performed at both ends of the data transmission, and the two result values are compared. If the results disagree, then an error occurred during the transmission.
For more details, see the VAX Architecture Reference Manual.
This section discusses routines that allocate processwide resources to a single operating system process. The processwide resources discussed here are:
The resource allocation routines are provided so that user routines can use the processwide resources without conflicting with one another.
In general, you must use run-time library resource allocation routines when your program needs processwide resources. This allows RTL routines supplied by Compaq, and user routines that you write to perform together within a process.
If your called routine includes a call to any RTL routine that frees a processwide resource, and that called routine fails to execute normally, the resource will not be freed. Thus, your routine should establish a condition handler that frees the allocated resource before resignaling or unwinding. For information about condition handling, see Chapter 9.
Table 24-8 list routines that perform processwide resource allocation.
24.4.1 Allocating Logical Unit Numbers
BASIC and Fortran use a logical unit number (LUN) to define the file or device a program uses to perform input and output. For a routine to be modular, it does not need to know the LUNs being used by other routines that are running at the same time. For this reason, logical units are allocated and deallocated at run time. You can use LIB$GET_LUN and LIB$FREE_LUN to obtain the next available number. This ensures that your BASIC or Fortran routine does not use a logical unit that is already being used by a calling program. Therefore, you should use this routine whenever your program calls or is called by another program that also allocates LUNs. Logical unit numbers 100 to 119 are available to modular routines through these entry points.
To allocate an LUN, call LIB$GET_LUN and use the value returned as the LUN for your I/O statements. If no LUNs are available, an error status is returned and the logical unit is set to -1 . When the program unit exits, it should use LIB$FREE_LUN to free any LUNs that have been allocated by LIB$GET_LUN. If it does not free any LUNs, the available pool of numbers is freed for use.
If your called routine contains a call to LIB$FREE_LUN to free the LUNs upon exit, and your routine fails to execute normally, the LUNs will not be freed. For this reason, you should make sure to establish a condition handler to call LIB$FREE_LUN before resignaling or unwinding. Otherwise, the allocated LUN is lost until the image exits.