197 lines
7.6 KiB
XML
197 lines
7.6 KiB
XML
<?xml version="1.0" encoding='UTF-8'?>
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<!DOCTYPE sect1 PUBLIC "-//OASIS//DTD DocBook V4.5//EN"
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"http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd">
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<sect1 id="using-textbinary"><title>Text and Binary modes</title>
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<sect2 id="textbin-issue"> <title>The Issue</title>
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<para>On a UNIX system, when an application reads from a file it gets
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exactly what's in the file on disk and the converse is true for writing.
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The situation is different in the DOS/Windows world where a file can
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be opened in one of two modes, binary or text. In the binary mode the
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system behaves exactly as in UNIX. However on writing in text mode, a
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NL (\n, ^J) is transformed into the sequence CR (\r, ^M) NL.
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</para>
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<para>This can wreak havoc with the seek/fseek calls since the number
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of bytes actually in the file may differ from that seen by the
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application.</para>
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<para>The mode can be specified explicitly as explained in the Programming
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section below. In an ideal DOS/Windows world, all programs using lines as
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records (such as <command>bash</command>, <command>make</command>,
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<command>sed</command> ...) would open files (and change the mode of their
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standard input and output) as text. All other programs (such as
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<command>cat</command>, <command>cmp</command>, <command>tr</command> ...)
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would use binary mode. In practice with Cygwin, programs that deal
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explicitly with object files specify binary mode (this is the case of
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<command>od</command>, which is helpful to diagnose CR problems). Most
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other programs (such as <command>sed</command>, <command>cmp</command>,
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<command>tr</command>) use the default mode.</para>
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</sect2>
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<sect2 id="textbin-default"><title>The default Cygwin behavior</title>
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<para>The Cygwin system gives us some flexibility in deciding how files
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are to be opened when the mode is not specified explicitly.
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The rules are evolving, this section gives the design goals.</para>
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<orderedlist numeration="loweralpha">
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<listitem>
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<para>If the filename is specified as a POSIX path and it appears to
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reside on a file system that is mounted (i.e. if its pathname starts
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with a directory displayed by <command>mount</command>), then the
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default is specified by the mount flag. If the file is a symbolic link,
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the mode of the target file system applies.</para>
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</listitem>
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<listitem>
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<para>If the file is specified via a MS-DOS pathname (i.e., it contains a
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backslash or a colon), the default is binary.
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</para>
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</listitem>
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<listitem>
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<para>Pipes, sockets and non-file devices are opened in binary mode.
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For pipes opened through the pipe() system call you can use the setmode()
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function (see <xref linkend="textbin-devel"></xref> to switch to textmode.
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For pipes opened through popen(), you can simply specify text or binary
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mode just like in calls to fopen().</para>
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</listitem>
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<listitem>
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<para>Sockets and other non-file devices are always opened in binary mode.
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</para>
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</listitem>
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<listitem>
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<para> When redirecting, the Cygwin shells uses rules (a-d).
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Non-Cygwin shells always pipe and redirect with binary mode. With
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non-Cygwin shells the commands <command> cat filename | program </command>
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and <command> program < filename </command> are not equivalent when
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<filename>filename</filename> is on a text-mounted partition. </para>
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<para>The programs <command>u2d</command> and <command>d2u</command> can
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be used to add or remove CR's from a file. <command>u2d</command> add's CR's before a NL.
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<command>d2u</command> removes CR's. Use the --help option to these commands
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for more information.
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</para>
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</listitem>
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</orderedlist>
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</sect2>
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<sect2 id="textbin-question"><title>Binary or text?</title>
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<para>UNIX programs that have been written for maximum portability
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will know the difference between text and binary files and act
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appropriately under Cygwin. Most programs included in the official
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Cygwin distributions should work well in the default mode. </para>
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<para>Binmode is the best choice usually since it's faster and
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easier to handle, unless you want to exchange files with native Win32
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applications. It makes most sense to keep the Cygwin distribution
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and your Cygwin home directory in binmode and generate text files in
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binmode (with UNIX LF lineendings). Most Windows applications can
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handle binmode files just fine. A notable exception is the mini-editor
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<command>Notepad</command>, which handles UNIX lineendings incorrectly
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and only produces output files with DOS CRLF lineendings.</para>
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<para>You can convert files between CRLF and LF lineendings by using
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certain tools in the Cygwin distribution like <command>d2u</command> and
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<command>u2d</command> from the cygutils package. You can also specify
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a directory in the mount table to be mounted in textmode so you can use
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that directory for exchange purposes.</para>
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<para>As application programmer you can decide on a file by file base,
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or you can specify default open modes depending on the purpose for which
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the application open files. See the next section for a description of
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your choices.</para>
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</sect2>
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<sect2 id="textbin-devel"><title>Programming</title>
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<para>In the <function>open()</function> function call, binary mode can be
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specified with the flag <literal>O_BINARY</literal> and text mode with
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<literal>O_TEXT</literal>. These symbols are defined in
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<filename>fcntl.h</filename>.</para>
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<para>The <function>mkstemp()</function> and <function>mkstemps()</function>
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calls force binary mode. Use <function>mkostemp()</function> or
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<function>mkostemps()</function> with the same flags
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as <function>open()</function> for more control on temporary files.</para>
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<para>In the <function>fopen()</function> and <function>popen()</function>
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function calls, binary mode can be specified by adding a <literal>b</literal>
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to the mode string. Text mode is specified by adding a <literal>t</literal>
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to the mode string.</para>
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<para>The mode of a file can be changed by the call
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<function>setmode(fd,mode)</function> where <literal>fd</literal> is a file
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descriptor (an integer) and <literal>mode</literal> is
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<literal>O_BINARY</literal> or <literal>O_TEXT</literal>. The function
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returns <literal>O_BINARY</literal> or <literal>O_TEXT</literal> depending
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on the mode before the call, and <literal>EOF</literal> on error.</para>
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<para>There's also a convenient way to set the default open modes used
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in an application by just linking against various object files provided
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by Cygwin. For instance, if you want to make sure that all files are
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always opened in binary mode by an application, regardless of the mode
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of the underlying mount point, just add the file
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<filename>/lib/binmode.o</filename> to the link stage of the application
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in your project, like this:</para>
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<screen>
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$ gcc my_tiny_app.c /lib/binmode.o -o my_tiny_app
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</screen>
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<para>Even simpler:</para>
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<screen>
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$ gcc my_tiny_app.c -lbinmode -o my_tiny_app
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</screen>
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<para>This adds code which sets the default open mode for all files
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opened by <command>my_tiny_app</command> to binary for reading and
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writing.</para>
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<para>Cygwin provides the following libraries and object files to set the
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default open mode just by linking an application against them:</para>
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<itemizedlist mark="bullet">
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<listitem>
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<screen>
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/lib/libautomode.a - Open files for reading in textmode,
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/lib/automode.o open files for writing in binary mode
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</screen>
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</listitem>
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<listitem>
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<screen>
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/lib/libbinmode.a - Open files for reading and writing in binary mode
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/lib/binmode.o
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</screen>
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</listitem>
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<listitem>
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<screen>
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/lib/libtextmode.a - Open files for reading and writing in textmode
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/lib/textmode.o
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</screen>
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</listitem>
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<listitem>
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<screen>
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/lib/libtextreadmode.a - Open files for reading in textmode,
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/lib/textreadmode.o keep default behaviour for writing.
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</screen>
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</listitem>
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</itemizedlist>
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</sect2>
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</sect1>
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