HP OpenVMS Systems Documentation

OpenVMS Programming Concepts Manual

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 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) partly on the stack. The argument information (AI) register contains the argument count that specifies the number of 64-bit argument items.

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 and Alpha 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 OpenVMS Calling Standard.

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:

• Read only. Data upon which a routine operates, or data needed by the routine to perform its operation, must be read by the called routine. Such data is also called input data. When an argument specifies input data, the access entry is read only.
  The term only is present to indicate that the called routine does not both read and write (that is, modify) the input data. Thus, input data supplied by a variable is preserved when the called routine completes execution.
• Write only. Data that the called routine returns to the calling program must be written into a location where the calling program can access it. Such data is also called output data. When an argument specifies output data, the access entry is write only.
  In this context, the term only is present to indicate that the called routine does not read the contents of the location either before or after it writes into the location.
• Modify. When an argument specifies data that is both read and written by the called routine, the access entry is modify. In this case, the called routine reads the input data, which it uses in its operation, and then overwrites the input data with the results (the output data) of the operation. Thus, when the called routine completes execution, the input data specified by the argument is lost.

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

• Read only
• Write only
• Modify
• Function call (before return)
• JMP after unwind
• Call after stack unwind
• Call without stack unwind

For more information, see the 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:

• By value. When the argument in the argument list contains the actual data to be used by the routine, the actual data is said to be passed to the routine by value. In this case, the argument is the actual data.
• By reference. When the argument in the argument list contains the address of the data to be used by the routine, the data is said to be passed by reference. In this case, the argument is a pointer to the data.
• By descriptor. When the argument in the argument list contains the address of a descriptor, the data is said to be passed by descriptor. A descriptor consists of two or more longwords (depending on the type of descriptor used) that describe the location, length, and the OpenVMS standard data type of the data to be used by the called routine. In this case, the argument is a pointer to a descriptor that points to the actual data.
  There are several kinds of descriptors. Each one contains a value, or class, in the fourth byte of the first longword. The class identifies the type of descriptor it is. Each class has a symbolic code.
  Table 17-4 lists the types of descriptors and their corresponding code names. See the OpenVMS Calling Standard for a detailed description of each descriptor class.

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.

A condition value is a longword that has the following uses on the OpenVMS VAX and OpenVMS Alpha systems:

• Indicates the success or failure of a called procedure
• Describes an exception condition when an exception is signaled
• Identifies system messages
• Reports program success or failure to the command level

The OpenVMS Calling Standard explains in detail the uses for the condition value and depicts its format and contents.

The Condition Values Returned heading describes the condition values that are returned by the routine when it completes execution without generating an exception condition. These condition values describe the completion status of the operation.

If a called routine generates an exception condition during execution, the exception condition is signaled; the exception condition is then handled by a condition handler (either user supplied or system supplied). Depending on the nature of the exception condition and on the condition handler, the called routine either continues normal execution or terminates abnormally.

If a called routine executes without generating an exception condition, the called routine returns a condition value in one or two of the following ways:

• Condition Values Returned
• Condition Values Returned in an I/O Status Block
• Condition Values Returned in a Mailbox
• Condition Values Signaled

The method used to return the condition value is indicated under the Condition Values Returned heading in the documentation of each routine. These methods are discussed individually in the following subsections.

Under these headings, a two-column list shows the symbolic code for each condition value the routine can return and an accompanying description. The description explains whether the condition value indicates success or failure and, if failure, what user action might have caused the failure and what to do to correct it. Condition values that indicate success are listed first.

Symbolic codes for condition values are defined by the system. Though the condition value consists of several fields, each of which can be interpreted individually for specific information, the entire condition value itself can be interpreted as an integer, and this integer has an equivalent symbolic code.

The three sections that follow discuss the ways in which the called routine returns condition values.

17.5.1 Condition Values Returned

The possible condition values that the called routine can return in general register R0 are listed under the Condition Values Returned heading in the documentation. Most routines return a condition value in this way.

In the documentation of system services that complete asynchronously, both the Condition Values Returned and Condition Values Returned in the I/O Status Block headings are used. Under the Condition Values Returned heading, the condition values returned by the asynchronous service refer to the success or failure of the system service request---that is, to the status associated with the correctness of the syntax of the call, in contrast to the final status associated with the completion of the service operation. For asynchronous system services, condition values describing the success or failure of the actual service operation---that is, the final completion status---are listed under the Condition Values Returned in the I/O Status Block heading.

17.5.2 Condition Values Returned in an I/O Status Block

The possible condition values that the called routine can return in an I/O status block are listed under the Condition Values Returned in the I/O Status Block heading.

The routines that return condition values in the I/O status block are the system services that are completed asynchronously. Each of these asynchronous system services returns to the caller as soon as the service call is queued. This allows the continued use of the calling program during the execution of the service operations. System services that are completed asynchronously all have arguments that specify an I/O status block. When the system service operation is completed, a condition value specifying the completion status of the operation is written in the first word of this I/O status block.

Representing a condition value in a word-length field is possible for system services because the high-order segment of all system service condition values is 0. See cond_value in Table E-1 or Section 18.11 for the field detail of the condition value structure.

17.5.3 Condition Values Returned in a Mailbox

The possible condition values that the called routine can return in a mailbox are listed under the Condition Values Returned in a Mailbox heading.

Routines such as SYS$SNDOPR that return condition values in a mailbox send information to another process to perform a task. The receiving process performs the action and returns the status of the task to the mailbox of the sending process.

17.5.4 Condition Values Signaled

The possible condition values that the called routine can signal (instead of returning them in R0) are listed under the Condition Values Signaled heading.

Routines that signal condition values as a way of indicating the completion status do so because these routines are returning actual data as the value of the routine.

As mentioned, the signaling of condition values occurs whenever a routine generates an exception condition, regardless of how the routine returns its completion status under normal circumstances.