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 <h4 class="subsection">13.14.1 File Access Modes</h4>

 <p>The file access modes allow a file descriptor to be used for reading,
 writing, or both.  (In the GNU system, they can also allow none of these,
 and allow execution of the file as a program.)  The access modes are chosen
 when the file is opened, and never change.

 <!-- fcntl.h -->
 <!-- POSIX.1 -->
 <div class="defun">
 &mdash; Macro: int <b>O_RDONLY</b><var><a name="index-O_005fRDONLY-1330"></a></var><br>
 <blockquote><p>Open the file for read access.
 </p></blockquote></div>

 <!-- fcntl.h -->
 <!-- POSIX.1 -->
 <div class="defun">
 &mdash; Macro: int <b>O_WRONLY</b><var><a name="index-O_005fWRONLY-1331"></a></var><br>
 <blockquote><p>Open the file for write access.
 </p></blockquote></div>

 <!-- fcntl.h -->
 <!-- POSIX.1 -->
 <div class="defun">
 &mdash; Macro: int <b>O_RDWR</b><var><a name="index-O_005fRDWR-1332"></a></var><br>
 <blockquote><p>Open the file for both reading and writing.
 </p></blockquote></div>

    <p>In the GNU system (and not in other systems), <code>O_RDONLY</code> and
 <code>O_WRONLY</code> are independent bits that can be bitwise-ORed together,
 and it is valid for either bit to be set or clear.  This means that
 <code>O_RDWR</code> is the same as <code>O_RDONLY|O_WRONLY</code>.  A file access
 mode of zero is permissible; it allows no operations that do input or
 output to the file, but does allow other operations such as
 <code>fchmod</code>.  On the GNU system, since &ldquo;read-only&rdquo; or &ldquo;write-only&rdquo;
 is a misnomer, <samp><span class="file">fcntl.h</span></samp> defines additional names for the file
 access modes.  These names are preferred when writing GNU-specific code.
 But most programs will want to be portable to other POSIX.1 systems and
 should use the POSIX.1 names above instead.

 <!-- fcntl.h -->
 <!-- GNU -->
 <div class="defun">
 &mdash; Macro: int <b>O_READ</b><var><a name="index-O_005fREAD-1333"></a></var><br>
 <blockquote><p>Open the file for reading.  Same as <code>O_RDONLY</code>; only defined on GNU.
 </p></blockquote></div>

 <!-- fcntl.h -->
 <!-- GNU -->
 <div class="defun">
 &mdash; Macro: int <b>O_WRITE</b><var><a name="index-O_005fWRITE-1334"></a></var><br>
 <blockquote><p>Open the file for writing.  Same as <code>O_WRONLY</code>; only defined on GNU.
 </p></blockquote></div>

 <!-- fcntl.h -->
 <!-- GNU -->
 <div class="defun">
 &mdash; Macro: int <b>O_EXEC</b><var><a name="index-O_005fEXEC-1335"></a></var><br>
 <blockquote><p>Open the file for executing.  Only defined on GNU.
 </p></blockquote></div>

    <p>To determine the file access mode with <code>fcntl</code>, you must extract
 the access mode bits from the retrieved file status flags.  In the GNU
 system, you can just test the <code>O_READ</code> and <code>O_WRITE</code> bits in
 the flags word.  But in other POSIX.1 systems, reading and writing
 access modes are not stored as distinct bit flags.  The portable way to
 extract the file access mode bits is with <code>O_ACCMODE</code>.

 <!-- fcntl.h -->
 <!-- POSIX.1 -->
 <div class="defun">
 &mdash; Macro: int <b>O_ACCMODE</b><var><a name="index-O_005fACCMODE-1336"></a></var><br>
 <blockquote><p>This macro stands for a mask that can be bitwise-ANDed with the file
 status flag value to produce a value representing the file access mode.
 The mode will be <code>O_RDONLY</code>, <code>O_WRONLY</code>, or <code>O_RDWR</code>.
 (In the GNU system it could also be zero, and it never includes the
 <code>O_EXEC</code> bit.)
 </p></blockquote></div>

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	Copyright (C) 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2001, 2002,
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	<h4 class="subsection">13.14.1 File Access Modes</h4>

	<p>The file access modes allow a file descriptor to be used for reading,
	writing, or both. (In the GNU system, they can also allow none of these,
	and allow execution of the file as a program.) The access modes are chosen
	when the file is opened, and never change.

	<!-- fcntl.h -->
	<!-- POSIX.1 -->
	<div class="defun">
	— Macro: int <b>O_RDONLY</b><var><a name="index-O_005fRDONLY-1330"></a></var><br>
	<blockquote><p>Open the file for read access.
	</p></blockquote></div>

	<!-- fcntl.h -->
	<!-- POSIX.1 -->
	<div class="defun">
	— Macro: int <b>O_WRONLY</b><var><a name="index-O_005fWRONLY-1331"></a></var><br>
	<blockquote><p>Open the file for write access.
	</p></blockquote></div>

	<!-- fcntl.h -->
	<!-- POSIX.1 -->
	<div class="defun">
	— Macro: int <b>O_RDWR</b><var><a name="index-O_005fRDWR-1332"></a></var><br>
	<blockquote><p>Open the file for both reading and writing.
	</p></blockquote></div>

	<p>In the GNU system (and not in other systems), <code>O_RDONLY</code> and
	<code>O_WRONLY</code> are independent bits that can be bitwise-ORed together,
	and it is valid for either bit to be set or clear. This means that
	<code>O_RDWR</code> is the same as <code>O_RDONLY\|O_WRONLY</code>. A file access
	mode of zero is permissible; it allows no operations that do input or
	output to the file, but does allow other operations such as
	<code>fchmod</code>. On the GNU system, since “read-only” or “write-only”
	is a misnomer, <samp><span class="file">fcntl.h</span></samp> defines additional names for the file
	access modes. These names are preferred when writing GNU-specific code.
	But most programs will want to be portable to other POSIX.1 systems and
	should use the POSIX.1 names above instead.

	<!-- fcntl.h -->
	<!-- GNU -->
	<div class="defun">
	— Macro: int <b>O_READ</b><var><a name="index-O_005fREAD-1333"></a></var><br>
	<blockquote><p>Open the file for reading. Same as <code>O_RDONLY</code>; only defined on GNU.
	</p></blockquote></div>

	<!-- fcntl.h -->
	<!-- GNU -->
	<div class="defun">
	— Macro: int <b>O_WRITE</b><var><a name="index-O_005fWRITE-1334"></a></var><br>
	<blockquote><p>Open the file for writing. Same as <code>O_WRONLY</code>; only defined on GNU.
	</p></blockquote></div>

	<!-- fcntl.h -->
	<!-- GNU -->
	<div class="defun">
	— Macro: int <b>O_EXEC</b><var><a name="index-O_005fEXEC-1335"></a></var><br>
	<blockquote><p>Open the file for executing. Only defined on GNU.
	</p></blockquote></div>

	<p>To determine the file access mode with <code>fcntl</code>, you must extract
	the access mode bits from the retrieved file status flags. In the GNU
	system, you can just test the <code>O_READ</code> and <code>O_WRITE</code> bits in
	the flags word. But in other POSIX.1 systems, reading and writing
	access modes are not stored as distinct bit flags. The portable way to
	extract the file access mode bits is with <code>O_ACCMODE</code>.

	<!-- fcntl.h -->
	<!-- POSIX.1 -->
	<div class="defun">
	— Macro: int <b>O_ACCMODE</b><var><a name="index-O_005fACCMODE-1336"></a></var><br>
	<blockquote><p>This macro stands for a mask that can be bitwise-ANDed with the file
	status flag value to produce a value representing the file access mode.
	The mode will be <code>O_RDONLY</code>, <code>O_WRONLY</code>, or <code>O_RDWR</code>.
	(In the GNU system it could also be zero, and it never includes the
	<code>O_EXEC</code> bit.)
	</p></blockquote></div>

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