ch07_02.htm

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

></TR
><TR
CLASS="row"
VALIGN="TOP"
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
>+&gt; <I
CLASS="filename"
>file</I
></P
></TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
>yes</P
></TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
>yes</P
></TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
>no</P
></TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
>yes</P
></TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
>yes</P
></TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
>RDWR TRUNC CREAT</P
></TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
><CODE
CLASS="literal"
>&quot;w+&quot;</CODE
></P
></TD
></TR
><TR
CLASS="row"
VALIGN="TOP"
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
>+&gt;&gt; <I
CLASS="filename"
>file</I
></P
></TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
>yes</P
></TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
>yes</P
></TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
>yes</P
></TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
>yes</P
></TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
>no</P
></TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
>RDWR APPEND CREAT</P
></TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
><CODE
CLASS="literal"
>&quot;a+&quot; </CODE
></P
></TD
></TR
></TBODY
></TABLE
><P
CLASS="para"
>Here's a tip: you almost never want to use +&gt; or +&gt;&gt;. The first clobbers your file before you can read it, and the second one is confusing because your read pointer can be anywhere, but on many systems, the writer always jumps to the end of the file.</P
><P
CLASS="para"
>The <CODE
CLASS="literal"
>sysopen</CODE
> function takes three or four arguments:</P
><PRE
CLASS="programlisting"
>sysopen(FILEHANDLE, $name, $flags)         or die &quot;Can't open $name : $!&quot;;
sysopen(FILEHANDLE, $name, $flags, $perms) or die &quot;Can't open $name : $!&quot;;</PRE
><P
CLASS="para"
><CODE
CLASS="literal"
>$name</CODE
> is the name of the file, without any &lt; or <CODE
CLASS="literal"
>+</CODE
> funny business. <CODE
CLASS="literal"
>$flags</CODE
> is a number, formed by ORing together separate mode values for <CODE
CLASS="literal"
>O_CREAT</CODE
><A
CLASS="indexterm"
NAME="ch07-idx-1000009586-0"
></A
>, <CODE
CLASS="literal"
>O_WRONLY</CODE
>, <CODE
CLASS="literal"
>O_TRUNC</CODE
>, etc. The exact availability of <CODE
CLASS="literal"
>O_*</CODE
> constants depends on your operating system, so consult the online documentation for this (usually <I
CLASS="filename"
>open </I
>(2), but not always), or look in <EM
CLASS="emphasis"
>/usr/include/fcntl.h</EM
>. Common ones are:</P
><TABLE
CLASS="informaltable"
BORDER="1"
CELLPADDING="3"
><TBODY
CLASS="tbody"
><TR
CLASS="row"
VALIGN="TOP"
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
><CODE
CLASS="literal"
>O_RDONLY</CODE
></P
></TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
>Read only</P
></TD
></TR
><TR
CLASS="row"
VALIGN="TOP"
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
><CODE
CLASS="literal"
>O_WRONLY</CODE
></P
></TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
>Write only</P
></TD
></TR
><TR
CLASS="row"
VALIGN="TOP"
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
><CODE
CLASS="literal"
>O_RDWR</CODE
></P
></TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
>Read and write</P
></TD
></TR
><TR
CLASS="row"
VALIGN="TOP"
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
><CODE
CLASS="literal"
>O_CREAT</CODE
></P
></TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
>Create the file if it doesn't exist</P
></TD
></TR
><TR
CLASS="row"
VALIGN="TOP"
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
><CODE
CLASS="literal"
>O_EXCL</CODE
></P
></TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
>Fail if the file already exists</P
></TD
></TR
><TR
CLASS="row"
VALIGN="TOP"
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
><CODE
CLASS="literal"
>O_APPEND</CODE
></P
></TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
>Append to the file</P
></TD
></TR
><TR
CLASS="row"
VALIGN="TOP"
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
><CODE
CLASS="literal"
>O_TRUNC</CODE
></P
></TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
>Truncate the file</P
></TD
></TR
><TR
CLASS="row"
VALIGN="TOP"
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
><CODE
CLASS="literal"
>O_NONBLOCK</CODE
></P
></TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
>Non-blocking access</P
></TD
></TR
></TBODY
></TABLE
><P
CLASS="para"
>Less common <CODE
CLASS="literal"
>O_*</CODE
> flags sometimes available include <CODE
CLASS="literal"
>O_SHLOCK</CODE
>, <CODE
CLASS="literal"
>O_EXLOCK</CODE
>, <CODE
CLASS="literal"
>O_BINARY</CODE
>, <CODE
CLASS="literal"
>O_NOCTTY</CODE
>, and <CODE
CLASS="literal"
>O_SYNC</CODE
>. Consult your <I
CLASS="filename"
>open </I
>(2) manpage or its local equivalent for details.</P
><P
CLASS="para"
>If you omit the <CODE
CLASS="literal"
>$perms</CODE
> argument to <CODE
CLASS="literal"
>sysopen</CODE
>, Perl uses the octal value <CODE
CLASS="literal"
>0666</CODE
>. These permissions values need to be in octal and are modified by your process's current <CODE
CLASS="literal"
>umask</CODE
>. A <CODE
CLASS="literal"
>umask</CODE
><A
CLASS="indexterm"
NAME="ch07-idx-1000009585-0"
></A
> value is a number representing disabled permissions bits &nbsp;-  if your <CODE
CLASS="literal"
>umask</CODE
> were 027 (group can't write; others can't read, write, or execute), then passing <CODE
CLASS="literal"
>sysopen</CODE
> 0666 would create a file with mode 0640 (mathematically: <CODE
CLASS="literal"
>0666</CODE
> <CODE
CLASS="literal"
>&amp;~</CODE
> <CODE
CLASS="literal"
>027</CODE
> is 0640).</P
><P
CLASS="para"
>If <CODE
CLASS="literal"
>umask</CODE
> seems confusing, here's some advice: supply a creation mode of 0666 for regular files and one of 0777 for directories and executable files. This gives users a choice: if they want protected files, they can choose process umasks of 022, 027, or even the particularly antisocial mask of 077. Programs should rarely if ever make policy decisions better left to the user. One exception is when writing files that should be kept private: mail files, web browser cookies, <EM
CLASS="emphasis"
>.rhosts</EM
> files, and so on. In short, seldom if ever use 0644 as argument to <CODE
CLASS="literal"
>sysopen</CODE
> because that takes away the user's option to have a more permissive umask.</P
><P
CLASS="para"
>Here are examples of <CODE
CLASS="literal"
>open</CODE
> and <CODE
CLASS="literal"
>sysopen</CODE
> in action.</P
><P
CLASS="para"
>To open file for reading:</P
><PRE
CLASS="programlisting"
>open(FH, &quot;&lt; $path&quot;)                                 or die $!;
sysopen(FH, $path, O_RDONLY)                        or die $!;</PRE
><P
CLASS="para"
>To open file for writing, create new file if needed, or else truncate old file:</P
><PRE
CLASS="programlisting"
>open(FH, &quot;&gt; $path&quot;)                                 or die $!;
sysopen(FH, $path, O_WRONLY|O_TRUNC|O_CREAT)        or die $!;
sysopen(FH, $path, O_WRONLY|O_TRUNC|O_CREAT, 0600)  or die $!;</PRE
><P
CLASS="para"
>To open file for writing, create new file, file must not exist:</P
><PRE
CLASS="programlisting"
>sysopen(FH, $path, O_WRONLY|O_EXCL|O_CREAT)         or die $!;
sysopen(FH, $path, O_WRONLY|O_EXCL|O_CREAT, 0600)   or die $!;</PRE
><P
CLASS="para"
>To open file for appending, create if necessary:</P
><PRE
CLASS="programlisting"
>open(FH, &quot;&gt;&gt; $path&quot;)                                or die $!;
sysopen(FH, $path, O_WRONLY|O_APPEND|O_CREAT)       or die $!;
sysopen(FH, $path, O_WRONLY|O_APPEND|O_CREAT, 0600) or die $!;</PRE
><P
CLASS="para"
>To open file for appending, file must exist:</P
><PRE
CLASS="programlisting"
>sysopen(FH, $path, O_WRONLY|O_APPEND)               or die $!;</PRE
><P
CLASS="para"
>To open file for update, file must exist:</P
><PRE
CLASS="programlisting"
>open(FH, &quot;+&lt; $path&quot;)                                or die $!;
sysopen(FH, $path, O_RDWR)                          or die $!;</PRE
><P
CLASS="para"
>To open file for update, create file if necessary:</P
><PRE
CLASS="programlisting"
>sysopen(FH, $path, O_RDWR|O_CREAT)                  or die $!;
sysopen(FH, $path, O_RDWR|O_CREAT, 0600)            or die $!;</PRE
><P
CLASS="para"
>To open file for update, file must not exist:</P
><PRE
CLASS="programlisting"
>sysopen(FH, $path, O_RDWR|O_EXCL|O_CREAT)           or die $!;
sysopen(FH, $path, O_RDWR|O_EXCL|O_CREAT, 0600)     or die $!;<A
CLASS="indexterm"
NAME="ch07-idx-1000009581-0"
></A
></PRE
><P
CLASS="para"
>We demonstrate using a creation mask of 0600 here only to show how to create a private file. The argument is normally omitted.</P
></DIV
><DIV
CLASS="sect2"
><H3
CLASS="sect2"
><A
CLASS="title"
NAME="ch07-pgfId-372"
>See Also</A
></H3
><P
CLASS="para"
>The <CODE
CLASS="literal"
>open</CODE
>, <CODE
CLASS="literal"
>sysopen</CODE
>, and <CODE
CLASS="literal"
>umask</CODE
> functions in <I
CLASS="filename"
>perlfunc </I
>(1) and <A
CLASS="olink"
HREF="../prog/ch03_01.htm"
>Chapter 3</A
> of <A
CLASS="citetitle"
HREF="../prog/index.htm"
TITLE="Programming Perl"
><CITE
CLASS="citetitle"
>Programming Perl</CITE
></A
>; the documentation for the standard IO::File and Fcntl modules (also in <A
CLASS="olink"
HREF="../prog/ch07_01.htm"
>Chapter 7</A
> of <A
CLASS="citetitle"
HREF="../prog/index.htm"
TITLE="Programming Perl"
><CITE
CLASS="citetitle"
>Programming Perl</CITE
></A
>); your system's <I
CLASS="filename"
>open </I
>(2), <I
CLASS="filename"
>fopen </I
>(3), and <I
CLASS="filename"
>umask </I
>(2) manpages; <A
CLASS="xref"
HREF="ch07_03.htm"
TITLE="Opening Files with Unusual Filenames"
>Recipe 7.2</A
></P
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