From: dpm@access2.digex.net (David P. Murphy)
Subject: Re: FILE * -> FAB or RAB?
Date: 1996/07/31
Message-ID: <dpm.838832687@access2>#1/1
references: <4tmig3$fko@newsbf02.news.aol.com>
organization: Phase of the Moon Software Inc.    Alexandria, VA
newsgroups: comp.os.vms



mschill513@aol.com (MSchill513) writes:

>Is there any way using DEC C  and given a FILE structure represneting an
>open file to find the underlying RMS structures?

yes, there is.  it is not supported and it is probably quite version-dependent,
but here it is.  you can see from looking in <stdio.h> that a FILE * is really
a handle to a struct _iobuf, which is defined as a sixteen-byte structure.
what you _don't_ know is that that structure is in fact much larger,
and --- ta da --- contains pointers to no fewer than five RMS structures.
six, if you count the NAM block pointed to by the FAB.

the problem is finding where they are:  they definitely changed at least once
between version V6.1's DECC$SHR and version V7.0's DECC$SHR.  and of course
the arrangement is different for the VAXCRTL.

the following code was compiled, linked and run under ALPHA V6.1 against
version T02.0-018 of DECC$SHR dated april 1994.  it is based upon the V7.0
source listings [V70.CRTL.LIS]VAXCIO.SRC module, and i make no claims
for any other versions than those mentioned.  in fact, i'd bet US$10
that it would not be correct.  mike, could you tell us _why_ you need this?

$ type findrms.c
#include <stdio.h>
#include <stddef.h>
#include <stdlib.h>
#include <rms.h>

/* the first seven members of this structure are cut&pasted
 * from the _iobuf definition in the official STDIO.H header
 */
struct DECCRTL_IOBUF {
        int _cnt;
        char *_ptr;
        char *_base;
        unsigned char _flag;
        unsigned char _file;
        unsigned char _pad1;
        unsigned char _pad2;
        int stuff[14];
        int morestuff[5];       /* should be 6 in V7.0? */
        char *vms_name;
        int evenmorestuff[7];   /* should be 5 in V7.0? */
        struct RAB *rabptr;     /* RAB and FAB are reversed in V7.0? */
        struct FAB *fabptr;
        struct XABDAT *xabdatptr;
        struct XABFHC *xabfhcptr;
        struct XABPRO *xabproptr;
};

#define SHOWBLOCK(label, ptr, id, code)  \
                printf("\n%s=%08X", label, xyzzy->ptr);  \
                if (xyzzy->ptr != NULL) { \
                        printf(" CODE=%2d (%2d)", xyzzy->ptr->id, code); \
                }

int main(
        int argc,
        char *argv[]
)
{
        FILE *fp;
        struct DECCRTL_IOBUF *xyzzy;

        if ((fp = fopen("SYS$LOGIN:LOGIN.COM", "r")) != NULL) {
                xyzzy = *((struct DECCRTL_IOBUF **) fp);
                printf("fp=%08X true=%08X name=%08X", fp, xyzzy, xyzzy->vms_name);
                if (xyzzy->vms_name != NULL) {
                        printf(" %s", xyzzy->vms_name);
                }
                SHOWBLOCK("FAB   ", fabptr,    fab$b_bid, FAB$C_BID);
                SHOWBLOCK("RAB   ", rabptr,    rab$b_bid, RAB$C_BID);
                SHOWBLOCK("XABDAT", xabdatptr, xab$b_cod, XAB$C_DAT);
                SHOWBLOCK("XABFHC", xabfhcptr, xab$b_cod, XAB$C_FHC);
                SHOWBLOCK("XABPRO", xabproptr, xab$b_cod, XAB$C_PRO);
                printf("\n");
                if (xyzzy->fabptr->fab$l_nam != NULL) {
                        if (xyzzy->fabptr->fab$l_nam->nam$l_rsa != NULL) {
                                printf("resfs=%-*s\n",
                                        (int) xyzzy->fabptr->fab$l_nam->nam$b_rsl,
                                        xyzzy->fabptr->fab$l_nam->nam$l_rsa);
                        }
                }
        }

        exit(1);
}
$ cc findrms
$ link findrms
$ run findrms
fp=7FE70D2C true=00065528 name=00065808 SYS$LOGIN:LOGIN.COM
FAB   =00065608 CODE= 3 ( 3)
RAB   =000657F8 CODE= 1 ( 1)
XABDAT=000656C8 CODE=18 (18)
XABFHC=00065710 CODE=29 (29)
XABPRO=00065748 CODE=19 (19)
resfs=USR:[MURPHY]LOGIN.COM;109
$ 

ok
dpm
-- 
David P. Murphy                 mailto:murphy@connor.datametrics.com     (work)
systems programmer              mailto:dpm@access.digex.net          (personal)
                                http://www.access.digex.net/~dpm
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