[Info-vax] Volatile, was: Re: yet another sys$qiow question

[VAXman- at SendSpamHere.ORG]

In article <fcdd3e23-53c1-4eb2-a5bb-c9339ec4f6b4 at googlegroups.com>, John Reagan <xyzzy1959 at gmail.com> writes:
>On Wednesday, August 19, 2015 at 12:58:05 PM UTC-4, VAXman- wrote:
>> In article <>, Simon Clubley <> writes:
>> >On 2015-08-19, VAXman-  @SendSpamHere.ORG <> wrote:
>> >> Question:  If "volatile" is necessary, shouldn't this code keep spinni=
>ng its
>> >> tail???
>> >>
>> >
>> >No, because you only have a function call in the loop (sys$qiow())
>> >(and the function call additionally directly references iosb BTW).
>> >
>> >This means that regardless of whether the compiler terminates the
>> >caching scope for iosb at any function call or whether it only
>> >terminates the caching scope when a function call references iosb,
>> >your code still causes the compiler to generate code to re-read the
>> >current contents of the iosb variable after the function call returns.
>> >
>> >The current discussion is based around the belief C compilers terminate
>> >the caching scope at any function call. I don't know if this is true
>> >for all the other languages VMS supports.
>>=20
>> So, I should put the $QIO in a subroutine.
>>=20
>
>Well, the compiler saw that somebody took the address of the IOSB and passe=
>d it along to $QIO.  The compiler doesn't know if that address was saved in=
> some global variable.  So the call to your jacket routine is still probabl=
>y sufficient to tell the compiler (this routine call might write into every=
> possible global variable and write into every variable whose address was e=
>ver taken).

OK.  Got it!  This should hide the 'iosb' use -- and it does. ;)

#pragma environment save
#ifndef __NEW_STARLET
#define __NEW_STARLET New_and_Improved
#endif

#include <builtins.h>		// DECC built-in function definitions
#include <c_asm.h>              // DECC asm() intrinsic function definiitons
#include <descrip.h>		// OpenVMS descriptor structure definitions  
#include <efndef.h>		// OpenVMS event flag number definitions
#include <iodef.h>		// OpenVMS $QIO I/O function code definitions
#include <iosbdef.h>		// OpenVMS I/O status block structure definitions
#include <starlet.h>		// OpenVMS system service prototype & codes
#include <stsdef.h>		// OpenVMS system message format definitions
#include <vadef.h>              // OpenVMS virtual address vield definitions

    int sts = 0, ctr = 0;

    unsigned short chan;
    $DESCRIPTOR(sys$output,"SYS$OUTPUT");

    char star[] ={'|','/','-','\\'};

    void *sp;

int main()
  {
      /////////////////////////////////////////////////////////////////////////
     //Remove the '//' comment to enable 'volatile' type qualifier on IOSB. //
    /////////////////////////////////////////////////////////////////////////

//  volatile  
    IOSB iosb = { 0, 0, 0};

    sp = (void *)asm("bis %r29,%r31,%r0;");	// get FP address                        
    sp = &((char*)sp)[VA$S_VADEF];		// skip PDSC address
						// usp is now at iosb

    if (!((sts=SYS$ASSIGN(
				&sys$output,		// device name
				&chan,			// channel
				0,			// access mode
				0			// mailbox name
			 ))&STS$M_SUCCESS)) return sts;

    while (iosb.iosb$w_status == 0)
      {
	if (!((sts=SYS$QIO(
				EFN$C_ENF,		// event flag number
				chan,			// channel
				IO$_WRITEVBLK,		// QIO function
				(IOSB*)sp,		// I/O status block
				0,			// AST address
				0,			// AST parameter
				&star[(ctr++)&3],	// P1: VA of buffer
				1,			// P2: number of bytes
				0,			// P3: n/a
				0x8D000000,		// P4: carriage control
				0,			// P5: n/a
				0			// P6: n/a
			   ))&STS$M_SUCCESS)) return sts;
      }
  }

#pragma environment restore


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VAXman- A Bored Certified VMS Kernel Mode Hacker    VAXman(at)TMESIS(dot)ORG

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