Liberty BASIC Community Wiki - API file truncate

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tsh73 Oct 18, 2011
==API file truncate==

There is an API call for truncating file.
With that, file gets shorter (truncated) instantly, with the "tail" portion lost.

Of cource you might use old way and: 
* copy part of the file you need to a  new file, 
* then delete old file, 
* then rename new file to old name.
But that would take time. And you will need extra disk space.

(Info taken from [[FileAPI|API-Based File Operations]] by DennisMcK, and MSDN on SetEndOfFile, SetFilePointer)

So. Here is a function truncateFile( fileName$, newSize) and using demo.

[[code format="lb"]] 
'api file truncate

fileName$ = "test.dat"
'create file
open fileName$ for output as #1
    for i = 1 to 10
        print #1, using("########", i)
    next
close #1

'check size, show contents
gosub [checkFile]

if truncateFile( fileName$, 50) then
    print "File truncated OK"
    'check size, show contents
    gosub [checkFile]
else
    print "Failed to open file with API calls"
end if

end

[checkFile]
    open fileName$ for input as #1
        print "File length ";lof(#1)
        a$=input$(#1, lof(#1))
    close #1
    print "Data: ------------"
    print a$
    print "//----------------"
return

function truncateFile( fileName$, newSize)
'truncate file (API calls)
'API consts
    hFile = 0       'file handle
    INVALID.HANDLE.VALUE = -1

    calldll #kernel32, "CreateFileA", fileName$ as ptr, _GENERIC_WRITE as ulong, _
        0 as ulong, 0 as long, _OPEN_ALWAYS as ulong, _
        _FILE_ATTRIBUTE_NORMAL as ulong, 0 as long, hFile as long

    if hFile = INVALID.HANDLE.VALUE then
        truncateFile = 0
        exit function
    end if
    'hFile <> INVALID.HANDLE.VALUE then 'file opened successfully

    'set the file pointer to the newSize
    calldll #kernel32,"SetFilePointer", hFile as long, newSize as long, 0 as long, _
        _FILE_BEGIN as ulong, r as ulong

    '"commit" newSize
    calldll #kernel32, "SetEndOfFile", hFile as long, r as ulong

    'close file
    calldll #kernel32, "CloseHandle", hFile as long, r as boolean

    truncateFile = 1

end function

[[code]]