Dynamic Dynamic strings and the misuse of STRPTR.

[Bradley Olson]

Hey Folks,

The dynamic string allocation question and my own "String Quickie" raise some interesting questions about string pointers. And I think there's a good deal of confusion about them.

I'm talking about type STRING, PB's dynamic strings (allocated by OLE, stored in ASCII, and managed by an internal handle-based system). I'll call them pbstrings for short.

The PB manuals tell us, correctly, that if we want a pointer to a pbstring's data, to use STRPTR, not VARPTR. It's exactly what you want when passing a data address to C or whatever.

But it's exactly what you DON'T want if you're using it for internal dreferencing to pbStrings. I have code to prove this following this message.

If you want to dereference a string pointer as ASCIIZ data, then you want STRPTR. But if you want to dereference it as the pbstring it is, you want VARPTR.

The syntax can seem counterintuitive:

Given:
local pS as string ptr, pZ as asciiz ptr, S as string

pS = strptr( S) 'THIS IS WRONG!
pS = varptr( S) 'THIS IS RIGHT
pZ = strptr( S) 'THIS IS RIGHT

You'd think you want to use STRPTR() to assign to a STRING POINTER, but not so. We don't want a pointer to the data, we want the internal handle so PB can handle it correctly.

Tested code follows in first reply.

The upshot of all this is that pb has a nice string subsystem. It allocates and disposes of OLE strings for us, but engine isn't exposed. It'd be great, for dynamic allocation needs, if we could tell PB, "I need another pstring. Give me the handle, and YOU remember to destroy it along with the rest when my program finishes."

Regards,
Brad


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[This message has been edited by Bradley Olson (edited December 14, 2000).]

[Bradley Olson]

'Test of string pointers
'Brad Olson, Dec 2000

$COMPILE EXE
$DIM ALL
$DEBUG ERROR ON

%nStrings = 3000
%SizeStrings = 70
%printN = 1000 'Print every n strings for progress feedback

GLOBAL S() AS STRING 'Test source array of strings
GLOBAL S2() AS STRING 'Test destination array of strings
GLOBAL pS() AS STRING PTR 'Pointers assigned with STRPTR
GLOBAL pV() AS STRING PTR 'Pointers assigned with VARPTR
GLOBAL pA() AS ASCIIZ PTR 'Pointers assigned by STRPTR

FUNCTION PBMAIN
LOCAL A$, i&, n&
DIM S( %nStrings), S2( %nStrings)
DIM pS( %nStrings), pV( %nStrings), pA( %nStrings)
PRINT "Beginning test of string pointers."
PRINT "Fill source array with PB dynamic string's."
FOR i& = 1 TO %nStrings
A$ = "This is" + STR$( i&) + ". "
n& = %sizeStrings \ LEN( A$)
S( i&) = REPEAT$( n&, a$)
IF i& MOD %printN = 0 THEN PRINT " "; s(i&)
NEXT
PRINT "Assign pointers to pointer arrays."
FOR i& = 1 TO %nStrings
pS( i&) = STRPTR( s( i&))
pA( i&) = STRPTR( s( i&))
pV( i&) = VARPTR( s( i&))
NEXT

$IF 0 'THIS BOMBS, change the zero to 1 to prove it.
PRINT "String=String assign via STRPTR."
FOR i& = 1 TO %nStrings
S2( i&) = @pS( i&)
IF i& MOD %printN = 0 THEN PRINT s2( i&)
NEXT
$ENDIF

$IF 1
PRINT "String=String assign via VARPTR."
FOR i& = 1 TO %nStrings
S2( i&) = @pV( i&)
IF i& MOD %printN = 0 THEN PRINT s2( i&)
NEXT
$ENDIF

$IF 1
PRINT "String=ASCIIZ assign via STRPTR."
FOR i& = 1 TO %nStrings
S2( i&) = @pA( i&)
IF i& MOD %printN = 0 THEN PRINT s2( i&)
NEXT
$ENDIF

END FUNCTION


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[Steve Hutchesson]

Brad,

The distinction between StrPtr() and VarPtr() is clear enough, its pretty
much a case of know what you are getting, StrPtr() returns the ADDRESS
of the character data in the string, VarPtr() returns the ADDRESS of the
handle of the string.

When you use the POINTER operator in PowerBASIC on a dynamic string handle
you are getting what used to be a C runtime error "Different levels of
indirection". Basically a pointer to a pointer which is fine if thats what
you need but a problem if its not.

===========================================================================
The upshot of all this is that pb has a nice string subsystem. It
allocates and disposes of OLE strings for us, but engine isn't exposed.
It'd be great, for dynamic allocation needs, if we could tell PB, "I need
another pstring. Give me the handle, and YOU remember to destroy it along
with the rest when my program finishes."
===========================================================================

This is in fact easy to do, just use the 2 API calls from oleaut32,
SysAllocStringByteLen() and SysFreeString() and you can manage your own
with no problems at all.

Regards,

[email protected]

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[Charles Dietz]

I have to agree with Bradley... The following code shows four ways of
using string pointers. But notice the confusing syntax of the third
way. Intuition would lead one to want to declare p3 as a STRING PTR,
since we use p3 = STRPTR(s3), but doing that fails.

#COMPILE EXE
#DIM ALL
FUNCTION PBMAIN
   'Four ways to use string pointers
   LOCAL s1 AS STRING, p1 AS STRING PTR
   LOCAL s2 AS STRING, p2 AS STRING PTR
   LOCAL s3 AS STRING, p3 AS ASCIIZ PTR
   LOCAL s4 AS ASCIIZ *40, p4 AS ASCIIZ PTR
   '1st way
      p1 = STRPTR(s1)
      @p1 = "This is the first way"
      MSGBOX @p1
   '2nd way
      s2 = "This is the second way"
      p2 = VARPTR(s2)
      MSGBOX @p2
   '3rd way
      s3 = "This is the third way"
      p3 = STRPTR(s3)
      MSGBOX @p3
   '4th way
      s4 = "This is the fourth way"
      p4 = VARPTR(s4)
      MSGBOX @p4
END FUNCTION

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