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HP OpenVMS Programming Concepts Manual

HP OpenVMS Programming Concepts Manual

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17.2.2 JSB Call Format (VAX only)

The JSB call format indicates that the named routine is called using the VAX JSB instruction. The routine returns using Return from Subroutine (RSB). You can use the JSB call format with only the VAX MACRO and VAX BLISS languages.

Explanatory Text

Explanatory text might follow the procedure call format or the JSB call format, or both. This text is present only when needed to clarify the format. For example, in the call format, you indicate that arguments are optional by enclosing them in brackets ([]). However, brackets alone cannot convey all the important information that might apply to optional arguments. For example, in some routines that have many optional arguments, if you select one optional argument, you must also select another optional argument. In such cases, text following the format clarifies this.

17.3 Returns Heading

The Returns heading contains a description of any information returned by the routine to the caller. A routine can return information to the caller in various ways. The following subsections discuss each possibility and then describe how this returned information is presented.

17.3.1 Condition Values Returned in a Register

Most routines return a condition value in register R0. This condition value contains various kinds of information, the most important for the caller (in bits <3:0>) being the completion status of the operation. You test the condition value to determine whether the routine completed successfully. On OpenVMS I64, the calling standard specifies that return status is returned in R8. As an aid to portable code, the MACRO complier automatically maps R0 to R8. See the HP OpenVMS MACRO Compiler Porting and User's Guide for additional information.

On Alpha and I64 processors, a 32-bit condition value is represented in the Alpha register sign-extended to 64 bits.

If you program in high-level languages for OpenVMS environments, the fact that status information is returned by means of a condition value and that it is returned in a hardware register is of little importance because you receive this status information in the return (or status) variable. The run-time environment established for the high-level language program allows the status information in R0 (R8, R9 for I64) to be moved automatically to the user's return variable.

Nevertheless, for routines that return a condition value, the Returns heading in the documentation contains the following information:

OpenVMS usage: cond_value 
type:          longword (unsigned) 
access:        write only 
mechanism:     by value 

The OpenVMS usage entry specifies the OpenVMS data type of the information returned. Because a condition value in any OpenVMS operating system environment is returned in a specific condition value structure, the OpenVMS usage entry is cond_value.

The type entry specifies the standard data type of the information returned. Because the condition value structure is 32 bits, the type heading is longword (unsigned).

The access entry specifies the way in which the called routine accesses the object. Because the called routine is returning the condition value, the routine writes the value into R0 (R8, R9 for I64), so the access heading is write only.

The mechanism heading specifies the passing mechanism used by the called routine in returning the condition value. Because the called routine is writing the condition value directly into R0 (R8, R9 for I64), the mechanism heading is by value. (If the called routine had written the address of the condition value into R0 (R8, R9 for I64), the passing mechanism would have been by reference.)

Note that if a routine returns a condition value, another main heading in the documentation format (Condition Values Returned) describes the possible condition values that the routine can return.

17.3.2 Other Returned Values

If a routine returns actual data, the Returns heading in the documentation of that routine contains the following information (for example, from a math routine):

OpenVMS usage: floating_point 
type:          G_floating 
access:        write only 
mechanism:     by value 

In this mathematics routine notation, the OpenVMS data type is floating_point and the standard data type is G_floating point. The meaning of the contents of the access and mechanism headings is discussed in Sections 17.4.3 and 17.4.4.

The registers used to return values vary with the type of the result and the specific hardware environment. For more information, see the HP OpenVMS Calling Standard.

In addition, under the Returns heading, some text can be provided after the information about the type, access, and mechanism. This text explains other relevant information about what the routine is returning.

For example, because the routine is returning actual data in the VAX, Alpha, or I64 registers, the registers cannot be used to convey completion status information. All routines that return actual data in VAX, Alpha, or I64 registers must signal the condition value, which contains the completion status. Thus, the text under the Returns heading points out that the routine signals its completion status.

17.3.3 Condition Values Signaled

Although most routines return condition values, some routines choose to signal their condition values using the OpenVMS signaling mechanism. Routines can signal their completion status whether or not they are returning actual data in the hardware registers, but all routines that return actual data in the hardware registers must signal their completion status if they are to return this status information at all.

If a routine signals its completion status, text under the Returns heading explains this, and the Condition Values Signaled heading in the documentation format describes the possible condition values that the routine can signal.

HP's system routines never signal condition values indicating success. Only error condition values are signaled.

17.4 Arguments Heading

Detailed information about each argument is listed in the call format under the Arguments heading. Arguments are described in the order in which they appear in the call format. If the routine has no arguments, the word None appears.

The following format is used to describe each argument:

OpenVMS usage:  OpenVMS data type 
type:           argument data type 
access:         argument access 
mechanism:      argument passing mechanism 

A paragraph of structured text describing the arguments follows the argument format along with additional information, if needed.

17.4.1 OpenVMS Usage Entry

The purpose of the OpenVMS usage entry is to facilitate the coding of source-language data type declarations in application programs. Ordinarily, the standard data type, discussed in Section 17.4.2, is sufficient to describe the type of data passed by an argument. However, within the OpenVMS operating system environment, many system routines contain arguments whose conceptual nature or complexity requires additional explanation. For instance, when an argument passes the name of an event flag, the type entry longword (unsigned) alone does not indicate the nature of the value. In this instance, an accompanying OpenVMS usage entry, denoting the OpenVMS data type ef_number, further explains the actual usage.

See Table E-1 for a list of the possible OpenVMS usage entries and their definitions. Refer to the appropriate language implementation table in Appendix E to determine the correct syntax of the type declaration in the language you are using.

Note that the OpenVMS usage entry is not a traditional data type (such as the standard data types of byte, word, longword, and so on). It is significant only within the context of the OpenVMS operating system and is intended solely to expedite data declarations within application programs.

17.4.2 Type Entry

In actuality, an argument does not have a data type; rather, the data specified by an argument has a data type. The argument is merely the vehicle for passing data to the called routine. Nevertheless, the phrase argument data type is used to describe the standard data type of the data specified by the argument.

Procedure calls result in the construction of an argument list. (This process is described in the HP OpenVMS Calling Standard.) An argument list is a sequence of entries together with a count of the number of entries.

On VAX systems, an argument list is represented as a vector of longwords, where the first longword contains the count and each remaining longword contains one argument.

On Alpha systems, an argument list is represented as quadword entities that comprise an argument item sequence, partly in hardware registers and (when there are more than six arguments for Alpha) partly on the stack. The argument information (AI) register contains the argument count that specifies the number of 64-bit argument items.

For I64 systems, parameters are passed in a combination of general registers, floating-point registers, and memory, as described in Chapter 18, and as illustrated in Figure 18-9. The parameter list is formed by placing each individual parameter into fixed-size elements of the parameter list, referred to as parameter slots. Each parameter slot is 64 bits wide; parameters larger than 64 bits are placed in as many consecutive parameter slots as are needed to contain the entire parameter. The rules for allocation and alignment of parameter slots are described in Section The contents of the first eight parameter slots are always passed in registers, while the remaining parameters are always passed on the memory stack, beginning at the caller's stack pointer plus 16 bytes.

When arguments are passed by descriptors, these standard data types are defined with symbolic codes. Table 17-3 lists the standard data types for VAX, Alpha, and I64 systems that can appear for the type entry in an argument description, along with their symbolic code (DTYPE) used in argument descriptors.

For a detailed description of each of the following symbolic codes, see the HP OpenVMS Calling Standard.

Table 17-3 Standard Data Types and Their Descriptor Field Symbols
Data Type Symbolic Code
Absolute date and time DSC$K_DTYPE_ADT
Byte integer (signed) DSC$K_DTYPE_B
Bound label value DSC$K_DTYPE_BLV
Bound procedure value 1 DSC$K_DTYPE_BPV
Byte (unsigned) DSC$K_DTYPE_BU
COBOL intermediate temporary DSC$K_DTYPE_CIT
D_floating DSC$K_DTYPE_D
D_floating complex DSC$K_DTYPE_DC
Descriptor DSC$K_DTYPE_DSC
F_floating DSC$K_DTYPE_F
F_floating complex DSC$K_DTYPE_FC
G_floating DSC$K_DTYPE_G
G_floating complex DSC$K_DTYPE_GC
H_floating 1 DSC$K_DTYPE_H
H_floating complex 1 DSC$K_DTYPE_HC
S_floating (32-bit IEEE) 2 DSC$K_DTYPE_FS
T_floating (64-bit IEEE) 2 DSC$K_DTYPE_FT
X_floating (128-bit IEEE) 2 DSC$K_DTYPE_FX
S_floating complex 2 DSC$K_DTYPE_FSC
T_floating complex 2 DSC$K_DTYPE_FTC
X_floating complex 2 DSC$K_DTYPE_FXC
Longword integer (signed) DSC$K_DTYPE_L
Longword (unsigned) DSC$K_DTYPE_LU
Numeric string, left separate sign DSC$K_DTYPE_NL
Numeric string, left overpunched sign DSC$K_DTYPE_NLO
Numeric string, right separate sign DSC$K_DTYPE_NR
Numeric string, right overpunched sign DSC$K_DTYPE_NRO
Numeric string, unsigned DSC$K_DTYPE_NU
Numeric string, zoned sign DSC$K_DTYPE_NZ
Octaword integer (signed) DSC$K_DTYPE_O
Octaword (unsigned) DSC$K_DTYPE_OU
Packed decimal string DSC$K_DTYPE_P
Quadword integer (signed) DSC$K_DTYPE_Q
Quadword (unsigned) DSC$K_DTYPE_QU
Character string DSC$K_DTYPE_T
Aligned bit string DSC$K_DTYPE_V
Varying character string DSC$K_DTYPE_VT
Unaligned bit string DSC$K_DTYPE_VU
Word integer (signed) DSC$K_DTYPE_W
Word (unsigned) DSC$K_DTYPE_WU
Unspecified DSC$K_DTYPE_Z
Procedure entry mask 1 DSC$K_DTYPE_ZEM
Sequence of instruction 1 DSC$K_DTYPE_ZI

1VAX specific.
2Alpha and I64 specific.

17.4.3 Access Entry

The access entry describes the way in which the called routine accesses the data specified by the argument, or access method. The following methods of access are most common:

Following is a complete list of access methods that can appear under the access entry in an argument description:

For more information, see the HP OpenVMS Calling Standard.

17.4.4 Mechanism Entry

The way in which an argument specifies the actual data to be used by the called routine is defined in terms of the argument passing mechanism. There are three basic passing mechanism types:

Table 17-4 Descriptor Classes of Passing Mechanisms
Passing Mechanism Descriptor Symbolic Code
By descriptor, fixed-length (scalar) DSC$K_CLASS_S
By descriptor, dynamic string DSC$K_CLASS_D
By descriptor, array DSC$K_CLASS_A
By descriptor, procedure DSC$K_CLASS_P
By descriptor, decimal string DSC$K_CLASS_SD
By descriptor, noncontiguous array DSC$K_CLASS_NCA
By descriptor, varying string DSC$K_CLASS_VS
By descriptor, varying string array DSC$K_CLASS_VSA
By descriptor, unaligned bit string DSC$K_CLASS_UBS
By descriptor, unaligned bit array DSC$K_CLASS_UBA
By descriptor, string with bounds DSC$K_CLASS_SB
By descriptor, unaligned bit string with bounds DSC$K_CLASS_UBSB

17.4.5 Explanatory Text

For each argument, one or more paragraphs of explanatory text follow the OpenVMS usage, type, access, and mechanism entries. The first paragraph is highly structured and always contains information in the following sequence:

  1. A sentence or a sentence fragment that describes (1) the nature of the data specified by the argument, and (2) the way in which the routine uses this data. For example, if an argument were supplying a number, which the routine converts to another data type, the argument description would contain the following sentence fragment:

    Integer to be converted to an F_floating point number
  2. A sentence that expresses the relationship between the argument and the data that it specifies. This relationship is the passing mechanism used to pass the data and, for a given argument, is expressed in one of the following ways:
    1. If the passing mechanism is by value, the sentence should read as follows:

      The attrib argument is a longword that contains (or is) the bit mask specifying the attributes.
    2. If the passing mechanism is by reference, the sentence should read as follows:

      The objtyp argument is the address of a longword containing a value indicating whether the object is a file or a device.
    3. If the passing mechanism is by descriptor, the sentence should read as follows:

      The devnam argument is the address of a string descriptor of a logical name denoting a device name.
  3. Additional explanatory paragraphs that appear for each argument, as needed. For example, some arguments specify complex data consisting of many discrete fields, each of which has a particular purpose and use. In such cases, additional paragraphs provide detailed descriptions of each such field, symbolic names for the fields, if any, and guidance on their use.

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