ch08_01.htm

By Tom Christiansen and Nathan Torkington ISBN 1-56592-243-3 First Edition, published August 1998

>-00</B
>, Perl will set <CODE
CLASS="literal"
>$/</CODE
> to <CODE
CLASS="literal"
>&quot;&quot;</CODE
>. The limit of a single octal value means you can't set <CODE
CLASS="literal"
>$/</CODE
> to a multibyte string, for instance, <CODE
CLASS="literal"
>&quot;%%\n&quot;</CODE
> to read <EM
CLASS="emphasis"
>fortune</EM
> files. Instead, you must use a <CODE
CLASS="literal"
>BEGIN</CODE
> block:</P
><PRE
CLASS="programlisting"
>% perl -ne 'BEGIN { $/=&quot;%%\n&quot; } chomp; print if /Unix/i' fortune.dat</PRE
><P
CLASS="para"
>Use <CODE
CLASS="literal"
>print</CODE
> to write a line or any other data. The <CODE
CLASS="literal"
>print</CODE
> function writes its arguments one after another and doesn't automatically add a line or record terminator by default.</P
><PRE
CLASS="programlisting"
>print HANDLE &quot;One&quot;, &quot;two&quot;, &quot;three&quot;; # &quot;Onetwothree&quot;
print &quot;Baa baa black sheep.\n&quot;;     # Sent to default output handle</PRE
><P
CLASS="para"
>There is no comma between the filehandle and the data to print. If you put a comma in there, Perl gives the error message <CODE
CLASS="literal"
>&quot;No</CODE
> <CODE
CLASS="literal"
>comma</CODE
> <CODE
CLASS="literal"
>allowed</CODE
> <CODE
CLASS="literal"
>after</CODE
> <CODE
CLASS="literal"
>filehandle&quot;</CODE
>. The default output handle is STDOUT. Change it with the <CODE
CLASS="literal"
>select</CODE
> function. (See the introduction to <A
CLASS="xref"
HREF="ch07_01.htm"
TITLE="File Access"
>Chapter 7, <CITE
CLASS="chapter"
>File Access</CITE
></A
>.)</P
><P
CLASS="para"
>All systems use the virtual <CODE
CLASS="literal"
>&quot;\n&quot;</CODE
> to represent a line terminator, called a <EM
CLASS="emphasis"
>newline</EM
>. There is no such thing as a newline character. It is an illusion that the operating system, device drivers, C libraries, and Perl all conspire to preserve. Sometimes, this changes the number of characters in the strings you read and write. The conspiracy is revealed in <A
CLASS="xref"
HREF="ch08_12.htm"
TITLE="Processing Binary Files"
>Recipe 8.11</A
>.</P
><P
CLASS="para"
>Use the <CODE
CLASS="literal"
>read</CODE
><A
CLASS="indexterm"
NAME="ch08-idx-1000004606-0"
></A
> function to read a <A
CLASS="indexterm"
NAME="ch08-idx-1000004763-0"
></A
><A
CLASS="indexterm"
NAME="ch08-idx-1000004763-1"
></A
><A
CLASS="indexterm"
NAME="ch08-idx-1000004763-2"
></A
>fixed-length record. It takes three arguments: a filehandle, a scalar variable, and the number of bytes to read. It returns <CODE
CLASS="literal"
>undef</CODE
> if an error occurred or else the number of bytes read. To write a fixed-length record, just use <CODE
CLASS="literal"
>print</CODE
>.</P
><PRE
CLASS="programlisting"
>$rv = read(HANDLE, $buffer, 4096)
        or die &quot;Couldn't read from HANDLE : $!\n&quot;;
# $rv is the number of bytes read,
# $buffer holds the data read</PRE
><P
CLASS="para"
>The <CODE
CLASS="literal"
>truncate</CODE
><A
CLASS="indexterm"
NAME="ch08-idx-1000004607-0"
></A
> function changes the length of a file, which can be specified as a filehandle or as a filename. It returns true if the file was successfully truncated, false otherwise:</P
><PRE
CLASS="programlisting"
>truncate(HANDLE, $length)
    or die &quot;Couldn't truncate: $!\n&quot;;
truncate(&quot;/tmp/$$.pid&quot;, $length)
    or die &quot;Couldn't truncate: $!\n&quot;;</PRE
><P
CLASS="para"
>Each filehandle keeps track of where it is in the file. Reads and writes occur from this point, unless you've specified the <CODE
CLASS="literal"
>O_APPEND</CODE
> flag (see <A
CLASS="xref"
HREF="ch07_02.htm"
TITLE="Opening a File"
>Recipe 7.1</A
>). Fetch the file position for a filehandle with <CODE
CLASS="literal"
>tell</CODE
><A
CLASS="indexterm"
NAME="ch08-idx-1000004608-0"
></A
><A
CLASS="indexterm"
NAME="ch08-idx-1000004608-1"
></A
>, and set it with <CODE
CLASS="literal"
>seek</CODE
>. Because the stdio library rewrites data to preserve the illusion that <CODE
CLASS="literal"
>&quot;\n&quot;</CODE
> is the line terminator, you cannot portably <CODE
CLASS="literal"
>seek</CODE
> to offsets calculated by counting characters. Instead, only <CODE
CLASS="literal"
>seek</CODE
> to offsets returned by <CODE
CLASS="literal"
>tell</CODE
>.</P
><PRE
CLASS="programlisting"
>$pos = tell(DATAFILE);
print &quot;I'm $pos bytes from the start of DATAFILE.\n&quot;;</PRE
><P
CLASS="para"
>The <CODE
CLASS="literal"
>seek</CODE
> function takes three arguments: the filehandle, the offset (in bytes) to go to, and a numeric argument indicating how to interpret the offset. 0 indicates an offset from the start of the file (the kind of value returned by <CODE
CLASS="literal"
>tell</CODE
>); 1, an offset from the current location (a negative number means move backwards in the file, a positive number means move forward); and 2, an offset from end of file.</P
><PRE
CLASS="programlisting"
>seek(LOGFILE, 0, 2)         or die &quot;Couldn't seek to the end: $!\n&quot;;
seek(DATAFILE, $pos, 0)     or die &quot;Couldn't seek to $pos: $!\n&quot;;
seek(OUT, -20, 1)           or die &quot;Couldn't seek back 20 bytes: $!\n&quot;;</PRE
><P
CLASS="para"
><A
CLASS="indexterm"
NAME="ch08-idx-1000004609-0"
></A
><A
CLASS="indexterm"
NAME="ch08-idx-1000004609-1"
></A
><A
CLASS="indexterm"
NAME="ch08-idx-1000004609-2"
></A
>So far we've been describing buffered I/O. That is, <CODE
CLASS="literal"
>&lt;FH&gt;</CODE
>, <CODE
CLASS="literal"
>print</CODE
>, <CODE
CLASS="literal"
>read</CODE
>, <CODE
CLASS="literal"
>seek</CODE
>, and <CODE
CLASS="literal"
>tell</CODE
> are all operations that use buffers for speed. Perl also provides unbuffered I/O operations: <CODE
CLASS="literal"
>sysread</CODE
>, <CODE
CLASS="literal"
>syswrite</CODE
>, and <CODE
CLASS="literal"
>sysseek</CODE
>, all discussed in <A
CLASS="xref"
HREF="ch07_01.htm"
TITLE="File Access"
>Chapter 7</A
>.</P
><P
CLASS="para"
>The <CODE
CLASS="literal"
>sysread</CODE
><A
CLASS="indexterm"
NAME="ch08-idx-1000004610-0"
></A
><A
CLASS="indexterm"
NAME="ch08-idx-1000004610-1"
></A
> and <CODE
CLASS="literal"
>syswrite</CODE
> functions are different from their <CODE
CLASS="literal"
>&lt;FH&gt;</CODE
> and <CODE
CLASS="literal"
>print</CODE
> counterparts. They both take a filehandle to act on, a scalar variable to either read into or write out from, and the number of bytes to read or write. They can also take an optional fourth argument, the offset in the scalar variable to start reading or writing at:</P
><PRE
CLASS="programlisting"
>$written = syswrite(DATAFILE, $mystring, length($mystring));
die &quot;syswrite failed: $!\n&quot; unless $written == length($mystring);
$read = sysread(INFILE, $block, 256, 5);
warn &quot;only read $read bytes, not 256&quot; if 256 != $read;</PRE
><P
CLASS="para"
>The <CODE
CLASS="literal"
>syswrite</CODE
> call sends the contents of <CODE
CLASS="literal"
>$mystring</CODE
> to <CODE
CLASS="literal"
>DATAFILE</CODE
>. The <CODE
CLASS="literal"
>sysread</CODE
> call reads 256 bytes from <CODE
CLASS="literal"
>INFILE</CODE
> and stores them 5 characters into <CODE
CLASS="literal"
>$block</CODE
>, leaving its first 5 characters intact. Both <CODE
CLASS="literal"
>sysread</CODE
> and <CODE
CLASS="literal"
>syswrite</CODE
> return the number of bytes transferred, which could be different than the amount of data you were attempting to transfer. Maybe the file didn't have all the data you thought it did, so you got a short read. Maybe the filesystem that the file lives on filled up. Maybe your process was interrupted part of the way through the write. Stdio takes care of finishing the transfer in cases of interruption, but if you use the <CODE
CLASS="literal"
>sysread</CODE
> and <CODE
CLASS="literal"
>syswrite</CODE
> calls, you must do it yourself. See <A
CLASS="xref"
HREF="ch09_04.htm"
TITLE="Copying or Moving a File"
>Recipe 9.3</A
> for an example of this.</P
><P
CLASS="para"
>The <CODE
CLASS="literal"
>sysseek</CODE
><A
CLASS="indexterm"
NAME="ch08-idx-1000004611-0"
></A
> function doubles as an unbuffered replacement for both <CODE
CLASS="literal"
>seek</CODE
> and <CODE
CLASS="literal"
>tell</CODE
>. It takes the same arguments as <CODE
CLASS="literal"
>seek</CODE
>, but it returns either the new position if successful or <CODE
CLASS="literal"
>undef</CODE
> on error. To find the current position within the file:</P
><PRE
CLASS="programlisting"
>$pos = sysseek(HANDLE, 0, 1);       # don't change position
die &quot;Couldn't sysseek: $!\n&quot; unless defined $pos;</PRE
><P
CLASS="para"
>These are the basic operations available to you. The art and craft of programming lies in using these basic operations to solve complex problems like finding the number of lines in a file, reversing the order of lines in a file, randomly selecting a line from a file, building an index for a file, and so on.</P
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