📄 ch9.htm
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The <TT>open()</TT> fuNCtion has many
variations to let you access files in different ways. Table 9.4
shows all of the different methods used to open a file.<BR>
<P>
<CENTER><B>Table 9.4 The Different Ways to Open a File</B></CENTER>
<p>
<CENTER>
<TABLE BORDERCOLOR=#000000 BORDER=1 WIDTH=80%>
<TR><TD WIDTH=175><I>Open Statement</I></TD><TD WIDTH=415><I>Description</I>
</TD></TR>
<TR><TD WIDTH=175><TT>open</TT>(<TT>FILE_HANDLE</TT>);
</TD><TD WIDTH=415>Opens the file named in <TT>$FILE_HANDLE</TT> and connect to it using <TT>FILE_HANDLE</TT> as the file handle. The file will be opened for input only.
</TD></TR>
<TR><TD WIDTH=175><TT>open</TT>(<TT>FILE_HANDLE</TT>, <TT><I>FILENAME.EXT</I></TT>);
</TD><TD WIDTH=415>Opens the file called <TT><I>FILENAME.EXT</I></TT> for input using <TT>FILE_HANDLE</TT> as the file handle.
</TD></TR>
<TR><TD WIDTH=175><TT>open</TT>(<TT>FILE_HANDLE</TT>, +<TT><I><FILENAME.EXT</I></TT>);
</TD><TD WIDTH=415>Opens <TT><I>FILENAME.EXT</I></TT> for input using <TT>FILE_HANDLE</TT> as the file handle.
</TD></TR>
<TR><TD WIDTH=175><TT>open</TT>(<TT>FILE_HANDLE</TT>, <TT><I>>FILENAME.EXT</I></TT>);
</TD><TD WIDTH=415>Opens <TT><I>FILENAME.EXT</I></TT> for output using <TT>FILE_HANDLE</TT> as the file handle.
</TD></TR>
<TR><TD WIDTH=175><TT>open</TT>(<TT>FILE_HANDLE</TT>, <TT><I>-</I></TT>);
</TD><TD WIDTH=415>Opens standard input.</TD></TR>
<TR><TD WIDTH=175><TT>open</TT>(<TT>FILE_HANDLE</TT>, <TT><I>>-</I></TT>);
</TD><TD WIDTH=415>Opens standard output.</TD></TR>
<TR><TD WIDTH=175><TT>open</TT>(<TT>FILE_HANDLE</TT>, <TT><I>>>FILENAME.EXT</I></TT>);
</TD><TD WIDTH=415>Opens <TT><I>FILENAME.EXT</I></TT> for appending using <TT>FILE_HANDLE</TT> as the file handle.
</TD></TR>
<TR><TD WIDTH=175><TT>open</TT>(<TT>FILE_HANDLE</TT>, <TT><I>+<FILENAME.EXT</I></TT>);
</TD><TD WIDTH=415>Opens <TT><I>FILENAME.EXT</I></TT> for both input and output using <TT>FILE_HANDLE</TT> as the file handle.
</TD></TR>
<TR><TD WIDTH=175><TT>open</TT>(<TT>FILE_HANDLE</TT>, <TT><I>+>FILENAME.EXT</I></TT>);
</TD><TD WIDTH=415>Opens <TT><I>FILENAME.EXT</I></TT> for both input and output using <TT>FILE_HANDLE</TT> as the file handle.
</TD></TR>
<TR><TD WIDTH=175><TT>open</TT>(<TT>FILE_HANDLE</TT>, <TT><I>+>>FILENAME.EXT</I></TT>);
</TD><TD WIDTH=415>Opens <TT><I>FILENAME.EXT</I></TT> for both input and output using <TT>FILE_HANDLE</TT> as the file handle.
</TD></TR>
<TR><TD WIDTH=175><TT>open</TT>(<TT>FILE_HANDLE</TT>, <TT><I>| PROGRAM</I></TT>)
</TD><TD WIDTH=415>Sends the output printed to <TT>FILE_HANDLE</TT> to another program.
</TD></TR>
<TR><TD WIDTH=175><TT>open</TT>(<TT>FILE_HANDLE</TT>, <TT><I>PROGRAM |</I></TT>)
</TD><TD WIDTH=415>Reads the output from another program using <TT>FILE_HANDLE</TT>.
</TD></TR>
</TABLE>
</CENTER>
<P>
<p>
<CENTER>
<TABLE BORDERCOLOR=#000000 BORDER=1 WIDTH=80%>
<TR><TD><B>Note</B></TD></TR>
<TR><TD>
<BLOCKQUOTE>
I am currently researching the differeNCes between +<, +>, and +>>. The research should be available by 12/1/97 as a link from </BLOCKQUOTE>
<BLOCKQUOTE>
<TT>http:\\www.mtolive.com\pbe\index.html.</TT>
</BLOCKQUOTE>
</TD></TR>
</TABLE>
</CENTER>
<P>
<P>
For information about handling failures while opening files, see
<A HREF="ch13.htm" tppabs="http://cheminf.nankai.edu.cn/~eb~/Perl%205%20By%20Example/ch13.htm" >Chapter 13</A>, "Handling Errors and Signals."
<P>
By prefixing the file name with a > character you open the
file for output. This next example opens a file that will hold
a log of messages.
<P>
<IMG SRC="pseudo.gif" tppabs="http://cheminf.nankai.edu.cn/~eb~/Perl%205%20By%20Example/pseudo.gif" BORDER=1 ALIGN=RIGHT><p>
<BLOCKQUOTE>
<I>Call the </I><TT><I>open()</I></TT><I>
fuNCtion to open the </I><TT><I>MESSAGE.LOG</I></TT><I>
file for writing with </I><TT><I>LOGFILE</I></TT><I>
as the file handle. If the open was successful, a true value will
be returned and the statement block will be executed.<BR>
Send the first message to the </I><TT><I>MESSAGE.LOG</I></TT><I>
file using the </I><TT><I>print()</I></TT><I>
fuNCtion. Notice that an alternate method is being used to call
</I><TT><I>print()</I></TT><I>.<BR>
Send the second message to the </I><TT><I>MESSAGE.LOG</I></TT><I>
file.<BR>
Close the file.</I>
</BLOCKQUOTE>
<BLOCKQUOTE>
<PRE>
if (open(LOGFILE, ">message.log")) {
print LOGFILE ("This is message number 1.\n");
print LOGFILE ("This is message number 2.\n");
close(LOGFILE);
}
</PRE>
</BLOCKQUOTE>
<P>
This program displays nothing. Instead, the output from the <TT>print()</TT>
fuNCtion is sent directly to the <TT>MESSAGE.LOG</TT>
file using the connection established by the <TT>open()</TT>
fuNCtion.
<P>
In this example, the <TT>print()</TT>
fuNCtion uses the first parameter as a file handle and the second
parameter as a list of things to print. You can find more information
about printing in the section, "Example: Printing Revisited,"
later in this chapter.
<P>
If you needed to add something to the end of the MESSAGE.LOG file,
you use <TT>>></TT> as the file
name prefix when opening the file. For example:
<P>
<IMG SRC="pseudo.gif" tppabs="http://cheminf.nankai.edu.cn/~eb~/Perl%205%20By%20Example/pseudo.gif" BORDER=1 ALIGN=RIGHT><p>
<BLOCKQUOTE>
<I>Call the </I><TT><I>open()</I></TT><I>
fuNCtion to open the </I><TT><I>MESSAGE.LOG</I></TT><I>
file for appending with </I><TT><I>LOGFILE</I></TT><I>
as the file handle. If the file does not exist, it will be created;
otherwise, anything printed to </I><TT><I>LOGFILE</I></TT><I>
will be added to the end of the file.<BR>
Send a message to the </I><TT><I>MESSAGE.LOG</I></TT><I>
file.<BR>
Send a message to the </I><TT><I>MESSAGE.LOG</I></TT><I>
file.<BR>
Close the file.</I>
</BLOCKQUOTE>
<BLOCKQUOTE>
<PRE>
if (open(LOGFILE, ">>message.log")) {
print LOGFILE ("This is message number 3.\n");
print LOGFILE ("This is message number 4.\n");
close(LOGFILE);
}
</PRE>
</BLOCKQUOTE>
<P>
Now, when <TT>MESSAGE.LOG</TT> is
viewed, it contains the following lines:
<BLOCKQUOTE>
<PRE>
This is message number 1.
This is message number 2.
This is message number 3.
This is message number 4.
</PRE>
</BLOCKQUOTE>
<H3><A NAME="ExampleBinaryFiles">
Example: Binary Files</A></H3>
<P>
When you need to work with data files, you will need to know what
binary mode is. There are two major differeNCes between binary
mode and text mode:
<UL>
<LI>In DOS and Windows, line endings are indicated by two characters-the
newline and carriage return characters. When in text mode, these
characters are input as a single character, the newline character.
In binary mode, both characters can be read by your program. UNIX
systems only use one character, the newline, to indicate line
endings.
<LI>In DOS and Windows, the end of file character is 26. When
a byte with this value is read in text mode, the file is considered
ended and your program cannot read any more information from the
file. UNIX considers the end-of-file character to be 4. For both
operating systems, binary mode will let the end-of-file character
be treated as a regular character.
</UL>
<P>
<p>
<CENTER>
<TABLE BORDERCOLOR=#000000 BORDER=1 WIDTH=80%>
<TR><TD><B>Note</B></TD></TR>
<TR><TD>
<BLOCKQUOTE>
The examples in this section relate to the DOS operating system.</BLOCKQUOTE>
</TD></TR>
</TABLE>
</CENTER>
<P>
<P>
In order to demonstrate these differeNCes, we'll use a data file
called BINARY.DAT with the following contents:
<BLOCKQUOTE>
<PRE>
01
02
03
</PRE>
</BLOCKQUOTE>
<P>
First, we'll read the file in the default text mode.
<P>
<IMG SRC="pseudo.gif" tppabs="http://cheminf.nankai.edu.cn/~eb~/Perl%205%20By%20Example/pseudo.gif" BORDER=1 ALIGN=RIGHT><p>
<BLOCKQUOTE>
<I>Initialize a buffer variable. Both </I><TT><I>read()</I></TT><I>
and </I><TT><I>sysread()</I></TT><I>
need their buffer variables to be initialized before the fuNCtion
call is executed.<BR>
Open the </I><TT><I>BINARY.DAT</I></TT><I>
file for reading.<BR>
Read the first 20 characters of the file using the </I><TT><I>read()</I></TT><I>
fuNCtion.<BR>
Close the file.<BR>
Create an array out of the characters in the </I><TT><I>$buffer</I></TT><I>
variable and iterate over that array using a </I><TT><I>foreach</I></TT><I>
loop.<BR>
Print the value of the current array element in hexadecimal format.
<BR>
Print a newline character. The current array element is a newline
character.</I>
</BLOCKQUOTE>
<HR>
<BLOCKQUOTE>
<B>Listing 9.8 09LST08.PL-Reading a File to Show Text
Mode Line Endings<BR>
</B>
</BLOCKQUOTE>
<BLOCKQUOTE>
<PRE>
$buffer = "";
open(FILE, ">binary.dat");
read(FILE, $buffer, 20, 0);
close(FILE);
foreach (split(//, $buffer)) {
printf("%02x ", ord($_));
print "\n" if $_ eq "\n";
}
</PRE>
</BLOCKQUOTE>
<HR>
<P>
This program displays:
<BLOCKQUOTE>
<PRE>
30 31 0a
30 32 0a
30 33 0a
</PRE>
</BLOCKQUOTE>
<P>
This example does a couple of things that haven't been seen yet
in this book. The <TT>Read()</TT>
fuNCtion is used as an alternative to the line-by-line input done
with the diamond operator. It will read a specified number of
bytes from the input file and assign them to a buffer variable.
The fourth parameter specifies an offset at which to start reading.
In this example, we started at the beginning of the file.
<P>
The <TT>split()</TT> fuNCtion in the
<TT>foreach</TT> loop breaks a string
into pieces and places those pieces into an array. The double
slashes indicate that each character in the string should be an
element of the new array.
<P>
For more information about the <TT>split()</TT>
fuNCtion, see <A HREF="ch5.htm" tppabs="http://cheminf.nankai.edu.cn/~eb~/Perl%205%20By%20Example/ch5.htm" >Chapter 5</A> "FuNCtions," and <A HREF="ch10.htm" tppabs="http://cheminf.nankai.edu.cn/~eb~/Perl%205%20By%20Example/ch10.htm" >Chapter 10</A>,
"Regular Expressions."
<P>
ONCe the array of characters has been created, the <TT>foreach</TT>
loop iterates over the array. The <TT>printf()</TT>
statement converts the ordinal value of the character into hexadecimal
before displaying it. The <I>ordinal</I> value of a character
is the value of the ASCII representation of the character. For
example, the ordinal value of '0' is 0x30 or 48.
<P>
The next line, the print statement, forces the output onto a new
line if the current character is a newline character. This was
done simply to make the output display look a little like the
input file.
<P>
For more information about the <TT>printf()</TT>
fuNCtion, see the section, "Example: Printing Revisited,"
later in this chapter.
<P>
Now, let's read the file in binary mode and see how the output
is changed.
<P>
<IMG SRC="pseudo.gif" tppabs="http://cheminf.nankai.edu.cn/~eb~/Perl%205%20By%20Example/pseudo.gif" BORDER=1 ALIGN=RIGHT><p>
<BLOCKQUOTE>
<I>Initialize a buffer variable.<BR>
Open the </I><TT><I>BINARY.DAT</I></TT><I>
file for reading.<BR>
Change the mode to binary.<BR>
Read the first 20 characters of the file using the </I><TT><I>read()</I></TT><I>
fuNCtion.<BR>
Close the file.<BR>
Create an array out of the characters in the </I><TT><I>$buffer</I></TT><I>
variable and iterate over that array using a </I><TT><I>foreach</I></TT><I>
loop.<BR>
Print the value of the current array element in hexadecimal format.
<BR>
Print a newline character. The current array element is a newline
character.</I>
</BLOCKQUOTE>
<HR>
<BLOCKQUOTE>
<B>Listing 9.9 09LST09.PL-Reading a File to Show Binary
Mode Line Endings<BR>
</B>
</BLOCKQUOTE>
<BLOCKQUOTE>
<PRE>
$buffer = "";
open(FILE, "<binary.dat");
binmode(FILE);
read(FILE, $buffer, 20, 0);
close(FILE);
foreach (split(//, $buffer)) {
printf("%02x ", ord($_));
print "\n" if $_ eq "\n";
}
</PRE>
</BLOCKQUOTE>
<HR>
<P>
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