newlib-cygwin/winsup/doc/textbinary.sgml

<sect1 id="using-textbinary"><title>Text and Binary modes</title>

<sect2 id="textbin-issue"> <title>The Issue</title>

<para>On a UNIX system, when an application reads from a file it gets
exactly what's in the file on disk and the converse is true for writing.
The situation is different in the DOS/Windows world where a file can
be opened in one of two modes, binary or text.  In the binary mode the
system behaves exactly as in UNIX.  However on writing in text mode, a
NL (\n, ^J) is transformed into the sequence CR (\r, ^M) NL.
</para>

<para>This can wreak havoc with the seek/fseek calls since the number
of bytes actually in the file may differ from that seen by the
application.</para>

<para>The mode can be specified explicitly as explained in the Programming
section below.  In an ideal DOS/Windows world, all programs using lines as
records (such as <command>bash</command>, <command>make</command>,
<command>sed</command> ...) would open files (and change the mode of their
standard input and output) as text.  All other programs (such as
<command>cat</command>, <command>cmp</command>, <command>tr</command> ...)
would use binary mode.  In practice with Cygwin, programs that deal
explicitly with object files specify binary mode (this is the case of
<command>od</command>, which is helpful to diagnose CR problems).  Most
other programs (such as <command>cat</command>, <command>cmp</command>,
<command>tr</command>) use the default mode.</para>

</sect2>

<sect2 id="textbin-default"><title>The default Cygwin behavior</title>

<para>The Cygwin system gives us some flexibility in deciding how files 
are to be opened when the mode is not specified explicitly. 
The rules are evolving, this section gives the design goals.</para>
<orderedlist numeration="loweralpha">
<listitem>
<para>If the filename is specified as a POSIX path and it appears to
reside on a file system that is mounted (i.e.  if its pathname starts
with a directory displayed by <command>mount</command>), then the
default is specified by the mount flag.  If the file is a symbolic link,
the mode of the target file system applies.</para>
</listitem>
<listitem>
<para>If the file is specified via a MS-DOS pathname (i.e., it contains a
backslash or a colon), the default is binary.
</para>
</listitem>
<listitem>
<para>Pipes and non-file devices are opened in binary mode,
except if the <envar>CYGWIN</envar> environment variable contains
<literal>nobinmode</literal>.  Sockets are always opened in binary
mode.</para>
</listitem>

<listitem>
<para> When redirecting, the Cygwin shells uses rules (a-e). For
these shells the relevant value of <envar>CYGWIN</envar> is that at the time
the shell was launched and not that at the time the program is executed.
Non-Cygwin shells always pipe and redirect with binary mode. With
non-Cygwin shells the commands <command> cat filename | program </command>
and <command> program &lt; filename </command> are not equivalent when
<filename>filename</filename> is on a text-mounted partition. </para>
</listitem>
</orderedlist>
</sect2>

<sect2 id="textbin-example"><title>Example</title>
<para>To illustrate the various rules, we provide scripts to delete CRs
from files by using the <command>tr</command> program, which can only write
to standard output. 
The script</para>
<screen>
<![CDATA[
#!/bin/sh
# Remove \r from the file given as argument
tr -d '\r' < "$1" > "$1".nocr
]]>
</screen>
<para> will not work on a text mounted systems because the \r will be 
reintroduced on writing. However scripts such as </para>
<screen>
<![CDATA[
#!/bin/sh
# Remove \r from the file given as argument
tr -d '\r' | gzip | gunzip > "$1".nocr
]]>
</screen>
<para>and the .bat file</para>
<screen>
<![CDATA[
REM Remove \r from the file given as argument
@echo off
tr -d \r < %1 > %1.nocr
]]>
</screen>
<para> work fine. In the first case (assuming the pipes are binary)
we rely on <command>gunzip</command> to set its output to binary mode,
possibly overriding the mode used by the shell.
In the second case we rely on the DOS shell to redirect in binary mode.
</para>
</sect2>

<sect2 id="textbin-question"><title>Binary or text?</title>

<para>UNIX programs that have been written for maximum portability
will know the difference between text and binary files and act
appropriately under Cygwin.  For those programs, the text mode default
is a good choice.  Programs included in official Cygwin distributions
should work well in the default mode. </para>

<para>Text mode makes it much easier to mix files between Cygwin and
Windows programs, since Windows programs will usually use the CRLF
format.  Unfortunately you may still have some problems with text
mode.  First, some of the utilities included with Cygwin do not yet
specify binary mode when they should.
Second, you will introduce CRs in text
files you write, which can cause problems when moving them back to a
UNIX system. </para>

<para>If you are mounting a remote file system from a UNIX machine,
or moving files back and forth to a UNIX machine, you may want to
access the files in binary mode. The text files found there will normally
be in UNIX NL format, and you would want any files put there by Cygwin
programs to be stored in a format understood by UNIX.
Be sure to remove CRs from all Makefiles and
shell scripts and make sure that you only edit the files with
DOS/Windows editors that can cope with and preserve NL terminated lines.
</para>

<para>Note that you can decide this on a disk by disk basis (for
example, mounting local disks in text mode and network disks in binary
mode).  You can also partition a disk, for example by mounting
<filename>c:</filename> in text mode, and <filename>c:\home</filename>
in binary mode.</para>

</sect2>

<sect2 id="textbin-devel"><title>Programming</title>

<para>In the <function>open()</function> function call, binary mode can be
specified with the flag <literal>O_BINARY</literal> and text mode with
<literal>O_TEXT</literal>. These symbols are defined in
<filename>fcntl.h</filename>.</para>

<para>In the <function>fopen()</function> function call, binary mode can be
specified by adding a <literal>b</literal> to the mode string. Text mode is specified
by adding a <literal>t</literal> to the mode string.</para>

<para>The mode of a file can be changed by the call
<function>setmode(fd,mode)</function> where <literal>fd</literal> is a file
descriptor (an integer) and <literal>mode</literal> is
<literal>O_BINARY</literal> or <literal>O_TEXT</literal>. The function
returns <literal>O_BINARY</literal> or <literal>O_TEXT</literal> depending
on the mode before the call, and <literal>EOF</literal> on error.</para>

</sect2>

</sect1>