Truncate a file

Truncate a file is a draft programming task. It is not yet considered ready to be promoted as a complete task, for reasons that should be found in its talk page.

The task is to demonstrate how to truncate a file to a specific length. This should be implemented as a routine that takes two parameters: the filename and the required file length (in bytes). The truncation can be achieved using system or library calls intended for such a task, if such methods exist, or by creating a temporary file of a reduced size and renaming it, after first deleting the original file, if no other method is possible. The file may contain non human readable binary data in an unspecified format, so the routine should be "binary safe", leaving the contents of the untruncated part of the file unchanged. If the specified filename does not exist, or the provided length is not less than the current file length, then the routine should raise an appropriate error condition and exit. On some systems, the provided file truncation facilities also extend the file, if the specified length is greater than the current length of the file. For the purpose of this task, it is permissible for such facilities to be used. If an increase in length occurs, it should be notes with the example. Optionally a warning message relating to the increase in file size may be implemented. In some languages, the provided truncation facilities might not change the file in this case. This should aslo be noted with the example.

Java

The built-in function for truncating a file in Java will leave the file unchanged if the specified size is larger than the file. This version expects the source file name and the new size as command line arguments (in that order). <lang java>import java.io.FileOutputStream; import java.io.IOException; import java.nio.channels.FileChannel;

public class TruncFile { public static void main(String[] args) throws IOException{ if(args.length < 2){ System.out.println("Usage: java TruncFile fileName newSize"); return; } //turn on "append" so it doesn't clear the file FileChannel outChan = new FileOutputStream(args[0], true).getChannel(); long newSize = Long.parseLong(args[1]); outChan.truncate(newSize); outChan.close(); } }</lang>

PureBasic

PureBasic has the internal function TruncateFile that cuts the file at the current file position and discards all data that follows. <lang PureBasic>Procedure SetFileSize(File$, length.q)

 Protected fh, pos, i
 If FileSize(File$) < length
   Debug "File to small, is a directory or does not exist."
   ProcedureReturn #False
 Else 
   fh = OpenFile(#PB_Any, File$)
   FileSeek(fh, length)
   TruncateFile(fh)
   CloseFile(fh)
 EndIf
 ProcedureReturn #True

EndProcedure</lang>

Ruby

This only works with some platforms. If truncation is not available, then Ruby raises NotImplementedError.

<lang ruby># Open a file for writing, and truncate it to 1234 bytes. File.open("file", "ab") { |f| f.truncate(1234) }

1. Trunate a file to 567 bytes.

File.truncate("file", 567)</lang>

Tcl

<lang tcl>package require Tcl 8.5

set f [open "file" r+] chan truncate $f 1234; # Truncate at a particular length close $f</lang>

UNIX Shell

<lang bash># Truncate a file named "myfile" to 1440 kilobytes. truncate -s 1440k myfile</lang>