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| <h4 class="subsection">13.14.4 Getting and Setting File Status Flags</h4> |
| |
| <p>The <code>fcntl</code> function can fetch or change file status flags. |
| |
| <!-- fcntl.h --> |
| <!-- POSIX.1 --> |
| <div class="defun"> |
| — Macro: int <b>F_GETFL</b><var><a name="index-F_005fGETFL-1362"></a></var><br> |
| <blockquote><p>This macro is used as the <var>command</var> argument to <code>fcntl</code>, to |
| read the file status flags for the open file with descriptor |
| <var>filedes</var>. |
| |
| <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. Since the file access modes are not single-bit values, |
| you can mask off other bits in the returned flags with <code>O_ACCMODE</code> |
| to compare them. |
| |
| <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_SETFL</b><var><a name="index-F_005fSETFL-1363"></a></var><br> |
| <blockquote><p>This macro is used as the <var>command</var> argument to <code>fcntl</code>, to set |
| the file status flags for the open file corresponding to the |
| <var>filedes</var> argument. This command requires a third <code>int</code> |
| argument to specify the new flags, so the call looks like this: |
| |
| <pre class="smallexample"> fcntl (<var>filedes</var>, F_SETFL, <var>new-flags</var>) |
| </pre> |
| <p>You can't change the access mode for the file in this way; that is, |
| whether the file descriptor was opened for reading or writing. |
| |
| <p>The normal return value from <code>fcntl</code> with this command is an |
| unspecified value other than -1, which indicates an error. The |
| error conditions are the same as for the <code>F_GETFL</code> command. |
| </p></blockquote></div> |
| |
| <p>If you want to modify the file status flags, you should get the current |
| flags with <code>F_GETFL</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>O_NONBLOCK</code> without |
| altering any other flags: |
| |
| <pre class="smallexample"> /* <span class="roman">Set the </span><code>O_NONBLOCK</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_nonblock_flag (int desc, int value) |
| { |
| int oldflags = fcntl (desc, F_GETFL, 0); |
| /* <span class="roman">If reading the flags failed, return error indication now.</span> */ |
| if (oldflags == -1) |
| return -1; |
| /* <span class="roman">Set just the flag we want to set.</span> */ |
| if (value != 0) |
| oldflags |= O_NONBLOCK; |
| else |
| oldflags &= ~O_NONBLOCK; |
| /* <span class="roman">Store modified flag word in the descriptor.</span> */ |
| return fcntl (desc, F_SETFL, oldflags); |
| } |
| </pre> |
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