How do I pre-extend a file to a specified size

[Anonymous Monk]

Some operating systems can optimize file creation (especially for fixed binary record type files) if the file is pre-extended to desired size. This eliminates multiple file extension requests and can produce less fragmented files systems on multi-user systems.

Any tricks to indicate desired size of file during open?

Originally posted as a Categorized Question.

[merlyn]

seek HANDLE, 0, $desired_size -1;
print HANDLE "\0";
seek HANDLE, 0, 0;

[Anonymous Monk]

I tried the following code segment on both Win98 and VMS and it failed to give the desired result:

open HANDLE, ">temp.tmp" || die;
$desired_size=1000000;
seek HANDLE, 0, $desired_size -1;
print HANDLE "\0";
seek HANDLE, 0, 0;
close HANDLE;

The resulting file on Win98 is one byte long!! On VMS it is one block (minimum file size). It would appear that the seek doesn't extend the file past its current size (initially 0 bytes). Any suggestion appreciated.

[chipmunk]

It is rather frustrating that the Q&A section is not properly threaded. I pointed out the bug in merlyn's code in Re: Answer: How do I pre-extend a file to a specified size, but looking at the node for the question, there's no indication that merlyn's answer even has a response. Meanwhile, looking at the node for merlyn's answer, there is no way to get back to the original question.

To fix the bug, swap the second and third arguments to seek:

seek HANDLE, $desired_size -1, 0;

[chipmunk]

That should be: seek HANDLE, $desired_size -1, 0; The second argument to seek is the number of bytes to move the position by, while the third argument is a number indicating where to start. 0 seeks from the beginning of the file, 1 seeks from the current location in the file, and 2 seeks from the end of the file.

[abhishek_akj]

open FILEHANDLE, " +< filename"        or die "Unable to open: $! \n";

truncate FILEHANDLE, $desired_length   or print "Unable to truncate: $
+! \n";

truncate just sets the filesize of the file opened by handle to whatever you specify; you can use it to remove characters from file as well as increase the filesize.

[Mr. Muskrat]

Actually, setting the length greater than the actual file size is undefined behavior.