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Compaq C

# Compaq C
User's Guide for OpenVMS Systems

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## 3.4.2 Sharing Program Sections with PL/I Externals

A Compaq PL/I variable with the EXTERNAL attribute corresponds to a FORTRAN common block and to a Compaq C `extern` variable in the `common_block` external model. Example 3_16 and Example 3-17 show how a program section is shared between Compaq C and Compaq PL/I.

A PL/I EXTERNAL CHARACTER attribute corresponds to a Compaq C `extern char` variable, but PL/I character strings are not necessarily null-terminated. In Example 3-16, Compaq C and Compaq PL/I use the same variable to manipulate the character string that resides in a program section named XYZ.

Example 3-16 Sharing Data with a PL/I Program in Named Program Sections

    /*  PL/I program PRSTRING.PLI contains the following lines of code:  */

    PRSTRING: PROCEDURE;

       DECLARE XYZ EXTERNAL CHARACTER(20);

       PUT SKIP LIST(XYZ);
       RETURN;

    END PRSTRING;

    /*  End of PL/I program  */

    /*  Compaq C program STRING.C contains the following lines of     *
     *  code:                                                      */

    main(void)
    {
       extern char xyz[20];

       strncpy(xyz,"This is a string    ", sizeof xyz);
       prstring();
    }

The PL/I procedure PRSTRING writes out the contents of the external variable `XYZ` .

PL/I also has a structure type similar (in its internal representation) to the `struct` keyword in Compaq C. Moreover, Compaq PL/I can output aggregates, such as structures and arrays, in fairly simple stream-output statements; consider Example 3-17.

Example 3-17 Sharing Data with a PL/I Program in a Compaq C Structure

    /*  PL/I program FNUM.PLI contains the following lines of code:  */

    FNUM:  PROCEDURE;
        /* EXTERNAL STRUCTURE CONTAINING THREE INTEGERS */
        DECLARE 1 NUMBERS EXTERNAL,
                2 FIRST FIXED(31),
                2 SECOND FIXED(31),
                2 THIRD FIXED(31);

        PUT SKIP LIST('Contents of structure:',NUMBERS);
        RETURN;
    END FNUM;

    /*  End of PL/I program   */

    /*  Compaq C program NUMBERS.C contains the following lines of    *
     *  code:                                                      */

    struct xs
       {
          int first;
          int second;
          int third;
       };

    main()
    {
       extern  struct  xs  numbers;

       numbers.first = 1;
       numbers.second = 2;
       numbers.third = 3;
       fnum();
    }

The PL/I procedure FNUM writes out the complete contents of the external structure NUMBERS; the structure members are written out in the order of their storage in memory, which is the same as for a Compaq C structure.

## 3.4.3 Sharing Program Sections with MACRO Programs

In a MACRO program, the .PSECT directive sets up a separate program section that can store data or MACRO instructions. The attributes in the .PSECT directive describe the contents of the program section.

Example 3-18 shows how to set up a psect in a MACRO program that allows data to be shared with a Compaq C program.

Example 3-18 Sharing Data with a MACRO Program in a Compaq C Structure

    ;  MACRO source file SET_VALUE.MAR contains the following lines of code:

               .entry  set_value,^M<>

               movl     #1,first
               movl     #2,second
               movl     #3,third
               ret

               .psect  example pic,usr,ovr,rel,gbl,noshr,-
                       noexe,rd,wrt,novec,long
    first:     .blkl
    second:    .blkl
    third:     .blkl

               .end

    ;  End of MACRO source file

    /*  Compaq C program NUMBERS.C contains the following lines of    *
     *  code:                                                      */

    #pragma extern_model common_block

    struct xs
       {
          int first;
          int second;
          int third;
       }  example;

    main()
    {
       set_value();

       printf("example.first = %d\n",  example.first);
       printf("example.second = %d\n", example.second);
       printf("example.third = %d\n",  example.third);
    }

The MACRO program initializes the locations first, second, and third in the psect named `example` and passes these values to the Compaq C program. The locations are referenced in the Compaq C program as members of the external structure named `example` .

Also, note the `#pragma extern_model common_block` preprocessor directive. This directive sets the model for external variables to the `common_block` model, which is the one used by VAX C. The default external model for Compaq C is the `relaxed_refdef` model. For more information on the `#pragma extern_model common_block` preprocessor directive, see Section 5.4.5.

# 3.5 OpenVMS Run-Time Library Routines

The OpenVMS Run-Time Library (RTL) is a library of prewritten, commonly used routines that perform a wide variety of functions. These routines are grouped according to the types of tasks they perform, and each group has a prefix that identifies those routines as members of a particular OpenVMS RTL facility. Table 3-10 lists all the language-independent, run-time library facility prefixes and the types of tasks each facility performs.

Table 3-10 OpenVMS Run-Time Library Facilities
Facility Prefix | Types of Tasks Performed
--- | ---
DTK$ | DECtalk routines that are used to control the DIGITAL DECtalk device.
LIB$ | Library routines that obtain records from devices, manipulate strings, convert data types for I/O, allocate resources, obtain system information, signal exceptions, establish condition handlers, enable detection of hardware exceptions, and process cross-reference data.
MTH$ | Mathematics routines that perform arithmetic, algebraic, and trigonometric calculations.
OTS$ | General-purpose routines that perform tasks such as data-type conversions as part of a compiler's generated code.
SMG$ | Screen management routines that are used in designing, composing, and keeping track of complex images on a video screen.
STR$ | String manipulation routines that perform such tasks as searching for substrings, concatenating strings, and prefixing and appending strings.

The OpenVMS run-time library routines are documented in detail in the following operating system documentation:

- OpenVMS RTL DECtalk (DTK$) Manual
- OpenVMS RTL Library (LIB$) Manual
- OpenVMS VAX RTL Mathematics (MTH$) Manual
- Digital Portable Mathematics Library
- OpenVMS RTL General Purpose (OTS$) Manual
- OpenVMS RTL Screen Management (SMG$) Manual
- OpenVMS RTL String Manipulation (STR$) Manual

# 3.6 OpenVMS System Services Routines

System services are prewritten system routines that perform a variety of tasks, such as controlling processes, communicating among processes, and coordinating I/O.

Unlike the OpenVMS Run-Time Library (RTL) routines, which are divided into groups by facility, all system services share the same facility prefix (SYS$). However, these services are logically divided into groups that perform similar tasks. Table 3-11 describes these groups.

Table 3-11 OpenVMS System Services
Group | Types of Tasks Performed
--- | ---
AST | Allows processes to control the handling of asynchronous system traps (ASTs).
Change mode | Changes the access mode of particular routines.
Condition handling | Designates condition handlers for special purposes.
Event flag | Clears, sets, reads, and waits for event flags, and associates with event flag clusters.
Information | Returns information about the system, queues, jobs, processes, locks, and devices.
Input/Output | Performs I/O directly without going through RMS.
Lock management | Enables processes to coordinate access to shareable system resources.
Logical names | Provides methods of accessing and maintaining pairs of character-string logical names and equivalence names.
Memory management | Increases or decreases available virtual memory, controls paging and swapping, and creates and accesses shareable files of code or data.
Process control | Creates, deletes, and controls execution of processes.
Security | Enhances the security of OpenVMS systems.
Time and Timing | Schedules events and obtains and formats binary time values.

System services are documented in detail in the OpenVMS System Services Reference Manual.

The routines that provide a programming interface to various OpenVMS utilities are described in the OpenVMS Utility Routines Manual.

# 3.7 Calling Routines

The basic steps for calling routines are the same whether you are calling a routine written in Compaq C, a routine written in some other OpenVMS language, a system service, or an OpenVMS Run-Time Library (RTL) routine. The following sections outline the procedures for calling non-Compaq C routines.

## 3.7.1 Determining the Type of Call

Before calling an external routine, you must first determine whether the call should be a procedure call or a function call. Call a routine as a procedure if it does not return a value. Call a routine as a function if it returns any type of value.

## 3.7.2 Declaring an External Routine and Its Arguments

To call an external routine or system routine, you need to declare it as an external function and to declare the names, data types, and passing mechanisms of its arguments. Arguments can be either required or optional.

Include the following information in a routine declaration:

- The name of the external routine
- The data types of all the routine parameters (optional)
- The data type of the return value if it is a function
- The `void` keyword if it is a procedure

The following example shows how to declare an external routine and its arguments:

    char func_name (int x, char y);

Header files are available to declare commonly used external routines. Using them will save you a lot of work. See Sections 1.3.1.1 and 1.3.1.2 in this manual for information on listing and including header files.

## 3.7.3 Calling the External Routine

After declaring an external routine, you can invoke it. To invoke a function, you must specify the name of the routine being invoked and all arguments required for that routine. Make sure the data types for the actual arguments you are passing coincide with those of the parameters you declared earlier, and with those declared in the routine. The following example shows how to invoke the function declared in Section 3.7.2:

    ret_status = func_name(1,'a');

## 3.7.4 System Routine Arguments

All system routine arguments are described in terms of the following information:

- OpenVMS usage
- Data type
- Type of access allowed
- Passing mechanism

OpenVMS usages are data structures that are layered on the standard OpenVMS data types. For example, the OpenVMS usage mask_longword signifies an unsigned longword integer that is used as a bit mask, and the OpenVMS usage floating_point represents any OpenVMS floating-point data type. Table 3-12 lists all the OpenVMS usages and the Compaq C types you need to implement them.

Table 3-12 Compaq C Implementation
OpenVMS Data Type | Compaq C Declaration
--- | ---
access_bit_names | user-defined 1
access_mode | unsigned char
address | int *pointer 2,4
address_range | int *array [2] 2,3,4
arg_list | user-defined 1
ast_procedure | pointer to a function 2
boolean | unsigned long int
byte_signed | char
byte_unsigned | unsigned char
channel | unsigned short int
char_string | char array[n] 3,5
complex_number | user-defined 1
cond_value | unsigned long int
context | unsigned long int
date_time | user-defined 1
device_name | char array[n] 3,5
ef_cluster_name | char array[n] 3,5
ef_number | unsigned long int
exit_handler_block | user-defined 1
fab | #include fab from text library struct FAB
file_protection | unsigned short int, or user-defined 1
floating_point | float or double
function_code | unsigned long int or user-defined 1
identifier | int *pointer 2,4
io_status_block | user-defined 1
item_list_2 | user-defined 1
item_list_3 | user-defined 1
item_list_pair | user-defined 1
item_quota_list | user-defined 1
lock_id | unsigned long int
lock_status_block | user-defined 1
lock_value_block | user-defined 1
logical_name | char array[n] 3,5
longword_signed | long int
longword_unsigned | unsigned long int
mask_byte | unsigned char
mask_longword | unsigned long int
mask_quadword | user-defined 1
mask_word | unsigned short int
null_arg | unsigned long int
octaword_signed | user-defined 1
octaword_unsigned | user-defined 1
page_protection | unsigned long int
procedure | pointer to function 2
process_id | unsigned long int
process_name | char array[n] 3,5
quadword_signed | user-defined 1
quadword_unsigned | user-defined 1
rights_holder | user-defined 1
rights_id | unsigned long int
rab | #include rab struct RAB
section_id | user-defined 1
section_name | char array[n] 3,5
system_access_id | user-defined 1
time_name | char array[n] 3,5
uic | unsigned long int
user_arg | user-defined 1
varying_arg | user-defined 1
vector_byte_signed | char array[n] 3,5
vector_byte_unsigned | unsigned char array[n] 3,5
vector_longword_signed | long int array[n] 3,5
vector_longword_unsigned | unsigned long int array[n] 3,5
vector_quadword_signed | user-defined 1
vector_quadword_unsigned | user-defined 1
vector_word_signed | short int array[n] 3,5
vector_word_unsigned | unsigned short int array[n] 3,5
word_signed | short int
word_unsigned | unsigned short int

1The declaration of a user-defined data structure depends on how the data will be used. Such data structures can be declared in a variety of ways, each of which is more suitable to specific applications.
2The term pointer refers to several declarations involving pointers. Pointers are declared with special syntax and are associated with the data type of the object being pointed to. This object is often user-defined.
3The term array denotes the syntax of a Compaq C array declaration.
4The data type specified can be changed to any valid Compaq C data type.
5The size of the array must be substituted for n.

If a system routine argument is optional, it will be indicated in the format section of the routine description in one of two ways, as follows:

- [,optional-argument]
- ,[optional-argument]

If the comma appears outside the brackets, you must pass a 0 by value to indicate the place of the omitted argument. If the comma appears inside the brackets, you can omit the argument if it is the last argument in the list.

For more information, see the OpenVMS Programming Interfaces: Calling a System Routine manual. This manual describes the OpenVMS programming interface and defines the standard conventions to call an OpenVMS system routine from a user procedure. The Alpha and VAX data type implementations for various high-level languages are also presented.

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