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| <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> |
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