ch07_01.htm

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

CLASS="indexterm"
NAME="ch07-idx-1000009573-4"
></A
>access mode. To open <EM
CLASS="emphasis"
>/tmp/log</EM
> for writing and to associate it with the filehandle <CODE
CLASS="literal"
>LOGFILE</CODE
>, say:</P
><PRE
CLASS="programlisting"
>open(LOGFILE, &quot;&gt; /tmp/log&quot;)     or die &quot;Can't write /tmp/log: $!&quot;;</PRE
><P
CLASS="para"
>The three most common access modes are &lt; for reading, &gt; for overwriting, and &gt;&gt; for appending. The <CODE
CLASS="literal"
>open</CODE
> function is discussed in more detail in <A
CLASS="xref"
HREF="ch07_02.htm"
TITLE="Opening a File"
>Recipe 7.1</A
>.</P
><P
CLASS="para"
>When opening a file or making virtually any other system call,[<A
CLASS="footnote"
HREF="#ch07-pgfId-1000008479"
>1</A
>] checking the return value is indispensable. Not every <CODE
CLASS="literal"
>open</CODE
> succeeds; not every file is readable; not every piece of data you <CODE
CLASS="literal"
>print</CODE
> can reach its destination. Most programmers check <CODE
CLASS="literal"
>open</CODE
>, <CODE
CLASS="literal"
>seek</CODE
>, <CODE
CLASS="literal"
>tell</CODE
>, and <CODE
CLASS="literal"
>close</CODE
> in robust programs. You might also want to check other functions. The Perl documentation lists return values from all functions and operators. If a system call fails, it returns <CODE
CLASS="literal"
>undef</CODE
>, except for <CODE
CLASS="literal"
>wait</CODE
>, <CODE
CLASS="literal"
>waitpid </CODE
>, and <CODE
CLASS="literal"
>syscall</CODE
>, which return <CODE
CLASS="literal"
>-1</CODE
> on failure. The system error message or number is available in the <CODE
CLASS="literal"
>$!</CODE
> variable. This is often used in <CODE
CLASS="literal"
>die</CODE
> or <CODE
CLASS="literal"
>warn</CODE
> messages.</P
><BLOCKQUOTE
CLASS="footnote"
><DIV
CLASS="footnote"
><P
CLASS="para"
><A
CLASS="footnote"
NAME="ch07-pgfId-1000008479"
>[1]</A
> The term <EM
CLASS="emphasis"
>system call</EM
> denotes a call into your operating system. It is unrelated to the C and Perl function that's actually named <CODE
CLASS="literal"
>system</CODE
>.</P
></DIV
></BLOCKQUOTE
><P
CLASS="para"
>To read a record in Perl, use the <A
CLASS="indexterm"
NAME="ch07-idx-1000009574-0"
></A
><A
CLASS="indexterm"
NAME="ch07-idx-1000009574-1"
></A
>circumfix operator <CODE
CLASS="literal"
>&lt;FILEHANDLE&gt;</CODE
>, whose behavior is also available through the <CODE
CLASS="literal"
>readline</CODE
> function. A record is normally a line, but you can change the record terminator, as detailed in <A
CLASS="xref"
HREF="ch08_01.htm"
TITLE="File Contents"
>Chapter 8</A
>. If <CODE
CLASS="literal"
>FILEHANDLE</CODE
> is omitted, Perl opens and reads from the filenames in <CODE
CLASS="literal"
>@ARGV</CODE
> or from STDIN if there aren't any. Customary and curious uses of this are described in <A
CLASS="xref"
HREF="ch07_08.htm"
TITLE="Writing a Filter"
>Recipe 7.7</A
>.</P
><P
CLASS="para"
>Abstractly, files are simply streams of bytes. Each filehandle has associated with it a number representing the current byte position in the file, returned by the <CODE
CLASS="literal"
>tell</CODE
> function and set by the <CODE
CLASS="literal"
>seek</CODE
> function. In <A
CLASS="xref"
HREF="ch07_11.htm"
TITLE="Modifying a File in Place Without a Temporary File"
>Recipe 7.10</A
>, we rewrite a file without closing and reopening by using <CODE
CLASS="literal"
>seek</CODE
> to move back to the start, rewinding it.</P
><P
CLASS="para"
>When you no longer have use for a filehandle, <CODE
CLASS="literal"
>close</CODE
> it. The <CODE
CLASS="literal"
>close</CODE
> function takes a single filehandle and returns true if the filehandle could be successfully flushed and closed, false otherwise. You don't need to explicitly close every filehandle. When you open a filehandle that's already open, Perl implicitly closes it first. When your program exits, any open filehandles also get closed.</P
><P
CLASS="para"
>These implicit closes are for convenience, not stability, because they don't tell you whether the system call succeeded or failed. Not all closes succeed. Even a <CODE
CLASS="literal"
>close</CODE
> on a read-only file can fail. For instance, you could lose access to the device because of a network outage. It's even more important to check the <CODE
CLASS="literal"
>close</CODE
> if the file was opened for writing. Otherwise you wouldn't notice if the disk filled up.</P
><PRE
CLASS="programlisting"
>close(FH)           or die &quot;FH didn't close: $!&quot;;</PRE
><P
CLASS="para"
>The prudent programmer even checks the <CODE
CLASS="literal"
>close</CODE
> on standard output stream at the program's end, in case STDOUT was redirected from the command line the output filesystem filled up. Admittedly, your run-time system should take care of this for you, but it doesn't. <A
CLASS="indexterm"
NAME="ch07-idx-1000009572-0"
></A
><A
CLASS="indexterm"
NAME="ch07-idx-1000009572-1"
></A
></P
><P
CLASS="para"
>Checking standard error, though, is probably of dubious value. After all, if STDERR fails to close, what are you planning to do about it?</P
><P
CLASS="para"
><A
CLASS="indexterm"
NAME="ch07-idx-1000009575-0"
></A
>STDOUT is the default destination for output from the <CODE
CLASS="literal"
>print</CODE
>, <CODE
CLASS="literal"
>printf</CODE
>, and <CODE
CLASS="literal"
>write</CODE
> functions. Change this with <CODE
CLASS="literal"
>select</CODE
><A
CLASS="indexterm"
NAME="ch07-idx-1000009576-0"
></A
>, which takes the new default output filehandle and returns the previous one. The new output filehandle should have been opened before calling <CODE
CLASS="literal"
>select</CODE
>:</P
><PRE
CLASS="programlisting"
>$old_fh = select(LOGFILE);                  # switch to LOGFILE for output
print &quot;Countdown initiated ...\n&quot;;
select($old_fh);                            # return to original output
print &quot;You have 30 seconds to reach minimum safety distance.\n&quot;;</PRE
><P
CLASS="para"
>Some of Perl's special variables change the behavior of the currently selected output filehandle. Most important is <CODE
CLASS="literal"
>$|</CODE
><A
CLASS="indexterm"
NAME="ch07-idx-1000009577-0"
></A
>, which controls output buffering for each filehandle. Buffering is explained in <A
CLASS="xref"
HREF="ch07_13.htm"
TITLE="Flushing Output"
>Recipe 7.12</A
>.</P
><P
CLASS="para"
><A
CLASS="indexterm"
NAME="ch07-idx-1000009578-0"
></A
><A
CLASS="indexterm"
NAME="ch07-idx-1000009578-1"
></A
><A
CLASS="indexterm"
NAME="ch07-idx-1000009578-2"
></A
>Perl provides functions for buffered and unbuffered input and output. Although there are some exceptions, you shouldn't mix calls to buffered and unbuffered I/O functions. The following table shows the two sets of functions you should not mix. Functions on a particular row are only loosely associated; for instance, <CODE
CLASS="literal"
>sysread</CODE
> doesn't have the same semantics as <CODE
CLASS="literal"
>&lt; &gt;</CODE
>, but they are on the same row because they both read input from a filehandle.</P
><TABLE
CLASS="informaltable"
BORDER="1"
CELLPADDING="3"
><THEAD
CLASS="thead"
><TR
CLASS="row"
VALIGN="TOP"
><TH
CLASS="entry"
ALIGN="LEFT"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
><B
CLASS="emphasis.bold"
>Action</B
></P
></TH
><TH
CLASS="entry"
ALIGN="LEFT"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
>Buffered</P
></TH
><TH
CLASS="entry"
ALIGN="LEFT"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
>Unbuffered</P
></TH
></TR
></THEAD
><TBODY
CLASS="tbody"
><TR
CLASS="row"
VALIGN="TOP"
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
>opening</P
></TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
><CODE
CLASS="literal"
>open,sysopen</CODE
></P
></TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
><CODE
CLASS="literal"
>sysopen </CODE
></P
></TD
></TR
><TR
CLASS="row"
VALIGN="TOP"
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
>closing</P
></TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
><CODE
CLASS="literal"
>close</CODE
></P
></TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
><CODE
CLASS="literal"
>close </CODE
></P
></TD
></TR
><TR
CLASS="row"
VALIGN="TOP"
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
>input</P
></TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
><CODE
CLASS="literal"
>&lt;FILE&gt;,readline</CODE
></P
></TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
><CODE
CLASS="literal"
>sysread </CODE
></P
></TD
></TR
><TR
CLASS="row"
VALIGN="TOP"
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
>output</P
></TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
><CODE
CLASS="literal"
>print</CODE
></P
></TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
><CODE
CLASS="literal"
>syswrite </CODE
></P
></TD
></TR
><TR
CLASS="row"
VALIGN="TOP"
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
>repositioning</P
></TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
><CODE
CLASS="literal"
>seek, tell</CODE
></P
></TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
><CODE
CLASS="literal"
>sysseek </CODE
></P
></TD
></TR
></TBODY
></TABLE
><P
CLASS="para"
>Repositioning is addressed in <A
CLASS="xref"
HREF="ch08_01.htm"
TITLE="File Contents"
>Chapter 8</A
>, but we also use it in <A
CLASS="xref"
HREF="ch07_11.htm"
TITLE="Modifying a File in Place Without a Temporary File"
>Recipe 7.10</A
>.<A
CLASS="indexterm"
NAME="ch07-idx-1000009568-0"
></A
></P
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