In article <5ekl3m$mnp$1@hammer.msfc.nasa.gov>, mallozzi@ssl.msfc.nasa.gov writes:
> Hi,
>
> Is it possible to pass the following data structures from C code back
> to IDL via CALL_EXTERNAL:
>
> 1. An array of strings?
> 2. A C structure, containing int, float, char
> 3. A C union
>
> I got my C code to pass back a single string, but could not do an
> array of strings. Really I would like to pass a structure back to
> IDL. Does anyone have any examples of this? The IDL examples treat
> only the simplest cases.
> PS I am using OpenVMS :-(
The answer is yes, you can do all of the above.
1) Arrays of strings work. However, note that IDL and PV-WAVE pass strings to
CALL_EXTERNAL differently. IDL passes strings by descriptor, PV-WAVE passes
them by reference. It is sometimes more convenient to convert IDL strings
to byte arrays before calling your C code and then convert them back to IDL
strings after the return from CALL_EXTERNAL. Byte arrays are handled the
same in IDL and PV-WAVE.
2) IDL structures are just like C structures. You can pass them back and forth
with no problem. In my experience IDL structures are aligned the same way
your C compiler will align them. This differs from one architecture to the
next, so there may be different amounts of padding on different machines.
This should be transparent, since both IDL and your C code are compiled and
running on the same architecture. Note that IDL does NOT guarantee the
way structures are stored and passed. This is the current behavior however.
3) A C union is just one location used to store 2 "things". IDL has no
equivalent.
Note that CALL_EXTERNAL on VMS simply passes the arguments (by reference or
descriptor) while on Unix the (argc, argv) mechanism is used.
Here is some C code which allows passing strings and works both on VMS and
Unix with both IDL and PV-WAVE.
/* ezcaIDL.c
*
* This file is an interface layer between IDL/PV-WAVE and EZCA. It mainly
* just converts parameter passing from the IDL/PV-WAVE mechanisms
* (most parameters passed by reference) to the mechanism required by EZCA.
* For a few the functions which are not supplied by EZCA this routine actually
* does the work, using the ezcaPvToChid to fetch the channel descriptor.
*
* Author: Mark Rivers
* Date: June 28, 1995
*/
/* The following macros allow for the differences in the way PV-WAVE and
* IDL pass character strings. PV-WAVE passes the address of the address
* of the string in argp[]. IDL passes the address of a string descriptor
* structure, which contains the address of the string as one of its elements */
#ifdef IDL
typedef struct {
unsigned short length;
short type;
char *address;
} IDL_STR_DESCR;
#define STRARG IDL_STR_DESCR
#define STRADDR(s) s->address
#define STRFIXLEN(s) s->length = strlen(s->address)
static char* str_array_addr[1000];
#define BUILD_STR_ARRAY(len, s) for (i=0; i<len; i++) str_array_addr[i]=s[i].address
#define STR_ARRAY_ADDR(s) str_array_addr
#else /*(PV-WAVE)*/
#define STRARG char*
#define STRADDR(s) *s
#define STRFIXLEN(s) (*s)[strlen(*s)] = (char) 32
#define BUILD_STR_ARRAY(len, s)
#define STR_ARRAY_ADDR(s) s
#endif
/* The following macros allow this source file to be used with
PV-WAVE and IDL on both Unix and VMS platforms. The difference
is that on VMS platforms arguments are passed directly
(by reference), while on Unix they are passed by the (argc, argp)
mechanism. These macros also simplify the code for each routine. */
#if defined (VMS)
# define WAVE_HEADER0(ftype, fname)\
ftype fname() {
# define WAVE_HEADER1(ftype, fname, type1, arg1)\
ftype fname(type1 *arg1) {
# define WAVE_HEADER2(ftype, fname, type1, arg1, type2, arg2)\
ftype fname(type1 *arg1, type2 *arg2) {
# define WAVE_HEADER3(ftype, fname, type1, arg1, type2, arg2, type3, arg3)\
ftype fname(type1 *arg1, type2 *arg2, type3 *arg3) {
# define WAVE_HEADER4(ftype, fname, type1, arg1, type2, arg2, type3, arg3, type4, arg4)\
ftype fname(type1 *arg1, type2 *arg2, type3 *arg3, type4 *arg4) {
# define WAVE_HEADER5(ftype, fname, type1, arg1, type2, arg2, type3, arg3, type4, arg4, type5, arg5)\
ftype fname(type1 *arg1, type2 *arg2, type3 *arg3, type4 *arg4, type5 *arg5) {
#else
# define WAVE_HEADER0(ftype, fname)\
ftype fname(argc, argp)\
int argc;\
void *argp[];\
{
# define WAVE_HEADER1(ftype, fname, type1, arg1)\
ftype fname(argc, argp)\
int argc;\
void *argp[];\
{\
type1 *arg1 = (type1 *) argp[0];
# define WAVE_HEADER2(ftype, fname, type1, arg1, type2, arg2)\
ftype fname(argc, argp)\
int argc;\
void *argp[];\
{\
type1 *arg1 = (type1 *) argp[0];\
type2 *arg2 = (type2 *) argp[1];
# define WAVE_HEADER3(ftype, fname, type1, arg1, type2, arg2, type3, arg3)\
ftype fname(argc, argp)\
int argc;\
void *argp[];\
{\
type1 *arg1 = (type1 *) argp[0];\
type2 *arg2 = (type2 *) argp[1];\
type3 *arg3 = (type3 *) argp[2];
# define WAVE_HEADER4(ftype, fname, type1, arg1, type2, arg2, type3, arg3, type4, arg4)\
ftype fname(argc, argp)\
int argc;\
void *argp[];\
{\
type1 *arg1 = (type1 *) argp[0];\
type2 *arg2 = (type2 *) argp[1];\
type3 *arg3 = (type3 *) argp[2];\
type4 *arg4 = (type4 *) argp[3];
# define WAVE_HEADER5(ftype, fname, type1, arg1, type2, arg2, type3, arg3, type4, arg4, type5, arg5)\
ftype fname(argc, argp)\
int argc;\
void *argp[];\
{\
type1 *arg1 = (type1 *) argp[0];\
type2 *arg2 = (type2 *) argp[1];\
type3 *arg3 = (type3 *) argp[2];\
type4 *arg4 = (type4 *) argp[3];\
type5 *arg5 = (type5 *) argp[4];
#endif
WAVE_HEADER4(int, ezcaIDLGet, STRARG, pvname, char, type, int, nelem, void, buff)
return(ezcaGet(STRADDR(pvname), *type, *nelem, buff));
}
WAVE_HEADER3(int, ezcaIDLGetControlLimits, STRARG, pvname, double, low, double, high)
return(ezcaGetControlLimits(STRADDR(pvname), low, high));
}
WAVE_HEADER3(int, ezcaIDLGetGraphicLimits, STRARG, pvname, double, low, double, high)
return(ezcaGetGraphicLimits(STRADDR(pvname), low, high));
}
WAVE_HEADER2(int, ezcaIDLGetPrecision, STRARG, pvname, short, precision)
return(ezcaGetPrecision(STRADDR(pvname), precision));
}
WAVE_HEADER4(int, ezcaIDLGetStatus, STRARG, pvname, TS_STAMP, timestamp, short, status, short, severity)
return(ezcaGetStatus(STRADDR(pvname), timestamp, status, severity));
}
WAVE_HEADER2(int, ezcaIDLGetUnits, STRARG, pvname, char, units)
return(ezcaGetUnits(STRADDR(pvname), units));
}
WAVE_HEADER1(int, ezcaIDLPerror, char, prefix)
ezcaPerror(prefix);
return EZCA_OK;
}
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