libc/winsup/doc/textbinary.xml

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<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>sed</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, sockets and non-file devices are opened in binary mode.
For pipes opened through the pipe() system call you can use the setmode()
function (see <xref linkend="textbin-devel"></xref> to switch to textmode.
For pipes opened through popen(), you can simply specify text or binary
mode just like in calls to fopen().</para>
</listitem>

<listitem>
<para>Sockets and other non-file devices are always opened in binary mode.
</para>
</listitem>

<listitem>
<para> When redirecting, the Cygwin shells uses rules (a-d).
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>
<para>The programs <command>u2d</command> and <command>d2u</command> can
be used to add or remove CR's from a file.  <command>u2d</command> add's CR's before a NL.
<command>d2u</command> removes CR's.  Use the --help option to these commands
for more information.
</para>
</listitem>
</orderedlist>
</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.  Most programs included in the official
Cygwin distributions should work well in the default mode. </para>

<para>Binmode is the best choice usually since it's faster and
easier to handle, unless you want to exchange files with native Win32
applications.  It makes most sense to keep the Cygwin distribution
and your Cygwin home directory in binmode and generate text files in
binmode (with UNIX LF lineendings).  Most Windows applications can
handle binmode files just fine.  A notable exception is the mini-editor
<command>Notepad</command>, which handles UNIX lineendings incorrectly
and only produces output files with DOS CRLF lineendings.</para>

<para>You can convert files between CRLF and LF lineendings by using
certain tools in the Cygwin distribution like <command>dos2unix</command> and
<command>unix2dos</command> from the dos2unix package.  You can also specify
a directory in the mount table to be mounted in textmode so you can use
that directory for exchange purposes.</para>

<para>As application programmer you can decide on a file by file base,
or you can specify default open modes depending on the purpose for which
the application open files.  See the next section for a description of
your choices.</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>The <function>mkstemp()</function> and <function>mkstemps()</function>
calls force binary mode.  Use <function>mkostemp()</function> or
<function>mkostemps()</function> with the same flags
as <function>open()</function> for more control on temporary files.</para>

<para>In the <function>fopen()</function> and <function>popen()</function>
function calls, 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>

<para>There's also a convenient way to set the default open modes used
in an application by just linking against various object files provided
by Cygwin.  For instance, if you want to make sure that all files are
always opened in binary mode by an application, regardless of the mode
of the underlying mount point, just add the file
<filename>/lib/binmode.o</filename> to the link stage of the application
in your project, like this:</para>

<screen>
  $ gcc my_tiny_app.c /lib/binmode.o -o my_tiny_app
</screen>

<para>Even simpler:</para>

<screen>
  $ gcc my_tiny_app.c -lbinmode -o my_tiny_app
</screen>

<para>This adds code which sets the default open mode for all files
opened by <command>my_tiny_app</command> to binary for reading and
writing.</para>

<para>Cygwin provides the following libraries and object files to set the
default open mode just by linking an application against them:</para>

<itemizedlist mark="bullet">

<listitem>
<screen>
/lib/libautomode.a      -  Open files for reading in textmode,
/lib/automode.o            open files for writing in binary mode
</screen>
</listitem>

<listitem>
<screen>
/lib/libbinmode.a       -  Open files for reading and writing in binary mode
/lib/binmode.o
</screen>
</listitem>

<listitem>
<screen>
/lib/libtextmode.a      -  Open files for reading and writing in textmode
/lib/textmode.o
</screen>
</listitem>

<listitem>
<screen>
/lib/libtextreadmode.a  -  Open files for reading in textmode,
/lib/textreadmode.o        keep default behaviour for writing.
</screen>
</listitem>

</itemizedlist>

</sect2>

</sect1>