176 lines
7.1 KiB
Plaintext
176 lines
7.1 KiB
Plaintext
<sect1 id="using-textbinary"><title>Text and Binary modes</title>
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<sect2> <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>cat</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><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 file appears to reside on a file system that is mounted
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(i.e. if its pathname starts with a directory displayed by
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<command>mount</command>), then the default is specified by the mount
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flag. If the file is a symbolic link, the mode of the target file system
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applies.</para>
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</listitem>
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<listitem>
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<para>If the file appears to reside on a file system that is not mounted
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(as can happen when the path contains a drive letter), the default is text.
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</para>
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</listitem>
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<listitem>
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<para>Pipes and non-file devices are opened in binary mode,
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except if the <EnVar>CYGWIN</EnVar> environment variable contains
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<literal>nobinmode</literal>.</para>
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<warning><Title>Warning!</Title><para>In b20.1 of 12/98, a file will be opened
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in binary mode if any of the following conditions hold:</para>
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<OrderedList Numeration="arabic" Spacing="Compact">
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<listitem><para>binary mode is specified in the open call</para>
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</listitem>
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<listitem><para><envar>CYGWIN</envar> contains <literal>binmode</literal></para>
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</listitem>
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<listitem><para>the file resides in a binary mounted partition</para>
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</listitem>
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<listitem><para>the file is not a disk file</para>
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</listitem>
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</OrderedList>
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</warning>
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</listitem>
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<listitem>
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<para>When a Cygwin program is launched by a shell, its standard input,
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output and error are in binary mode if the <envar>CYGWIN</envar> variable
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contains <literal>tty</literal>, else in text mode, except if they are piped
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or redirected.</para>
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<para> When redirecting, the Cygwin shells uses rules (a-c). For
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these shells the relevant value of <envar>CYGWIN</envar> is that at the time
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the shell was launched and not that at the time the program is executed.
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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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</listitem>
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</OrderedList>
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</sect2>
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<sect2><title>Example</title>
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<para>To illustrate the various rules, we provide scripts to delete CRs
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from files by using the <command>tr</command> program, which can only write
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to standard output.
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The script</para>
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<screen>
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<![CDATA[
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#!/bin/sh
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# Remove \r from the file given as argument
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tr -d '\r' < "$1" > "$1".nocr
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]]>
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</screen>
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<para> will not work on a text mounted systems because the \r will be
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reintroduced on writing. However scripts such as </para>
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<screen>
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<![CDATA[
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#!/bin/sh
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# Remove \r from the file given as argument
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tr -d '\r' | gzip | gunzip > "$1".nocr
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]]>
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</screen>
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<para>and the .bat file</para>
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<screen>
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<![CDATA[
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REM Remove \r from the file given as argument
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@echo off
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tr -d \r < %1 > %1.nocr
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]]>
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</screen>
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<para> work fine. In the first case (assuming the pipes are binary)
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we rely on <command>gunzip</command> to set its output to binary mode,
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possibly overriding the mode used by the shell.
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In the second case we rely on the DOS shell to redirect in binary mode.
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</para>
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</sect2>
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<sect2><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. For those programs, the text mode default
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is a good choice. Programs included in official Cygwin distributions
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should work well in the default mode. </para>
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<para>Text mode makes it much easier to mix files between Cygwin and
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Windows programs, since Windows programs will usually use the CRLF
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format. Unfortunately you may still have some problems with text
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mode. First, some of the utilities included with Cygwin do not yet
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specify binary mode when they should.
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Second, you will introduce CRs in text
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files you write, which can cause problems when moving them back to a
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UNIX system. </para>
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<para>If you are mounting a remote file system from a UNIX machine,
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or moving files back and forth to a UNIX machine, you may want to
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access the files in binary mode. The text files found there will normally
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be in UNIX NL format, and you would want any files put there by Cygwin
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programs to be stored in a format understood by UNIX.
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Be sure to remove CRs from all Makefiles and
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shell scripts and make sure that you only edit the files with
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DOS/Windows editors that can cope with and preserve NL terminated lines.
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</para>
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<para>Note that you can decide this on a disk by disk basis (for
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example, mounting local disks in text mode and network disks in binary
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mode). You can also partition a disk, for example by mounting
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<filename>c:</filename> in text mode, and <filename>c:\home</filename>
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in binary mode.</para>
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</sect2>
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<sect2><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>In the <function>fopen()</function> function call, binary mode can be
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specified by adding a <literal>b</literal> to the mode string. There is no
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direct way to specify text mode.</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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</sect2>
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</sect1>
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