xsh_chap02_05.html

posix标准英文,html格式

<p>was the most recent operation on that stream), no action need be taken.</p>
</li>

<li>
<p>If it is a stream which is open for writing or appending (but not also open for reading), the application shall either perform
an <a href="../functions/fflush.html"><i>fflush</i>()</a>, or the stream shall be closed.</p>
</li>

<li>
<p>If the stream is open for reading and it is at the end of the file ( <a href="../functions/feof.html"><i>feof</i>()</a> is
true), no action need be taken.</p>
</li>

<li>
<p>If the stream is open with a mode that allows reading and the underlying open file description refers to a device that is
capable of seeking, the application shall either perform an <a href="../functions/fflush.html"><i>fflush</i>()</a>, or the stream
shall be closed.</p>
</li>
</ul>

<p>Otherwise, the result is undefined.</p>

<p>For the second handle:</p>

<ul>
<li>
<p>If any previous active handle has been used by a function that explicitly changed the file offset, except as required above for
the first handle, the application shall perform an <a href="../functions/lseek.html"><i>lseek</i>()</a> or <a href=
"../functions/fseek.html"><i>fseek</i>()</a> (as appropriate to the type of handle) to an appropriate location.</p>
</li>
</ul>

<p>If the active handle ceases to be accessible before the requirements on the first handle, above, have been met, the state of the
open file description becomes undefined. This might occur during functions such as a <a href=
"../functions/fork.html"><i>fork</i>()</a> or <a href="../functions/_exit.html"><i>_exit</i>()</a>.</p>

<p>The <i><a href="../functions/exec.html">exec</a></i> functions make inaccessible all streams that are open at the time they are
called, independent of which streams or file descriptors may be available to the new process image.</p>

<p>When these rules are followed, regardless of the sequence of handles used, implementations shall ensure that an application,
even one consisting of several processes, shall yield correct results: no data shall be lost or duplicated when writing, and all
data shall be written in order, except as requested by seeks. It is implementation-defined whether, and under what conditions, all
input is seen exactly once.</p>

<p>If the rules above are not followed, the result is unspecified.</p>

<p>Each function that operates on a stream is said to have zero or more ``underlying functions''. This means that the stream
function shares certain traits with the underlying functions, but does not require that there be any relation between the
implementations of the stream function and its underlying functions.</p>

<h4><a name="tag_02_05_02">  2.5.2 </a>Stream Orientation and Encoding Rules</h4>

<p>For conformance to the ISO/IEC&nbsp;9899:1999 standard, the definition of a stream includes an ``orientation''. After a stream
is associated with an external file, but before any operations are performed on it, the stream is without orientation. Once a
wide-character input/output function has been applied to a stream without orientation, the stream shall become ``wide-oriented''.
Similarly, once a byte input/output function has been applied to a stream without orientation, the stream shall become
``byte-oriented''. Only a call to the <a href="../functions/freopen.html"><i>freopen</i>()</a> function or the <a href=
"../functions/fwide.html"><i>fwide</i>()</a> function can otherwise alter the orientation of a stream.</p>

<p>A successful call to <a href="../functions/freopen.html"><i>freopen</i>()</a> shall remove any orientation. The three predefined
streams <i>standard input</i>, <i>standard output</i>, and <i>standard error</i> shall be unoriented at program start-up.</p>

<p>Byte input/output functions cannot be applied to a wide-oriented stream, and wide-character input/output functions cannot be
applied to a byte-oriented stream. The remaining stream operations shall not affect and shall not be affected by a stream's
orientation, except for the following additional restriction:</p>

<ul>
<li>
<p>For wide-oriented streams, after a successful call to a file-positioning function that leaves the file position indicator prior
to the end-of-file, a wide-character output function can overwrite a partial character; any file contents beyond the byte(s)
written are henceforth undefined.</p>
</li>
</ul>

<p>Each wide-oriented stream has an associated <b>mbstate_t</b> object that stores the current parse state of the stream. A
successful call to <a href="../functions/fgetpos.html"><i>fgetpos</i>()</a> shall store a representation of the value of this
<b>mbstate_t</b> object as part of the value of the <b>fpos_t</b> object. A later successful call to <a href=
"../functions/fsetpos.html"><i>fsetpos</i>()</a> using the same stored <b>fpos_t</b> value shall restore the value of the
associated <b>mbstate_t</b> object as well as the position within the controlled stream.</p>

<p>Implementations that support multiple encoding rules associate an encoding rule with the stream. The encoding rule shall be
determined by the setting of the <i>LC_CTYPE</i> category in the current locale at the time when the stream becomes wide-oriented.
As with the stream's orientation, the encoding rule associated with a stream cannot be changed once it has been set, except by a
successful call to <a href="../functions/freopen.html"><i>freopen</i>()</a> which clears the encoding rule and resets the
orientation to unoriented.</p>

<p>Although wide-oriented streams are conceptually sequences of wide characters, the external file associated with a wide-oriented
stream is a sequence of (possibly multi-byte) characters generalized as follows:</p>

<ul>
<li>
<p>Multi-byte encodings within files may contain embedded null bytes (unlike multi-byte encodings valid for use internal to the
program).</p>
</li>

<li>
<p>A file need not begin nor end in the initial shift state.</p>
</li>
</ul>

<p>Moreover, the encodings used for characters may differ among files. Both the nature and choice of such encodings are
implementation-defined.</p>

<p>The wide-character input functions read characters from the stream and convert them to wide characters as if they were read by
successive calls to the <a href="../functions/fgetwc.html"><i>fgetwc</i>()</a> function. Each conversion shall occur as if by a
call to the <a href="../functions/mbrtowc.html"><i>mbrtowc</i>()</a> function, with the conversion state described by the stream's
own <b>mbstate_t</b> object, <sup>[<a href="javascript:open_code('CX')">CX</a>]</sup> <img src="../images/opt-start.gif" alt=
"[Option Start]" border="0"> &nbsp;except the encoding rule associated with the stream is used instead of the encoding rule implied
by the <i>LC_CTYPE</i> category of the current locale. <img src="../images/opt-end.gif" alt="[Option End]" border="0"></p>

<p>The wide-character output functions convert wide characters to (possibly multi-byte) characters and write them to the stream as
if they were written by successive calls to the <a href="../functions/fputwc.html"><i>fputwc</i>()</a> function. Each conversion
shall occur as if by a call to the <a href="../functions/wcrtomb.html"><i>wcrtomb</i>()</a> function, with the conversion state
described by the stream's own <b>mbstate_t</b> object, <sup>[<a href="javascript:open_code('CX')">CX</a>]</sup> <img src=
"../images/opt-start.gif" alt="[Option Start]" border="0"> &nbsp;except the encoding rule associated with the stream is used
instead of the encoding rule implied by the <i>LC_CTYPE</i> category of the current locale. <img src="../images/opt-end.gif" alt=
"[Option End]" border="0"></p>

<p>An ``encoding error'' shall occur if the character sequence presented to the underlying <a href=
"../functions/mbrtowc.html"><i>mbrtowc</i>()</a> function does not form a valid (generalized) character, or if the code value
passed to the underlying <a href="../functions/wcrtomb.html"><i>wcrtomb</i>()</a> function does not correspond to a valid
(generalized) character. The wide-character input/output functions and the byte input/output functions store the value of the macro
[EILSEQ] in <i>errno</i> if and only if an encoding error occurs.</p>



<hr size="2" noshade>
<center><font size="2"><!--footer start-->
UNIX &reg; is a registered Trademark of The Open Group.<br>
POSIX &reg; is a registered Trademark of The IEEE.<br>
[ <a href="../mindex.html">Main Index</a> | <a href="../basedefs/contents.html">XBD</a> | <a href=
"../utilities/contents.html">XCU</a> | <a href="../functions/contents.html">XSH</a> | <a href="../xrat/contents.html">XRAT</a>
]</font></center>

<!--footer end-->
<hr size="2" noshade>
</body>
</html>