Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!usc!elroy.jpl.nasa.gov!jpl-devvax!lwall
From: lwall@jpl-devvax.JPL.NASA.GOV (Larry Wall)
Newsgroups: comp.lang.perl
Subject: Re: PERL PL28 and reading raw devices (problem)
Message-ID: <9681@jpl-devvax.JPL.NASA.GOV>
Date: 26 Sep 90 17:57:50 GMT
References: <1990Sep26.122859.18935@uvaarpa.Virginia.EDU>
Reply-To: lwall@jpl-devvax.JPL.NASA.GOV (Larry Wall)
Organization: Jet Propulsion Laboratory, Pasadena, CA
Lines: 38

In article <1990Sep26.122859.18935@uvaarpa.Virginia.EDU> telxon!ping!gorpong@uunet.uu.net writes:
: I am writing a perl program which will handle tape copying from remote or
: local devices.  The scheme I use for a local source device is to open up
: the device via:
: 	open(SRC_TAPE, "< $SRC_TAPE");
: I then open up a pipe to write the remote tape (or pipe to read the remote).
: When going remote for the device, I use the 'dd' command to do the reading
: and/or writing.  Enough background, now to the question.
: 
: When copying a tape which is in boot format, the program tells me that it
: has read and written the exact same number of bytes.  When attempting to
: boot this tape, it fails every time, on every tape drive.  When forcing
: the program to use 'dd' to read the tape and write the tape, it also tells
: me that the correct number of bytes have been written.  However, the
: copy made with 'dd' on both ends boots correctly.
: 
: I am reading the tape with the perl read(SRC_TAPE, $_, $blksize); and
: SRC_TAPE is opened either "< device" or "dd if=device |"
: The exact same code works when the open is opening up a 'dd' command to
: read the device.  Is there something which perl is doing to the bytes
: read in from the raw device which it does not do when reading from a pipe?
: 
: I should note that when copying things like 'tar' format tapes, it is still
: in tar format, and it unarchives correctly.  It really screws up boot 
: format tapes.  My question is, why?  It is perl patchlevel 28 on a Sun 3/50
: or 3/anything (or 4/anything) running SunOS 4.1.  Any help you may be able
: to give me would be greatly appreciated.

The problem is that read is actually a call to standard I/O's fread(), and
how many bytes fread() chooses to read has little relation to how many
bytes you said to read.

For this very reason, Perl 4.0 has a sysread() and a syswrite() function.

For now, you can call a real read() using syscall(), if you have it
available.

Larry