HP OpenVMS Systems Documentation
OpenVMS Programming Concepts Manual
19.2.2 The RTL Call Entry
Arguments passed to a routine must be listed in your call entry in the order shown in the format section of the routine description. Each argument has four characteristics: OpenVMS usage, data type, access type, and passing mechanism. These characteristics are described in Chapter 17.
Some arguments are optional. Optional arguments are indicated by brackets in the routine descriptions. When your program invokes a run-time library routine using a CALL entry point, you can omit optional arguments at the end of the argument list. If the optional argument is not the last argument in the list, you must either pass a zero by value or use a comma to indicate the place of the omitted argument. Some languages, such as C, require that you pass zero by value for trailing optional arguments. See your language processor documentation for further information.
On VAX systems, the calling program passes an argument list of longwords to a called routine; each longword in the argument list specifies a single argument. Note that a 64-bit floating-point argument would count as 2 longword arguments in the list.
On Alpha systems, the calling program passes arguments in an argument item sequence; each quadword in the sequence specifies a single argument item. Note that the argument item sequence is formed using R16--21 or F16--21 (a register for each argument). The argument item sequence can have a mix of integer and floating-point items that use both register types but must not repeat the same number.
In the Alpha and VAX environments, the called routine interprets each argument using one of three standard passing mechanisms: by value, by reference, or by descriptor. For more information on arguments, see Sections 18.4 and 18.5.
A FORTRAN program could include any one of the following calls to this procedure:
The following examples illustrate the standard mechanism for calling an external procedure, subroutine, or function in most high-level languages.
In VAX MACRO, a calling sequence takes one of three forms, as illustrated by the following examples:
As these examples show, high-level languages use different forms of the
call statement. Each language's user guide gives specific information
about calling the run-time library from that language.
On VAX systems, JSB entry point names follow the naming conventions explained in Section 19.2.1, except that they include a suffix indicating the number of the highest register accessed or modified. This suffix helps ensure that the calling program and the called routine agree on the number of registers that the called routine is going to change.
The following example illustrates the VAX MACRO code that invokes the library routine MTH$SIN_R4 by means of a JSB instruction. As indicated in the JSB entry point name, this routine uses R0 through R4.
JSB entry points are available only to VAX MACRO and VAX BLISS
programs. No VAX high-level language provides a mechanism for accessing
JSB entry points.
On VAX systems, some run-time library routines return a function value. Typically on a VAX system, the return is in the form of a 32-bit value in register R0 or a 64-bit value in registers R0 and R1. In high-level languages, statuses or function return values in R0 appear as the function result. When a routine returns a function value in R0, it cannot also use R1 to return a status code. Therefore, such a procedure signals errors rather than returning a status. For more information, refer to the OpenVMS Calling Standard or the description of LIB$SIGNAL in the OpenVMS RTL Library (LIB$) Manual.
On Alpha systems, a standard function returns its function value in R0,
F0, or F0 and F1. A function value of less than 64 bits returned by
immediate value in R0 is zero-extended or sign-extended to a full
quadword as required by the data type. Note that a floating function
value is returned by immediate value in F0 or in F0 and F1.
Library return status and condition value symbols have the following general form:
The elements that make up this format represent the following:
Articles and prepositions are not considered significant words in this format. If a significant word is only two letters long, an underscore is used to fill out the third space. Some examples follow. Note that, in most facilities, the normal or success symbol is an exception to the convention described here.
19.3 Calling a Library Procedure in VAX MACRO (VAX Only)
This section describes how to code MACRO calls to library routines
using a CALLS, CALLG, or JSB instruction for VAX systems. The routine
descriptions that appear later in this manual describe the entry points
for each routine. You can use either a CALLS or a CALLG instruction to
invoke a procedure with a CALL entry point. You must use a JSB
instruction to invoke a procedure with a JSB entry point. All MACRO
calls are explicitly defined.
All run-time library routines have a CALL entry point. Some routines also have a JSB entry point. In MACRO, you invoke a CALL entry point with a CALLS or CALLG instruction. To access a JSB entry point, use a JSB instruction.
Arguments are passed to CALLS and CALLG entry points by a pointer to the argument list. The only difference between the CALLS and CALLG instructions is as follows:
Both of these instructions have the same effect on the called procedure.
JSB instructions execute faster than CALL instructions. They do not set up a new stack frame, do not change the enabling of hardware traps or faults, and do not preserve the contents of any registers before modifying them. For these reasons, you must be careful when invoking a JSB entry point in order to prevent the loss of information stored by the calling program.
Whichever type of call you use, the actual reference to the procedure entry point should use general-mode addressing (G^). This ensures that the linker and the image activator are able to locate the module within the shareable image.
In most cases, you have to tell a library routine where to find input values and store output values. You must select a data type for each argument when you code your program. Most routines accept and return 32-bit arguments.
For input arguments of byte, word, or longword values, you can supply a constant value, a variable name, or an expression in the run-time library routine call. If you supply a variable name for the argument, the data type of the variable must be as large as or larger than the data types that the called procedure requires. For example, if the called procedure expects a byte in the range 0 to 100, you can use a variable data type of a byte, word, or longword with a value between 0 and 100.
For each output argument, you must declare a variable of exactly the
length required to avoid extraneous data. For example, if the called
procedure returns a byte value to a word-length variable, the leftmost
8 bits of the variable <15:8> are not overwritten on output.
Conversely, if a procedure returns a longword value to a word-length
variable, it modifies variables in the next higher word.
Before executing a CALLS instruction, you must push the necessary arguments on the stack. Arguments are pushed in reverse order; the last argument listed in the calling sequence is pushed first. The following example shows how a MACRO program calls the procedure that allocates virtual memory in the program region for LIB$GET_VM.
Because the stack grows toward location 0, arguments are pushed onto
the stack in reverse order from the order shown in the general format
for the routine. Thus, the base-address argument, here
called START, is pushed first, and then the
number-bytes argument, called LEN. Upon return from
LIB$GET_VM, the calling program tests the return status
(ret-status), which is returned in R0 and branches to
an appropriate error routine if an error occurred.
When you use the CALLG instruction, the arguments are set up in any location, and the call includes a reference to the argument list. The following example of a CALLG instruction is equivalent to the preceding CALLS example.
19.3.4 VAX MACRO JSB Entry Points
A procedure's JSB entry point name indicates the highest numbered register that the procedure modifies. Thus, a procedure with a suffix Rn modifies registers R0 through Rn. (You should always assume that R0 and R1 are modified.) The calling program loads the arguments in the registers before the JSB instruction is executed.
A calling program must use a JSB instruction to invoke a run-time library routine by means of its JSB entry point. You pass arguments to a JSB entry point by placing them in registers in the following manner:
In this example, R4 in the entry point name indicates that MTH$SIN_R4 changes the contents of registers R0 through R4. The routine does not reference or change the contents of registers R5 through R11.
The entry mask of a calling procedure should specify all the registers to be saved if the procedure invokes a JSB routine. This step is necessary because a JSB procedure does not have an entry mask and thus has no way to specify registers to be saved or restored.
For example, consider program A calling procedure B by means of a CALL entry point.
A similar problem occurs if the stack is unwound, because unwinding the stack restores the contents of registers for each stack frame as it removes the previous frame. Because a JSB entry point does not create a stack frame, the contents of the registers before the JSB instruction will not be restored unless they were saved in the entry mask of the calling program. You do this by naming the registers to be saved in the calling program's entry mask, so a stack unwind correctly restores all registers from the stack. In the following example, the function Y=PROC(A,B) returns the value Y, where Y = SIN(A)*SIN(B):
19.3.5 Return Status
Your VAX MACRO program can test for errors by examining segments of the 32-bit status code returned by a run-time library routine.
To test for errors, check for a zero in bit 0 using a Branch on Low Bit Set (BLBS) or Branch on Low Bit Clear (BLBC) instruction.
To test for a particular condition value, compare the 32 bits of the return status with the appropriate return status symbol using a Compare Long (CMPL) instruction or the run-time library routine LIB$MATCH_COND.
There are three ways to define a symbol for the condition value returned by a run-time library routine so that you can compare the value in R0 with a particular error code:
The following example asks for the user's name. It then calls the run-time library routine LIB$GET_INPUT to read the user's response from the terminal. If the string returned is longer than 30 characters (the space allocated to receive the name), LIB$GET_INPUT returns in R0 the condition value equivalent to the error LIB$_INPSTRTRU, 'input string truncated.' This value is defined as a global symbol by default. The example then checks for the specific error by comparing LIB$_INPSTRTRU with the contents of R0. If LIB$_INPSTRTRU is the error returned, the program considers that the routine executed successfully. If any other error occurs, the program handles it as a true error.
19.3.6 Function Return Values in VAX MACRO
Function values are generally returned in R0 (32-bit values) or R0 and R1 (64-bit values). A MACRO program can access a function value by referencing R0 or R0 and R1 directly. For functions that return a string, the address of the string or the address of its descriptor is returned in R0. If a function needs to return a value larger than 64 bits, it must return the value by using an output argument.
Note the following exceptions to these rules: