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HP OpenVMS SystemsC Programming Language |
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HP C
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Example 3-4 shows a variant of the program in Example 3-3. Here, the $DESCRIPTOR macro is used to create a compile-time string descriptor and to pass it to the SYS$SETPRN system service routine. In Example 3-4, the program returns the status value returned by SYS$SETPRN to DCL for interpretation.
| Example 3-4 Passing Compile-Time String Descriptors |
|---|
/* This program returns the status value returned by *
* SYS$SETPRN. */
#include <descrip.h> /* Define $DESCRIPTOR macro. */
int SYS$SETPRN();
int main(void)
{
/* Initialize structure name_desc *
* as string descriptor. */
static $DESCRIPTOR(name_desc,"NEWPROC");
return SYS$SETPRN(&name_desc);
}
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To test the results of the preceding example, do the following:
$ SHOW PROCESS ! Note the process name. $ RUN example ! Run the example. $ SHOW PROCESS ! Note that the process name has changed. |
The $DESCRIPTOR macro is used in further examples in this chapter.
There are default parameter-passing mechanisms established for every data type you can use with HP C. Table 3-8 lists the HP C data types you can use with each parameter-passing mechanism. Asterisks appear next to the default parameter-passing mechanism for that particular data type.
You must use the appropriate parameter-passing mechanisms whenever you call a routine written in some other OpenVMS language or some prewritten system routine.
In HP C, you can call external routines written in other languages or HP C routines from routines written in other languages as either functions or subroutines. When you call an external routine as a function, a single value is returned. When you call an external routine as a subroutine (a void function), any values are returned in the argument list.
By default, HP C passes all arguments by immediate value with the exception of arrays and functions; these are passed by reference. Table 3-9 lists the default passing mechanisms for other OpenVMS languages.
The following sections describe the methods involved in using HP C with routines written in other OpenVMS languages.
When calling HP FORTRAN from HP C or vice versa, note these considerations. HP FORTRAN argument lists and argument descriptors are usually allocated statically. When it is possible, and to optimize space and time, the HP FORTRAN compiler pools the argument lists and initializes them at compile time. Sometimes several calls may use the same argument list.
In HP C, you often use arguments as local variables, and modify them at will. If a HP C routine that modifies an argument is called from a HP FORTRAN routine, unintended and incorrect side effects may occur.
The following example shows a HP C routine that is invalid when called from HP FORTRAN:
void f(int *x) /* x is a FORTRAN INTEGER passed by reference */
{
/* The next assignment is OK. It is permitted to modify what a
* FORTRAN argument list entry points to. */
*x = 0; /* ok */
/* The next assignment is invalid. It is not permitted to modify
* a FORTRAN argument list entry itself. */
x = x + 1; /* Invalid */
}
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Another problem is the semantic mismatch between strings in C and strings in HP FORTRAN. Strings in C vary in length and end in a null character. Strings in HP FORTRAN do not end in a null character and are padded with spaces to some fixed length. In general, this mismatch means that strings may not be passed between HP C and HP FORTRAN unless you do additional work. You may make a HP FORTRAN routine add a null character to a CHARACTER string before calling a HP C function. You may also write code that explicitly gets the length of a HP FORTRAN string from its descriptor and carefully pads the string with spaces after modifying it. An example later in this section shows a C function that carefully produces a proper string for HP FORTRAN.
Example 3-5 shows a HP C function calling a HP FORTRAN subprogram with a variety of data types. For most scalar types, HP FORTRAN expects arguments to be passed by reference but character data is passed by descriptor.
| Example 3-5 HP C Function Calling a HP FORTRAN Subprogram |
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/*
* Beginning of HP C function:
*/
#include <stdio.h>
#include <descrip.h> /* Get layout of descriptors */
extern int fort(); /* Declare FORTRAN function */
main(void)
{
int i = 508;
float f = 649.0;
double d = 91.50;
struct {
short s;
float f;
} s = {-2, -3.14};
auto $DESCRIPTOR(string1, "Hello, FORTRAN");
struct dsc$descriptor_s string2;
/* "string1" is a FORTRAN-style string declared and initialized using the
* $DESCRIPTOR macro. "string2" is also a FORTRAN-style string, but we are
* declaring and initializing it by hand. */
string2.dsc$b_dtype = DSC$K_DTYPE_T; /* Type is CHARACTER */
string2.dsc$b_class = DSC$K_CLASS_S; /* String descriptor */
string2.dsc$w_length = 3; /* Three characters in string */
string2.dsc$a_pointer = "bye"; /* Pointer to string value */
printf("FORTRAN result is %d\n", fort(&i, &f, &d, &s, &string1, &string2));
} /* End of HP C function */
C
C Beginning of FORTRAN subprogram:
C
INTEGER FUNCTION FORT(I, F, D, S, STRING1, STRING2)
INTEGER I
REAL F
DOUBLE PRECISION D
STRUCTURE /STRUCT/
INTEGER*2 SHORT
REAL FLOAT
END STRUCTURE
RECORD /STRUCT/ S
C You can tell FORTRAN to use the length in the descriptor
C as done here for STRING1, or you can tell FORTRAN to ignore the
C descriptor and assume the string has a particular length as done
C for STRING2. This choice is up to you.
CHARACTER*(*) STRING1
CHARACTER*3 STRING2
WRITE(5, 10) I, F, D, S.SHORT, S.FLOAT, STRING1, STRING2
10 FORMAT(1X, I3, F8.1, D10.2, I7, F10.3, 1X, A, 2X, A)
FORT = -15
RETURN
END
C End of FORTRAN subprogram
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Example 3-5 produces the following output:
508 649.0 0.92D+02 -2 -3.140 Hello, FORTRAN bye FORTRAN result is -15 |
Example 3-6 shows a HP FORTRAN subprogram calling a HP C function. Since the HP C function is called from HP FORTRAN as a subroutine and not as a function, the HP C function is declared to have a return value of void .
| Example 3-6 HP FORTRAN Subprogram Calling a HP C Function |
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C
C Beginning of FORTRAN subprogram:
C
INTEGER I
REAL F(3)
CHARACTER*10 STRING
C Since this program does not have a C main program and you want
C to use HP C RTL functions from the C subroutine, you must call
C DECC$CRTL_INIT to initialize the run-time library.
CALL DECC$CRTL_INIT
I = -617
F(1) = 3.1
F(2) = 0.04
F(3) = 0.0016
STRING = 'HELLO'
CALL CSUBR(I, F, STRING)
END
C End of FORTRAN subprogram
/*
* Beginning of HP C function:
*/
#include <stdio.h>
#include <descrip.h> /* Get layout of descriptors */
void csubr(int *i, /* FORTRAN integer, by reference */
float f[3], /* FORTRAN array, by reference */
struct dsc$descriptor_s *string) /* FORTRAN character, by descriptor */
{
int j;
printf("i = %d\n", *i);
for (j = 0; j < 3; ++j)
printf("f[%d] = %f\n", j, f[j]);
/* Since FORTRAN character data is not null-terminated, you must use
* a counted loop to print the string.
*/
printf("string = \"");
for (j = 0; j < string->dsc$w_length; ++j)
putchar(string->dsc$a_pointer[j]);
printf("\"\n");
} /* End of HP C function */
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Example 3-6 produces the following output:
i = -617 f[0] = 3.100000 f[1] = 0.040000 f[2] = 0.001600 string = "HELLO " |
Example 3-7 shows a C function that acts like a CHARACTER*(*) function in HP FORTRAN.
| Example 3-7 HP C Function Emulating a HP FORTRAN CHARACTER*(*) Function |
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C
C Beginning of FORTRAN program:
C
CHARACTER*9 STARS, C
C Call a C function to produce a string of three "*" left-justified
C in a nine-character field.
C = STARS(3)
WRITE(5, 10) C
10 FORMAT(1X, '"', A, '"')
END
C End of FORTRAN program
/*
* Beginning of HP C function:
*/
#include <descrip.h> /* Get layout of descriptors */
/* Routine "stars" is equivalent to a FORTRAN function declared as
* follows:
*
* CHARACTER*(*) FUNCTION STARS(NUM)
* INTEGER NUM
*
* Note that a FORTRAN CHARACTER function has an extra entry added to
* the argument list to represent the return value of the CHARACTER
* function. This entry, which appears first in the argument list,
* is the address of a completely filled-in character descriptor. Since
* the C version of a FORTRAN character function explicitly uses this
* extra argument list entry, the C version of the function is void!
*
* This example function returns a string that contains the specified
* number of asterisks (or "stars").
*
*/
void stars(struct dsc$descriptor_s *return_value, /* FORTRAN return value */
int *num_stars) /* Number of "stars" to create */
{
int i, limit;
/* A FORTRAN string is truncated if it is too large for the memory area
* allocated, and it is padded with spaces if it is too short. Set limit
* to the number of stars to put in the string given the size of the area
* used to store it. */
if (*num_stars < return_value->dsc$w_length)
limit = *num_stars;
else
limit = return_value->dsc$w_length;
/* Create a string of stars of the specified length up to the limit of the
* string size. */
for (i = 0; i < limit; ++i)
return_value->dsc$a_pointer[i] = '*';
/* Pad rest of string with spaces, if necessary. */
for (; i < return_value->dsc$w_length; ++i)
return_value->dsc$a_pointer[i] = ' ';
} /* End of HP C Function */
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Example 3-7 produces the following output:
"*** " |
You can call a VAX MACRO routine from HP C or vice versa. However, like all interlanguage calls, it is necessary to make sure that the actual arguments correspond to the expected formal parameter types. Also, it is necessary to remember that C strings are null-terminated and to take special action in either the MACRO routine or the C routine to allow for this.
Example 3-8 shows a MACRO routine that calls a C routine with three arguments, passing one by value, one by reference, and one by descriptor. It is followed by the source for the called C routine.
| Example 3-8 VAX MACRO Program Calling a HP C Function |
|---|
;-----------------------------------------------------------------------
; Beginning of MACRO program
;-----------------------------------------------------------------------
.extrn dbroutine ; The C routine
;-----------------------------------------------------------------------
; Local Data
;-----------------------------------------------------------------------
.psect data rd,nowrt,noexe
ft$$t_part_num: .ascii /WidgitGadget/
ft$$t_query_mode: .ascii /I/
ft$$s_query_mode = <. - ft$$t_query_mode>
ft$$l_protocol_buff: .blkl 1
ft$$kd_part_num_dsc:
.word 12
.word 0
.address ft$$t_part_num
;-----------------------------------------------------------------------
; Entry Point
;-----------------------------------------------------------------------
.psect ft_code rd,nowrt,exe
.entry dbtest ^m<r2,r3,r4,r5,r6,r7,r8>
;+
; call C routine for data base lookup
;-
movl #1,r3
pushal ft$$kd_part_num_dsc ; Descriptor for part number
pushal ft$$t_query_mode ; Mode to call
pushl #1 ; Status
calls #3, dbroutine ; Check the data base
99$:
ret
.end dbtest
;-----------------------------------------------------------------------
; End of MACRO program
;-----------------------------------------------------------------------
/*
* Beginning of HP C code for dbroutine:
*/
#include <stdio.h>
#include <descrip.h>
#include <stdlib.h>
#include <string.h>
/* Structure pn_desc is the format of the descriptor
passed by the macro routine. */
extern struct
mydescript {
short pn_len;
short pn_zero;
char *pn_addr;
};
int dbroutine (int status, /* Passed by value */
char *action, /* Passed by reference */
struct mydescript *name_dsc) /* Passed by descriptor */
{
char *part_name;
/* Allocate space to put the null-padded name string. */
part_name = malloc(name_dsc->pn_len + 1);
memcpy( part_name,name_dsc -> pn_addr ,name_dsc -> pn_len);
/* Remember that C array bounds start at 0 */
part_name[name_dsc -> pn_len] = '\0';
printf (" Status is %d\n", status);
printf (" Length is %d\n",name_dsc -> pn_len);
printf (" Part_name is %s\n",part_name);
printf (" Request is %c\n",*action);
status = 1;
return(status);
} /* End of HP C code for dbroutine */
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Example 3-8 produces the following output:
Status is 1 Length is 12 Part_name is WidgitGadget Request is I |
Example 3-9 shows a HP C program that calls a VAX MACRO program.
| Example 3-9 HP C Program Calling a VAX MACRO Program |
|---|
/* Beginning of HP C function */
#include <stdio.h>
#include <descrip.h>
int zapit( int status, int *action, struct dsc$descriptor_s *descript);
main(void)
{
int status=255, argh = 99;
int *action = &argh;
$DESCRIPTOR(name_dsc,"SuperEconomySize");
printf(" Before calling ZAPIT: \n");
printf(" Status was %d \n",status);
printf(" Action contained %d and *action contained %d \n" ,action, *action);
printf(" And the thing described by the descriptor was %s\n",
name_dsc.dsc$a_pointer);
if (zapit(status,action,&name_dsc) && 1)
{
printf(" Ack, the world has been zapped! \n");
printf(" Status is %d \n",status);
printf(" Action contains %d and *action contains %d \n" ,action, *action);
printf(" And the address of the thing described by the descriptor is %d\n",
name_dsc.dsc$a_pointer);
}
} /* End of HP C function */
;-----------------------------------------------------------------------
; Beginning of VAX MACRO source code for zapit
;-----------------------------------------------------------------------
; Entry Point
;-----------------------------------------------------------------------
.psect ft_code rd,nowrt,exe
.entry zapit ^m<r2,r3,r4,r5,r6,r7,r8>
;+
; Maliciously change parameters passed by the C routine.
;
; The first parameter is passed by value, the second by
; reference, and the third by descriptor.
;-
movl 4(ap), @8(ap) ;Change the by-reference parameter
;to the first parameter's value.
movl 12(ap), r2
movl #0,4(r2) ;Zap address of string in descriptor.
; Return -1 to signal successful destruction.
movl #-1,r0
ret
.end
;-----------------------------------------------------------------------
; End of VAX MACRO source code for zapit
;-----------------------------------------------------------------------
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Example 3-9 produces the following output:
Before calling ZAPIT: Status was 255 Action contained 2146269556 and *action contained 99 And the thing described by the descriptor was SuperEconomySize Ack, the world has been zapped! Status is 255 Action contains 2146269556 and *action contains 255 And the address of the thing described by the descriptor is 0 |
Calling routines written in HP BASIC from HP C programs or vice versa is straightforward. By default, HP BASIC passes arguments by reference, except for arrays and strings, which are passed by descriptor. In some cases, these defaults may be overridden by explicitly specifying the desired parameter-passing mechanisms in the HP BASIC program. However, if an argument is a constant or an expression, the actual argument passed refers to a local copy of the specified argument's value.
Strings in HP BASIC are not terminated by a null character, which is done by HP C. As a result, passing strings between HP BASIC and HP C routines requires you to do additional work. You may choose to add an explicit null character to a HP BASIC string before passing it to a HP C routine, or you may prefer to code the HP C routine to obtain the string's length from its descriptor.
Example 3-10 shows a HP C program that calls a HP BASIC function with a variety of argument data types.
| Example 3-10 HP C Function Calling a HP BASIC Function |
|---|
/*
* Beginning of HP C function:
*/
#include <stdio.h>
#include <descrip.h>
extern int basfunc ();
main(void)
{
int i = 508;
float f = 649.0;
double d = 91.50;
struct
{
short s;
float f;
} s = { -2, -3.14 };
$DESCRIPTOR (string1, "A C string");
printf ("BASIC returned %d\n",
basfunc (&i, &f, &d, &s, &string1, "bye"));
} /* End of HP C function */
! Beginning of the BASIC program
FUNCTION INTEGER basfunc (INTEGER i, REAL f, DOUBLE d, x s, &
STRING string1, &
STRING string2 = 3 BY REF)
RECORD x
WORD s
REAL f
END RECORD x
PRINT 'i = '; i
PRINT 'f = '; f
PRINT 'd = '; d
PRINT 's::s = '; s::s
PRINT 's::f = '; s::f
PRINT 'string1 = '; string1
PRINT 'string2 = '; string2
END FUNCTION -15
! End of the BASIC program
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Example 3-10 produces the following output:
i = 508
f = 649
d = 91.5
s::s = -2
s::f = -3.14
string1 = A C string
string2 = bye
BASIC returned -15
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Example 3-11 shows a HP BASIC program that calls a HP C function.
| Example 3-11 HP BASIC Program Calling a HP C Function |
|---|
! Beginning of the BASIC program:
PROGRAM example
EXTERNAL STRING FUNCTION cfunc (INTEGER BY VALUE, &
INTEGER BY VALUE, &
STRING BY DESC)
s$ = cfunc (5, 3, "abcdefghi")
PRINT "substring is "; s$
END PROGRAM
! End of the BASIC program
/*
* Beginning of HP C function:
*/
#include <descrip.h>
int str$copy_dx();
/*
* This routine simulates a BASIC function whose return
* value is a STRING. It returns the substring that is `length'
* characters long, starting from the offset `offset' (0-based)
* in the input string described by the descriptor pointed to
* by `in_str'.
*/
void cfunc (struct dsc$descriptor_s *out_str,
int offset,
int length,
struct dsc$descriptor_s *in_str)
{
/* Declare a string descriptor for the substring. */
struct dsc$descriptor temp;
/* Check that the desired substring is wholly
within the input string. */
if (offset + length > in_str -> dsc$w_length)
return;
/* Fill in the substring descriptor. */
temp.dsc$w_length = length;
temp.dsc$a_pointer = in_str -> dsc$a_pointer + offset;
temp.dsc$b_dtype = DSC$K_DTYPE_T;
temp.dsc$b_class = DSC$K_CLASS_S;
/* Copy the substring to the return string. */
str$copy_dx (out_str, & temp);
} /* End of HP C function */
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Example 3-11 produces the following output:
substring is fgh
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