| <html lang="en"> |
| <head> |
| <title>Descriptor Flags - The GNU C Library</title> |
| <meta http-equiv="Content-Type" content="text/html"> |
| <meta name="description" content="The GNU C Library"> |
| <meta name="generator" content="makeinfo 4.13"> |
| <link title="Top" rel="start" href="index.html#Top"> |
| <link rel="up" href="Low_002dLevel-I_002fO.html#Low_002dLevel-I_002fO" title="Low-Level I/O"> |
| <link rel="prev" href="Duplicating-Descriptors.html#Duplicating-Descriptors" title="Duplicating Descriptors"> |
| <link rel="next" href="File-Status-Flags.html#File-Status-Flags" title="File Status Flags"> |
| <link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage"> |
| <!-- |
| This file documents the GNU C library. |
| |
| This is Edition 0.12, last updated 2007-10-27, |
| of `The GNU C Library Reference Manual', for version |
| 2.8 (Sourcery G++ Lite 2011.03-41). |
| |
| Copyright (C) 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2001, 2002, |
| 2003, 2007, 2008, 2010 Free Software Foundation, Inc. |
| |
| Permission is granted to copy, distribute and/or modify this document |
| under the terms of the GNU Free Documentation License, Version 1.3 or |
| any later version published by the Free Software Foundation; with the |
| Invariant Sections being ``Free Software Needs Free Documentation'' |
| and ``GNU Lesser General Public License'', the Front-Cover texts being |
| ``A GNU Manual'', and with the Back-Cover Texts as in (a) below. A |
| copy of the license is included in the section entitled "GNU Free |
| Documentation License". |
| |
| (a) The FSF's Back-Cover Text is: ``You have the freedom to |
| copy and modify this GNU manual. Buying copies from the FSF |
| supports it in developing GNU and promoting software freedom.''--> |
| <meta http-equiv="Content-Style-Type" content="text/css"> |
| <style type="text/css"><!-- |
| pre.display { font-family:inherit } |
| pre.format { font-family:inherit } |
| pre.smalldisplay { font-family:inherit; font-size:smaller } |
| pre.smallformat { font-family:inherit; font-size:smaller } |
| pre.smallexample { font-size:smaller } |
| pre.smalllisp { font-size:smaller } |
| span.sc { font-variant:small-caps } |
| span.roman { font-family:serif; font-weight:normal; } |
| span.sansserif { font-family:sans-serif; font-weight:normal; } |
| --></style> |
| <link rel="stylesheet" type="text/css" href="../cs.css"> |
| </head> |
| <body> |
| <div class="node"> |
| <a name="Descriptor-Flags"></a> |
| <p> |
| Next: <a rel="next" accesskey="n" href="File-Status-Flags.html#File-Status-Flags">File Status Flags</a>, |
| Previous: <a rel="previous" accesskey="p" href="Duplicating-Descriptors.html#Duplicating-Descriptors">Duplicating Descriptors</a>, |
| Up: <a rel="up" accesskey="u" href="Low_002dLevel-I_002fO.html#Low_002dLevel-I_002fO">Low-Level I/O</a> |
| <hr> |
| </div> |
| |
| <h3 class="section">13.13 File Descriptor Flags</h3> |
| |
| <p><a name="index-file-descriptor-flags-1322"></a> |
| <dfn>File descriptor flags</dfn> are miscellaneous attributes of a file |
| descriptor. These flags are associated with particular file |
| descriptors, so that if you have created duplicate file descriptors |
| from a single opening of a file, each descriptor has its own set of flags. |
| |
| <p>Currently there is just one file descriptor flag: <code>FD_CLOEXEC</code>, |
| which causes the descriptor to be closed if you use any of the |
| <code>exec...</code> functions (see <a href="Executing-a-File.html#Executing-a-File">Executing a File</a>). |
| |
| <p>The symbols in this section are defined in the header file |
| <samp><span class="file">fcntl.h</span></samp>. |
| <a name="index-fcntl_002eh-1323"></a> |
| <!-- fcntl.h --> |
| <!-- POSIX.1 --> |
| |
| <div class="defun"> |
| — Macro: int <b>F_GETFD</b><var><a name="index-F_005fGETFD-1324"></a></var><br> |
| <blockquote><p>This macro is used as the <var>command</var> argument to <code>fcntl</code>, to |
| specify that it should return the file descriptor flags associated |
| with the <var>filedes</var> argument. |
| |
| <p>The normal return value from <code>fcntl</code> with this command is a |
| nonnegative number which can be interpreted as the bitwise OR of the |
| individual flags (except that currently there is only one flag to use). |
| |
| <p>In case of an error, <code>fcntl</code> returns -1. The following |
| <code>errno</code> error conditions are defined for this command: |
| |
| <dl> |
| <dt><code>EBADF</code><dd>The <var>filedes</var> argument is invalid. |
| </dl> |
| </p></blockquote></div> |
| |
| <!-- fcntl.h --> |
| <!-- POSIX.1 --> |
| <div class="defun"> |
| — Macro: int <b>F_SETFD</b><var><a name="index-F_005fSETFD-1325"></a></var><br> |
| <blockquote><p>This macro is used as the <var>command</var> argument to <code>fcntl</code>, to |
| specify that it should set the file descriptor flags associated with the |
| <var>filedes</var> argument. This requires a third <code>int</code> argument to |
| specify the new flags, so the form of the call is: |
| |
| <pre class="smallexample"> fcntl (<var>filedes</var>, F_SETFD, <var>new-flags</var>) |
| </pre> |
| <p>The normal return value from <code>fcntl</code> with this command is an |
| unspecified value other than -1, which indicates an error. |
| The flags and error conditions are the same as for the <code>F_GETFD</code> |
| command. |
| </p></blockquote></div> |
| |
| <p>The following macro is defined for use as a file descriptor flag with |
| the <code>fcntl</code> function. The value is an integer constant usable |
| as a bit mask value. |
| |
| <!-- fcntl.h --> |
| <!-- POSIX.1 --> |
| <div class="defun"> |
| — Macro: int <b>FD_CLOEXEC</b><var><a name="index-FD_005fCLOEXEC-1326"></a></var><br> |
| <blockquote><p><a name="index-close_002don_002dexec-_0028file-descriptor-flag_0029-1327"></a>This flag specifies that the file descriptor should be closed when |
| an <code>exec</code> function is invoked; see <a href="Executing-a-File.html#Executing-a-File">Executing a File</a>. When |
| a file descriptor is allocated (as with <code>open</code> or <code>dup</code>), |
| this bit is initially cleared on the new file descriptor, meaning that |
| descriptor will survive into the new program after <code>exec</code>. |
| </p></blockquote></div> |
| |
| <p>If you want to modify the file descriptor flags, you should get the |
| current flags with <code>F_GETFD</code> and modify the value. Don't assume |
| that the flags listed here are the only ones that are implemented; your |
| program may be run years from now and more flags may exist then. For |
| example, here is a function to set or clear the flag <code>FD_CLOEXEC</code> |
| without altering any other flags: |
| |
| <pre class="smallexample"> /* <span class="roman">Set the </span><code>FD_CLOEXEC</code><span class="roman"> flag of </span><var>desc</var><span class="roman"> if </span><var>value</var><span class="roman"> is nonzero,</span> |
| <span class="roman">or clear the flag if </span><var>value</var><span class="roman"> is 0.</span> |
| <span class="roman">Return 0 on success, or -1 on error with </span><code>errno</code><span class="roman"> set.</span> */ |
| |
| int |
| set_cloexec_flag (int desc, int value) |
| { |
| int oldflags = fcntl (desc, F_GETFD, 0); |
| /* <span class="roman">If reading the flags failed, return error indication now.</span> */ |
| if (oldflags < 0) |
| return oldflags; |
| /* <span class="roman">Set just the flag we want to set.</span> */ |
| if (value != 0) |
| oldflags |= FD_CLOEXEC; |
| else |
| oldflags &= ~FD_CLOEXEC; |
| /* <span class="roman">Store modified flag word in the descriptor.</span> */ |
| return fcntl (desc, F_SETFD, oldflags); |
| } |
| </pre> |
| </body></html> |
| |
