This chapter provides information on the following topics:

• Passing data arguments and function return values by using Compaq Fortran procedures and subprograms ( Section 10.1)
• Compaq Fortran argument-passing mechanisms ( Section 10.2)
• Using the cDEC$ ALIAS and cDEC$ ATTRIBUTES directives to specify an alternate name (alias) for external subprograms and change the way certain arguments are passed ( Section 10.3)
• The common language environment provided by the OpenVMS procedure calling standard ( Section 10.4)
• Understanding OpenVMS system routines ( Section 10.5)
• General considerations about calling routines ( Section 10.6)
• Calling OpenVMS system services ( Section 10.7)
• Passing data arguments and function return values between Compaq Fortran and Compaq Fortran 77 procedures and subprograms ( Section 10.8)
• Passing data arguments and function return values between Compaq Fortran and C functions on OpenVMS Alpha systems ( Section 10.9)

Compaq Fortran provides you with a variety of mechanisms for gaining access to procedures and system services external to your Compaq Fortran programs. By including CALL statements or function references in your source program, you can use procedures such as mathematical functions, OpenVMS system services, and routines written in such languages as Compaq Fortran 77 and Compaq C.

The Compaq Fortran compiler operates within the OpenVMS common language environment, which defines certain calling procedures and guidelines. These guidelines allow you to use Compaq Fortran to call OpenVMS system or library routines and routines written in different languages (usually called mixed-language programming).

This chapter provides information on the OpenVMS procedure-calling standard and how to access OpenVMS system services.

For More Information:

About calling and using the RMS (Record Management Services) system services, see Chapter 11.

10.1 Compaq Fortran Procedures and Argument Passing

The bounds of the main program are usually defined by using PROGRAM and END or END PROGRAM statements. Within the main program, you can define entities related to calling a function or subroutine, including modules and interface blocks.

A function or subroutine is considered a subprogram. A subprogram can accept one or more data values passed from the calling routine; the values are called arguments.

There are two types of arguments:

• Actual arguments are specified in the subprogram call.
• Dummy arguments are variables within the function or subroutine that receive the values (from the actual arguments).

The following methods define the interface between procedures:

• Declare and name a function with a FUNCTION statement and terminate the function definition with an END FUNCTION statement. Set the value of the data to be returned to the calling routine by using the function name as a variable in an assignment statement (or by specifying RESULT in a FUNCTION statement).
  Reference a function by using its name in an expression.
• Declare and name a subroutine with a SUBROUTINE statement and terminate the subroutine definition with an END SUBROUTINE statement. No value is returned by a subroutine.
  Reference a subroutine by using its name in a CALL statement (or use a defined assignment statement).
• For an external subprogram, depending on the type of arguments or function return values, you may need to declare an explicit interface to the arguments and function return value by using an interface block.
  Declare and name an interface block with an INTERFACE statement and terminate the interface block definition with an END INTERFACE statement. The interface body that appears between these two statements consists of function or subroutine specification statements.
• You can make data, specifications, definitions, procedure interfaces, or procedures globally available to the appropriate parts of your program by using a module (use association).
  Declare a module with a MODULE statement and terminate the module definition with an END MODULE statement. Include the definitions and other information contained within the module in appropriate parts of your program with a USE statement. A module can contain interface blocks, function and subroutine declarations, data declarations, and other information.

For More Information:

On the Compaq Fortran language, including statement functions and defined assignment statements not described in this manual, see the Compaq Fortran Language Reference Manual.

10.1.1 Explicit and Implicit Interfaces

An explicit interface occurs when the properties of the subprogram interface are known within the scope of the function or subroutine reference. For example, the function reference or CALL statement occurs at a point where the function or subroutine definition is known through host or use association. Intrinsic procedures also have an explicit interface.

An implicit interface occurs when the properties of the subprogram interface are not known within the scope of the function or subroutine reference. In this case, the procedure data interface is unknown to the compiler. For example, external routines (EXTERNAL statement) that have not been defined in an interface block have an implicit interface.

In most cases, you can use a procedure interface block to make an implicit interface an explicit one. An explicit interface provides the following advantages over an implicit interface:

• Better compile-time argument checking and fewer run-time errors
• In some cases, faster run-time performance
• Ease of locating problems in source files since the features help to make the interface self-documenting
• Allows use of some language features that require an explicit interface, such as array function return values.
• When passing certain types of arguments between Compaq Fortran and non-Fortran languages, an explicit interface may be needed. For example, detailed information about an assumed-shape array argument can be obtained from the passed array descriptor. The array descriptor is generated when an appropriate explicit interface is used for certain types of array arguments.

For More Information:

On Compaq Fortran array descriptors, see Section 10.1.7.

10.1.2 Types of Compaq Fortran Subprograms

There are three major types of subprograms:

• A subprogram might be local to a single program unit (known only within its host). Since the subprogram definition and all its references are contained within the same program unit, it is called an internal subprogram.
  An internal subprogram has an explicit interface.
• A subprogram needed in multiple program units should be placed within a module. To create a module subprogram within a module, add a CONTAINS statement followed by the subprogram code. A module subprogram can also contain internal subprograms.
  A module subprogram has an explicit interface in those program units that reference the module with a USE statement (unless it is declared PRIVATE).
• External subprograms are needed in multiple program units but cannot be placed in a module. This makes their procedure interface unknown in the program unit in which the reference occurs. Examples of external subprograms include general-purpose library routines in standard libraries and subprograms written in other languages, like C or Ada.
  Unless an external subprogram has an associated interface block, it has an implicit interface. To provide an explicit interface for an external subprogram, create a procedure interface block (see Section 10.1.3).
  For subprograms with no explicit interface, declare the subprogram name as external using the EXTERNAL statement within the program unit where the external subprogram reference occurs. This allows the linker to resolve the reference.
  An external subprogram must not contain PUBLIC or PRIVATE statements.

Procedure interface blocks allow you to specify an explicit interface for a subprogram as well as to define generic procedure names. This section limits discussion to those interface blocks used to provide an explicit subprogram interface. For complete information on interface blocks, see the Compaq Fortran Language Reference Manual.

The components of a procedure interface block follow:

• Begin a procedure interface block with an INTERFACE statement. Unless you are defining a generic procedure name, user-defined operator, or user-defined assignment, only the word INTERFACE is needed.
• To provide the interface body, copy the procedure specification statements from the actual subprogram, including:
  • The FUNCTION or SUBROUTINE statements.
  • The interface body. For a procedure interface block, this includes specification (declaration) statements for the dummy arguments and a function return value (omit data assignment, FORMAT, ENTRY, DATA, and related statements).
    The interface body can include USE statements to obtain definitions.
  • The END FUNCTION or END SUBROUTINE statements.
• Terminate the interface block with an END INTERFACE statement.
• To make the procedure interface block available to multiple program units, you can do one of the following:
  • Place the procedure interface block in a module. Reference the module with a USE statement in each program unit that references the subprogram (use association).
  • Place the procedure interface block in each program unit that references the subprogram.

For an example of a module that contains a procedure interface block, see Section 1.3.

10.1.4 Passing Arguments and Function Return Values

Compaq Fortran uses the same argument-passing conventions as Compaq Fortran 77 on OpenVMS Alpha systems for non-pointer scalar variables and explicit-shape and assumed-size arrays.

When calling Compaq Fortran subprograms, be aware that Compaq Fortran expects to receive arguments the same way it passes them.

The main points about argument passing and function return values are as follows:

• Arguments are generally passed by reference (arguments contain an address).
  An argument contains the address of the data being passed, not the data itself (unless explicitly specified otherwise, such as with the cDEC$ ATTRIBUTES directive or the %VAL built-in function).
  Assumed-shape arrays and deferred-shape arrays are passed by array descriptor.
  Character data is passed by character descriptor.¹
  Arguments omitted by adding an extra comma (,) are passed as a zero by immediate value (see Section 10.7.4). Such arguments include OPTIONAL arguments that are not passed.
  Any argument specified using the %DESCR built-in function is also passed by descriptor (see Section 10.2).
• Function return data is usually passed by immediate value (function return contains a value). Certain types of data (such as array-valued functions) are passed by other means.
  The value being returned from a function call usually contains the actual data, not the address of the data.
• Character variables, explicit-shape character arrays, and assumed-size character arrays are passed by a character descriptor (except for character constant actual arguments when /BY_REF_CALL is used).
  Dummy arguments for character data can use an assumed length.
  When passing character arguments to a C routine as strings, the character argument is not automatically null-terminated by the compiler. To null-terminate a string from Compaq Fortran, use the CHAR intrinsic function (described in the Compaq Fortran Language Reference Manual).

The arguments passed from a calling routine must match the dummy arguments declared in the called function or subroutine (or other procedure), as follows:

• Arguments must be in the same order, unless argument keywords are used.
  Arguments are kept in the same position as they are specified by the user. The exception to same position placement is the use of argument keywords to associate dummy and actual arguments.
• Each corresponding argument or function return value must at least match in data type, kind, and rank, as follows:
  • The primary Compaq Fortran intrinsic data types are character, integer, logical, real, and complex.
    To convert data from one data type to another, use the appropriate intrinsic procedures described in the Compaq Fortran Language Reference Manual.
    Also, certain attributes of a data item may have to match. For example, if a dummy argument has the POINTER attribute, its corresponding actual argument must also have the POINTER attribute (see Section 10.1.6).
  • You can use the kind parameter to specify the length of each numeric intrinsic type, such as INTEGER (KIND=8). For character lengths, use the LEN specifier, perhaps with an assumed length for dummy character arguments (LEN=*).
  • The rank (number of dimensions) of the actual argument is usually the same (or less than) the rank of the dummy argument, unless an assumed-size dummy array is used.
    When using an explicit interface, the rank of the actual argument must be the same as the rank of the dummy argument.
    For example, when passing a scalar actual argument to a scalar dummy argument (no more than one array element or a nonarray variable), the rank of both is 0.
    Other rules which apply to passing arrays and pointers are described in Section 10.1.5, Section 10.1.6, and in the Compaq Fortran Language Reference Manual.
• The means by which the argument is passed and received (passing mechanism) must match.
  By default, Compaq Fortran arguments are passed by reference. (See Table 10-1 for information about passing arguments with the C property.)
  When calling functions or other routines that are intended to be called from another language (such as C), be aware that these languages may require data to be passed by other means, such as by value.
  Most Compaq Fortran function return values are passed by value. Certain types of data (such as array-valued functions) are passed by other means.
  In most cases, you can change the passing mechanism of actual arguments by using the following Compaq extensions:
  • cDEC$ ATTRIBUTES directive (see Section 10.3.2)
  • Built-in functions (see Section 10.2)
• To explicitly specify the procedure (argument or function return) interface, provide an explicit interface.
  You can use interface blocks and modules to specify INTENT and other attributes of actual or dummy arguments.

For More Information:

• On passing arguments, function return values, and the contents of registers on OpenVMS Alpha systems, see the OpenVMS Calling Standard.
• On intrinsic data types, see Chapter 8 and the Compaq Fortran Language Reference Manual.
• On intrinsic procedures and attributes available for array use, see the Compaq Fortran Language Reference Manual.
• On explicit interfaces and when they are required, see the Compaq Fortran Language Reference Manual.
• On a Compaq Fortran example program that uses an external subprogram and a module that contains a procedure interface block, see Example 1-3.

Certain arguments or function return values require the use of an explicit interface, including assumed-shape dummy arguments, pointer dummy arguments, and function return values that are arrays. This is discussed in the Compaq Fortran Language Reference Manual.

When passing arrays as arguments, the rank and the extents (number of elements in a dimension) should agree, so the arrays have the same shape and are conformable. If you use an assumed-shape array, the rank is specified and extents of the dummy array argument are taken from the actual array argument.

If the rank and extent (shape) do not agree, the arrays are not conformable. The assignment of elements from the actual array to the noncomformable (assumed-size or explicit-shape) dummy array is done by using array element sequence association.

Certain combinations of actual and dummy array arguments are disallowed.

For More Information:

• On the types of arrays and passing array arguments, see the Compaq Fortran Language Reference Manual.
• On explicit interfaces and when they are required, see the Compaq Fortran Language Reference Manual.
• On array descriptors, see Section 10.1.7

Previous sections have discussed the case where the actual and dummy arguments have neither the POINTER attribute nor the TARGET attribute.

The argument passing rules of like type, kind, and rank (for conformable arrays) or array element sequence association (for noncomformable arrays) apply when:

• Both actual and dummy arguments have the POINTER attribute (and must be conformable)
• Dummy arguments have the TARGET attribute
• Both actual and dummy arguments have neither attribute

If you specify an actual argument of type POINTER and a dummy argument of type POINTER, the dummy argument receives the correct pointer value if you specify (in the code containing the actual argument) an appropriate explicit interface that defines the dummy argument with the POINTER attribute and follows certain rules.

However, if you specify an actual argument of type POINTER and do not specify an appropriate explicit interface (such as an interface block), it is passed as actual (target) data.

For More Information:

On using pointers and pointer arguments, see the Compaq Fortran Language Reference Manual.

10.1.7 Compaq Fortran Array Descriptor Format

When using an explicit interface (by association or procedure interface block), Compaq Fortran will generate a descriptor for the following types of dummy argument data structures:

• Pointers to arrays (array pointers)
• Assumed-shape arrays

To allow calling between Compaq Fortran 77 and Compaq Fortran, certain data structure arguments also supported by Compaq Fortran 77 do not use a descriptor, even when an appropriate explicit interface is provided. For example, since explicit-shape and assumed-size arrays are supported by both Compaq Fortran 77 and Compaq Fortran, an array descriptor is not used.

When calling between Compaq Fortran and a non-Fortran language (such as C), you can specify an appropriate explicit interface or use an implicit interface.

However, for cases where the called routine needs the information in the array descriptor, declare the routine with an assumed-shape argument and an explicit interface.

The array descriptor used by Compaq Fortran is the OpenVMS Noncontiguous Array Descriptor as described in the OpenVMS Calling Standard. In the DSC$B_AFLAGS byte, bit DSC$V_FL_UNALLOC specifies whether storage has or has not been set for this array. If this bit is set, the array has not yet been allocated.

For example, for 32-bit address access, consider the following array declaration:

INTEGER,TARGET :: A(10,10) INTEGER,POINTER :: P(:,:) P => A(9:1:-2,1:9:3) CALL F(P) . . .

The descriptor for actual argument P (using 32-bit addresses) would contain the following values:

• length (DSC$W_LENGTH) contains 4.
• dtype (DSC$B_DTYPE) contains DSC$K_DTYPE_L.
• class (DSC$B_CLASS) contains DSC$K_CLASS_NCA.
• pointer (DSC$A_POINTER) contains the address of A (9,1).
• scale (DSC$B_SCALE) contains 0.
• digits (DSC$B_DIGITS) contains 0.
• aflags (DSC$B_AFLAGS) contains 0, since A is allocated, the V_FL_UNALLOC bit is clear.
• dimen (DSC$B_DIMEN) contains 2.
• arsize (DSC$L_ARSIZE) contains 60.
• DSC$A_A0 contains the address of A(0,0).
• DSC$L_Si contains the stride of dimension i.
• DSC$L_Li contains the lower bound of dimension i.
• DSC$L_Ui contains the upper bound of dimension i.

For information about the Noncontiguous Array Descriptor when 64-bit addressing is requested (cDEC$ ATTRIBUTES ADDRESS64 directive), see the OpenVMS Calling Standard.

10.2 Argument-Passing Mechanisms and Built-In Functions

The OpenVMS procedure-calling standard defines three mechanisms by which arguments are passed to procedures:

• By immediate value---The argument list entry contains the value.
• By reference---The argument list entry contains the address of the value.
• By descriptor---The argument list entry contains the address of a descriptor of the value.

By default, Compaq Fortran uses the reference and descriptor mechanisms to pass arguments, depending on each argument's data type:

• The reference mechanism is used to pass all actual arguments that are numeric: logical, integer, real, and complex.
• The descriptor mechanism is used to pass all actual arguments that are character, Fortran 90 pointers, assumed-shape arrays, and deferred-shape arrays (except for character constant actual arguments when the Compaq Fortran program was compiled with /BY_REF_CALL).

When a Compaq Fortran program needs to call a routine written in a different language (or in some cases a Fortran 90 subprogram), there may be a need to use a form other the Compaq Fortran default mechanisms. For example, OpenVMS system services may require that certain numeric arguments be passed by immediate value instead of by reference.

For cases where you cannot use the default passing mechanisms, Compaq Fortran provides three built-in functions for passing arguments. It also provides a built-in function for computing addresses for use in argument lists. These built-in functions are as follows:

• %VAL, %REF, %DESCR---argument list built-in functions
• %LOC---general usage built-in function

Except for the %LOC built-in function, which can be used in any arithmetic expression, these functions can appear only as unparenthesized arguments in argument lists. The three argument list built-in functions and %LOC built-in function are rarely used to call a procedure written in Compaq Fortran.

The use of these functions in system service calls is described in Section 10.7.4. The sections that follow describe their use in general.

Instead of using the Compaq Fortran built-in functions, you can use the cDEC$ ATTRIBUTES directive to change the Compaq Fortran default passing mechanisms (see Section 10.3.2).

10.2.1 Passing Arguments by Descriptor---%DESCR Function

The %DESCR function passes its argument by descriptor. It has the following form:

%DESCR(arg)

The argument generated by the compiler is the address of a descriptor of the argument (arg). The argument value can be any Fortran 90 expression. The argument value must not be a derived type, record name, record array name, or record array element. The compiler can generate OpenVMS descriptors for all Fortran data types.

In Compaq Fortran, the descriptor mechanism is the default for passing character arguments, Fortran 90 pointers, assumed-shape arrays, and deferred-shape arrays. This is because the subprogram may need to know the length or other information about the character, pointer, or array argument. Compaq Fortran always generates code to refer to character dummy arguments through the addresses in their character descriptors.

For More Information:

On Compaq Fortran array descriptors, see Section 10.1.7.

10.2.2 Passing Addresses---%LOC Function

The %LOC built-in function computes the address of a storage element as an INTEGER (KIND=8) (64-bit) value. With 64-bit addressing (cDEC$ ATTRIBUTE ADDRESS64 directive specified), all 64-bits are used. With 32-bit addressing (cDEC$ ATTRIBUTE ADDRESS64 directive omitted), only the lower 32 bits are used. You can then use this value in an arithmetic expression. It has the following form:

%LOC(arg)

The %LOC function is particularly useful for certain system services or non-Fortran procedures that may require argument data structures containing the addresses of storage elements. In such cases, the data structures should be declared volatile to protect them from possible optimizations.

For More Information:

• On declaring volatile data structures, see the Compaq Fortran Language Reference Manual.
• On optimization and declaring volatile data, see Section 5.7.3.
• On an example that uses the %LOC function, see Example 10-4.

The %VAL function passes the argument list entry as a 64-bit immediate value. It has the following form:

%VAL(arg)

The argument-list entry generated by the compiler is the value of the argument (arg). The argument value can be a constant, variable, array element, or expression of type INTEGER, LOGICAL, REAL (KIND=4), REAL (KIND=8), COMPLEX (KIND=4), or COMPLEX (KIND=8).

If a COMPLEX (KIND=4) or COMPLEX (KIND=8) argument is passed by value, two REAL arguments (one contains the real part; the other the imaginary part) are passed by immediate value. If a COMPLEX parameter to a routine is specified as received by value (or given the C attribute), two REAL parameters are received and stored in the real and imaginary parts of the COMPLEX parameter specified.

If the value is a byte, word, or longword, it is sign-extended to a quadword (eight bytes).

To produce a zero-extended value rather than a sign-extended value, use the ZEXT intrinsic function.

For More Information:

• On intrinsic procedures, see the Compaq Fortran Language Reference Manual.
• On an example of passing integer data by value (using %VAL) and by reference (default) to a C function, see Section 10.9.7.
• On the ZEXT intrinsic function, see Compaq Fortran Language Reference Manual.
• On the %VAL built-in function, see the Compaq Fortran Language Reference Manual.