SetFilePointer API

[Paul Squires]

I am writing an xbase/b+tree database system and I am using the
Windows API for I/O. I need to use the SetFilePointer API to
position the file pointer before I do the WriteFile call. The
SetFilePointer API can take two LONG integers that specify the
byte position.

From reading past posts (using POFFS) I noticed that only the
first parameter is being passed with a LONG, the second ByVal
%NULL. This would restrict files to a size of 2,147,486,648
bytes. How can you pass a Quad Integer to the API so file sizes
could be enormous. The API alludes to the fact that it can
accept values in the QUAD range, but the API parameters are such
that you need to pass two LONGS.

Any idea how this is done??

Greatly appreciated for your time and trouble.




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Paul Squires
[email protected]

[Michael Mattias]

from MSDN:

DWORD SetFilePointer( HANDLE hFile, LONG lDistanceToMove,
PLONG lpDistanceToMoveHigh, DWORD dwMoveMethod );

Not tested with the API, but

Distance High = QuadValue \ 2,147,483,648
Distance Low = QuadValue MOD 2,147,483,648


Not that you pass the Distance Low by value and the Distance High by reference.

(Sheesh, that's weird, ain't it?)

MCM

[Semen Matusovski]

Dim pp As Quad ' <-------------- Position
ReDim pm(1 To 2) As Long At VarPtr(pp) ' <-- For API
pp = ...
SetFilePointer hFile, ByVal pm(1), pm(2), %FILE_BEGIN

Note, pm(2) is ByRef.

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E-MAIL: [email protected]

[Paul Squires]

Michael & Semen,

Thanks for the suggestions. I will try both suggestions once I
clear out enough room on my hard drive to test such a large
file.

The DIM AT function is very interesting. The more I read about
it, the more interesting things it seems you can do with it. This is
only my first week with PB - next week I need to learn more
about pointers (I gave up on "C" 10 years ago because pointers
confused me so much!).

Thanks,

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Paul Squires
[email protected]

[Steve Hutchesson]

Paul,

You will have fun with pointers as it allows you to do some interesting
things. The distinction is actually a simple one, a "pointer" to a value
tells you WHERE it is in memory.

Take a simple example like,

LOCAL var as LONG
LOCAL address as DWORD

var = 100 ' assign a value to a variable.
address = VarPtr(var) ' get the address in memory of the variable.

The result of VarPtr() is a POINTER to the variable "var"

If you look at both in a MsgBox,

MsgBox str$(var),0,str$(address)

You will get the value "100" and a large DWORD size number which is the
actual address of WHERE the variable is in memory. Now if you pass that
POINTER to a SUB or FUNCTION by value (ByVal), at the other end you get
the same large DWORD size number that is its address. Converting it back
to a value is easy with the POINTER operator on PowerBASIC.

FUNCTION TestFunc(ByVal longVar as LONG PTR) as LONG

    LOCAL var as LONG   ' make a variable of the right type

    var = @longVar      ' use the POINTER operator [ @ ] to access the value

    FUNCTION = var      ' return the value so you can test it

END FUNCTION

This example is very simple but you can do the same thing with an array or
UDT so that you can pass larger and more complex data in a single address.

You will find that pointers in basic are a lot easier to manage that in the
older dialects of C and they are very powerful once you get the swing of them.

Regards,

[email protected]

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[Paul Squires]

Thanks Steve,

That makes things clearer. I have know how to use VarPtr and
StrPtr since I absorbed Ethan Winer's book "BASIC Techniques
and Utilities" a few years ago. I never realized that pointers
were so similar. I have a few SUBS now where I use VarPtr or
StrPtr to get the address and then I pass that address to
the SUB via a DWORD parameter. I could do the same thing with
a pointer and then use the @ functionality in the SUB to
manipulate the data. Fantastic!

They should call this "AmazingBasic" because I am blown away as
each hour passes that I use this product! Also, the in-line
assembler is great. Even though I am not a very good assembly
language programmer, I have been able to "borrow" some in-line
code from this forum and it has sped up some of my time critical
code by leaps and bounds!

Thanks


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Paul Squires
[email protected]