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<H2><A NAME="Controlling Streams">Controlling Streams</A></H2>

<P><A HREF="stdio.html#fopen"><CODE>fopen</CODE></A>
returns the address of an object of type
<A HREF="stdio.html#FILE"><CODE>FILE</CODE></A>.
You use this address as the <CODE>stream</CODE> argument to several library
functions to perform various operations on an open file. For a byte
stream, all input takes place as if each character is read by calling
<A HREF="stdio.html#fgetc"><CODE>fgetc</CODE></A>,
and all output takes place as if each character is written by calling
<A HREF="stdio.html#fputc"><CODE>fputc</CODE></A>.
For a wide stream (with
<A HREF="lib_over.html#Amendment 1">Amendment 1</A>),
all input takes place as if each character is read by calling
<A HREF="wchar.html#fgetwc"><CODE>fgetwc</CODE></A>,
and all output takes place as if each character is written by calling
<A HREF="wchar.html#fputwc"><CODE>fputwc</CODE></A>.</P>

<P>You can
<B><A NAME="file close">close</A></B> a file by calling
<A HREF="stdio.html#fclose"><CODE>fclose</CODE></A>,
after which the address of the
<A HREF="stdio.html#FILE"><CODE>FILE</CODE></A> object is invalid.</P>

<P>A <A HREF="stdio.html#FILE"><CODE>FILE</CODE></A>
object stores the state of a stream, including:</P>

<UL>
<LI>an <B><A NAME="error indicator">error indicator</A></B> --
set nonzero by a function that encounters a read or write error</LI>

<LI>an <B><A NAME="end-of-file indicator">end-of-file indicator</A></B> --
set nonzero by a function that
encounters the end of the file while reading</LI>

<LI>a <B><A NAME="file-position indicator">file-position indicator</A></B> --
specifies the next byte in the stream to read or write,
if the file can support positioning requests</LI>

<LI>a <A HREF="#Stream States">stream state</A> --
specifies whether the stream will accept reads and/or writes and, with
<A HREF="lib_over.html#Amendment 1">Amendment 1</A>, whether the stream is
<A HREF="#unbound stream">unbound</A>,
<A HREF="#byte oriented">byte oriented</A>, or
<A HREF="#wide oriented">wide oriented</A></LI>

<LI>a <A HREF="charset.html#conversion state">conversion state</A> --
remembers the state of any partly assembled or generated
<A HREF="#generalized multibyte characters">
generalized multibyte character</A>, as well as
any shift state for the sequence of bytes in the file)</LI>

<LI>a <B><A NAME="file buffer">file buffer</A></B> --
specifies the address and size of an array object
that library functions can use to improve the performance
of read and write operations to the stream</LI>
</UL>

<P>Do not alter any value stored in a
<A HREF="stdio.html#FILE"><CODE>FILE</CODE></A> object or in
a file buffer that you specify for use with that object.
You cannot copy a
<A HREF="stdio.html#FILE"><CODE>FILE</CODE></A> object
and portably use the address of the copy
as a <CODE>stream</CODE> argument to a library function.</P>

<H2><A NAME="Stream States">Stream States</A></H2>

<P>The valid states, and state transitions, for a stream are
shown in the diagram.</P>

<P><IMG SRC="stream.gif"></P>

<P>Each of the circles denotes a stable
state. Each of the lines denotes a transition that can occur as the
result of a function call that operates on the stream. Five groups
of functions can cause state transitions.</P>

<P>Functions in the first three groups are declared in
<A HREF="stdio.html"><CODE>&lt;stdio.h&gt;</CODE></A>:</P>

<UL>
<LI>the <B><A NAME="byte read functions">byte read functions</A></B> --
<A HREF="stdio.html#fgetc"><CODE>fgetc</CODE></A>,
<A HREF="stdio.html#fgets"><CODE>fgets</CODE></A>,
<A HREF="stdio.html#fread"><CODE>fread</CODE></A>,
<A HREF="stdio.html#fscanf"><CODE>fscanf</CODE></A>,
<A HREF="stdio.html#getc"><CODE>getc</CODE></A>,
<A HREF="stdio.html#getchar"><CODE>getchar</CODE></A>,
<A HREF="stdio.html#gets"><CODE>gets</CODE></A>,
<A HREF="stdio.html#scanf"><CODE>scanf</CODE></A>,
and
<A HREF="stdio.html#ungetc"><CODE>ungetc</CODE></A></LI>

<LI>the <B><A NAME="byte write functions">byte write functions</A></B> --
<A HREF="stdio.html#fprintf"><CODE>fprintf</CODE></A>,
<A HREF="stdio.html#fputc"><CODE>fputc</CODE></A>,
<A HREF="stdio.html#fputs"><CODE>fputs</CODE></A>,
<A HREF="stdio.html#fwrite"><CODE>fwrite</CODE></A>,
<A HREF="stdio.html#printf"><CODE>printf</CODE></A>,
<A HREF="stdio.html#putc"><CODE>putc</CODE></A>,
<A HREF="stdio.html#putchar"><CODE>putchar</CODE></A>,
<A HREF="stdio.html#puts"><CODE>puts</CODE></A>,
<A HREF="stdio.html#vfprintf"><CODE>vfprintf</CODE></A>, and
<A HREF="stdio.html#vprintf"><CODE>vprintf</CODE></A></LI>

<LI>the <B><A NAME="position functions">position functions</A></B> --
<A HREF="stdio.html#fflush"><CODE>fflush</CODE></A>,
<A HREF="stdio.html#fseek"><CODE>fseek</CODE></A>,
<A HREF="stdio.html#fsetpos"><CODE>fsetpos</CODE></A>, and
<A HREF="stdio.html#rewind"><CODE>rewind</CODE></A></LI>
</UL>

<P>Functions in the remaining two groups are declared
in <CODE>&lt;wchar.h&gt;</CODE>:</P>

<UL>
<LI>the <B><A NAME="wide read functions">wide read functions</A></B> --
<A HREF="wchar.html#fgetwc"><CODE>fgetwc</CODE></A>,
<A HREF="wchar.html#fgetws"><CODE>fgetws</CODE></A>,
<A HREF="wchar.html#fwscanf"><CODE>fwscanf</CODE></A>,
<A HREF="wchar.html#getwc"><CODE>getwc</CODE></A>,
<A HREF="wchar.html#getwchar"><CODE>getwchar</CODE></A>,
<A HREF="wchar.html#ungetwc"><CODE>ungetwc</CODE></A>,
and
<A HREF="wchar.html#wscanf"><CODE>wscanf</CODE></A></LI>

<LI>the <B><A NAME="wide write functions">wide write functions</A></B> --
<A HREF="wchar.html#fwprintf"><CODE>fwprintf</CODE></A>,
<A HREF="wchar.html#fputwc"><CODE>fputwc</CODE></A>,
<A HREF="wchar.html#fputws"><CODE>fputws</CODE></A>,
<A HREF="wchar.html#putwc"><CODE>putwc</CODE></A>,
<A HREF="wchar.html#putwchar"><CODE>putwchar</CODE></A>,
<A HREF="wchar.html#vfwprintf"><CODE>vfwprintf</CODE></A>,
<A HREF="wchar.html#vwprintf"><CODE>vwprintf</CODE></A>, and
<A HREF="wchar.html#wprintf"><CODE>wprintf</CODE></A>,</LI>
</UL>

<P>For the stream <CODE>s</CODE>, the call
<CODE><A HREF="wchar.html#fwide">fwide</A>(s, 0)</CODE>
is always valid and never causes a change of state. Any other call to
<A HREF="wchar.html#fwide"><CODE>fwide</CODE></A>, or to any of the five
groups of functions described above, causes the state transition shown
in the state diagram. If no such transition is shown, the function
call is invalid.</P>

<P>The state diagram shows how to establish the orientation of
a stream:</P>

<UL>
<LI>The call
<CODE><A HREF="wchar.html#fwide">fwide</A>(s, -1)</CODE>,
or to a byte read or byte write function, establishes the stream as
<A HREF="#byte oriented">byte oriented</A>.</LI>

<LI>The call
<CODE><A HREF="wchar.html#fwide">fwide</A>(s, 1)</CODE>,
or to a wide read or wide write function, establishes the stream as
<A HREF="#wide oriented">wide oriented</A>.</LI>
</UL>

<P>The state diagram shows that you must call one of the position
functions between most write and read operations:</P>

<UL>
<LI>You cannot call a read function if the last operation on the
stream was a write.</LI>

<LI>You cannot call a write function if the last operation on the
stream was a read, unless that read operation set the
<A HREF="#end-of-file indicator">end-of-file indicator</A>.</LI>
</UL>

<P>Finally, the state diagram shows that a position operation never
<I>decreases</I> the number of valid function calls that can follow.</P>

<HR>
<P>See also the
<B><A HREF="index.html#Table of Contents">Table of Contents</A></B> and the
<B><A HREF="_index.html">Index</A></B>.</P>

<P><I>
<A HREF="crit_pb.html">Copyright</A> &#169; 1989-2002
by P.J. Plauger and Jim Brodie. All rights reserved.</I></P>

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