stdio.html

unix 下的C开发手册,还用详细的例程。

exactly as if it were a change of active handle.
(If the only action performed by one of the processes is one of the
<i>exec</i>
functions or
<i><a href="_exit.html">_exit()</a></i>
(not
<i><a href="exit.html">exit()</a></i>),
the handle is never accessed in that process.)
<p>
For the first handle, the first applicable condition below applies.
After the actions required below are taken, if the handle is still
open, the application can close it.
<p>
<ul>
<p>
<li>
If it is a file descriptor, no action is required.
<p>
<li>
If the only further action to be performed on any handle to this open
file descriptor is to close it, no action need be taken.
<p>
<li>
If it is a stream which is unbuffered, no action need be taken.
<p>
<li>
If it is a stream which is line buffered, and the last
byte written to the stream was a newline (that is, as if a:
<pre>
putc('\n')
</pre>
was the most recent operation on that stream),
no action need be taken.
<p>
<li>
If it is a stream which is open for writing or appending (but not also open
for reading), either an
<i><a href="fflush.html">fflush()</a></i>
must be done, or the stream must be closed.
<p>
<li>
If the stream is open for reading and it is at the end of the file (.Fn feof
is true), no action need be taken.
<p>
<li>
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, either
an
<i><a href="fflush.html">fflush()</a></i>
must occur or the stream must be closed.
<p>
</ul>
<p>
Otherwise, the result is undefined.
<p>
For the second handle:
<ul>
<p>
<li>
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 must perform an
<i><a href="lseek.html">lseek()</a></i>
or
<i><a href="fseek.html">fseek()</a></i>
(as appropriate to the type of handle)
to an appropriate location.
<p>
</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
<i><a href="fork.html">fork()</a></i>
or
<i><a href="_exit.html">_exit()</a></i>.
<p>
The
<i>exec</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>
When these rules are followed,
regardless of the sequence of handles used,
implementations will ensure that an application, even one consisting
of several processes, will yield correct results:
no data will be lost or duplicated when writing, and
all data will be written in order, except as requested by seeks.
It is implementation-dependent whether, and under what conditions,
all input is seen exactly once.
<p>
If the rules above are not followed, the result is unspecified.
<h4><a name = "tag_000_009_002">&nbsp;</a>Stream Orientation</h4>
<xref type="3" name="orientation"></xref>
For conformance to the Multibyte Support Extension, the definition of
a stream is adjusted to include an
<i>orientation</i>
for both text and binary streams.  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
becomes
<i>wide-orientated</i>.
Similarly, once a byte input/output function has been applied to a
stream without orientation, the stream becomes
<i>byte-orientated</i>.
Only a call to the
<i><a href="freopen.html">freopen()</a></i>
function or the
<i><a href="fwide.html">fwide()</a></i>
function can otherwise alter the orientation of a stream.
<p>
A successful call to
<i><a href="freopen.html">freopen()</a></i>
removes any orientation.  The three predefined streams
<i>standard input</i>,
<i>standard output</i>
and
<i>standard error</i>
are unorientated at program startup.
<p>
Byte input/output functions cannot be applied to a wide-orientated
stream, and wide-character input/output functions cannot be applied to
a byte-orientated stream.  The remaining stream operations do not
affect and are not affected by a stream's orientation, except for the
following additional restrictions:
<ul>
<p>
<li>
Binary wide-orientated streams have the file positioning restrictions
ascribed to both text and binary streams.
<p>
<li>
For wide-orientated 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>
</ul>
<p>
Each wide-orientated stream has an associated
<b>mbstate_t</b>
object that stores the current parse state of the stream.  A
successful call to
<i><a href="fgetpos.html">fgetpos()</a></i>
stores 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
<i><a href="fsetpos.html">fsetpos()</a></i>
using the same stored
<b>fpos_t</b>
value restores the value of the associated
<b>mbstate_t</b>
object as well as the position within the controlled stream.
<p>
Although both text and binary wide-orientated streams are conceptually
sequences of wide-characters, the external file associated with a
wide-orientated stream is a sequence of (possibly multibyte)
characters
generalised as follows:
<ul>
<p>
<li>
Multibyte encodings within files may contain embedded null bytes
(unlike multibyte encodings valid for use internal to the program).
<p>
<li>
A file need not begin nor end in the initial shift state.
<p>
</ul>
<p>
Moreover, the encodings used for characters may differ among
files.  Both the nature and choice of such encodings are
implementation-dependent.
<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
<i><a href="fgetwc.html">fgetwc()</a></i>
function.  Each conversion occurs as if by a call to the
<i><a href="mbrtowc.html">mbrtowc()</a></i>
function, with the conversion state described by the stream's own
<b>mbstate_t</b>
object.
<p>
The wide-character output functions convert wide-characters
to (possibly multibyte) characters and write them to the stream as
if they were written by successive calls to the
<i><a href="fputwc.html">fputwc()</a></i>
function.  Each conversion occurs as if by a call to the
<i><a href="wcrtomb.html">wcrtomb()</a></i>
function, with the conversion state described by the stream's own
<b>mbstate_t</b>
object.
<p>
An <i>encoding&nbsp;error</i> occurs if the character sequence presented to
the underlying
<i><a href="mbrtowc.html">mbrtowc()</a></i>
function does not form a valid (generalised) character, or
if the code value passed to the underlying
<i><a href="wcrtomb.html">wcrtomb()</a></i>
function does not correspond to a valid (generalised)
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.
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