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I am not a regular Perl programmer and I could not find anything about this in the forum or few books I have.

I am trying to write binary data to a file using the construct: print filehandle $record

I note that all of my records truncate when an x'0A' is encountered so apparently Perl uses the LF as and end of record indicator. How can I write the complete records, using for example, a length specifier? I am worried about Perl tampering with other binary "non printables" as well.

thanks Fritz

ikegami
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Fritz
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2 Answers2

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You want to use

open(my $fh, '<', $qfn) or die $!;
binmode($fh);

or

open(my $fh, '<:raw', $qfn) or die $!;

to prevent modifications. Same goes for output handles.

This "truncation at 0A" talk makes it sound like you're using readline and expect to do something other than read a line.

Well, actually, it can! You just need to tell readline you want it to read fix width records.

local $/ = \4096;
while (my $rec = <$fh>) {
   ...
}

The other alternative would be to use read.

while (1) {
   my $rv = read($fh, my $rec, 4096);
   die $! if !defined($rv);
   last if !$rv;
   ...
}
ikegami
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  • This is what I needed. I was using a read from an opened serial port "$inrec = " which was stopping at the \r\n bytes. I am rewriting the routine to use the binmode as you mention and read past the \r\n. I can sort out the individual (variable length) records in post processing. Unfortunately I cannot guarantee a fixed record length as the port sometime loses a byte or 2. – Fritz Aug 13 '13 at 22:21
  • @Fritz, `sysread` will read as much as is available and return that. You can accumulate what you read in a buf and check if you have a whole frame. (`my $buf, while (1) { my $rv = sysread($fh, $buf, 64*1024, length($buf)); die if !defined($rv); last if !$rv; ... }`) If that doesn't work, `$ser` might be magical, and it might work if you substitute `read` for `sysread`. – ikegami Aug 14 '13 at 02:09
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Perl is not "tampering" with your writes. If your records are being truncated when they encounter a line feed, then that's a problem with the code that reads them, not the code that writes them. (Unless the format specifies that line feeds must be escaped, in which case the "problem" with the code writing the file is that it doesn't tamper with the data (by escaping line feeds) and instead writes exactly what you tell it to.)

Please provide a small (but runnable) code sample demonstrating your issue, ideally including both reading and writing, along with the actual result and the desired result, and we'll be able to give more specific help.

Note, however, that \n does not map directly to a single data byte (ASCII character) unless you're in binary mode. If the file is being read or written in text mode, \n could be just a CR, just a LF, or a CRLF, depending on the operating system it's being run under.

Dave Sherohman
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  • Perl can indeed "tamper" with the reads and the writes. Two ways: LF⇔CRLF translation, and encoded⇔decoded translation. – ikegami Aug 13 '13 at 13:26