API file truncate

API file truncate

There is an API call for truncating file.
With that, file gets shorter (truncated) instantly, with the "tail" "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 API-Based File Operations by DennisMcK, and MSDN on SetEndOfFile, SetFilePointer)

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

[[code format="lb"]]

 'api file truncate 
 
 fileName$ = "test.dat" "test.dat" 
 'create file 
 open fileName$ for output as #1 
 for i = 1 to 10 
 print #1, using("########", using("########", i) 
 next 
 close #1 
 
 'check size, show contents 
 gosub [checkFile] 
 
 if truncateFile( fileName$, 50) then 
 print "File "File truncated OK" OK" 
 'check size, show contents 
 gosub [checkFile] 
 else 
 print "Failed "Failed to open file with API calls" calls" 
 end if 
 
 end 
 
 [checkFile] 
 open fileName$ for input as #1 
 print "File "File length ";lof(#1) ";lof(#1) 
 a$=input$(#1, lof(#1)) 
 close #1 
 print "Data: ------------" "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", "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", #kernel32,"SetFilePointer", hFile as long, newSize as long, 0 as long, _ 
 _FILE_BEGIN as ulong, r as ulong 
 
 '"commit" '"commit" newSize 
 calldll #kernel32, "SetEndOfFile", "SetEndOfFile", hFile as long, r as ulong 
 
 'close file 
 calldll #kernel32, "CloseHandle", "CloseHandle", hFile as long, r as boolean 
 
 truncateFile = 1 
 
 end function code