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    <p>It's important to distinguish between the <strong>file descriptor</strong>, which is a small integer that the process uses in its read and write calls to identify the file, and the <strong>file description</strong>, which is a structure in the kernel. The file offset is part of the file description. It lives in the kernel.</p> <p>As an example, let's use this program:</p> <pre><code>#include &lt;unistd.h&gt; #include &lt;fcntl.h&gt; #include &lt;sys/wait.h&gt; int main(void) { int fd; fd = open("output", O_CREAT|O_TRUNC|O_WRONLY, 0666); if(!fork()) { /* child */ write(fd, "hello ", 6); _exit(0); } else { /* parent */ int status; wait(&amp;status); write(fd, "world\n", 6); } } </code></pre> <p>(All error checking has been omitted)</p> <p>If we compile this program, call it <code>hello</code>, and run it like this:</p> <pre><code>./hello </code></pre> <p>here's what happens:</p> <p>The program opens the <code>output</code> file, creating it if it didn't exist already or truncating it to zero size if it did exist. The kernel creates a file description (in the Linux kernel this is a <code>struct file</code>) and associates it with a file descriptor for the calling process (the lowest non-negative integer not already in use in that process's file descriptor table). The file descriptor is returned and assigned to <code>fd</code> in the program. For the sake of argument suppose that <code>fd</code> is 3.</p> <p>The program does a fork(). The new child process gets a <em>copy</em> of its parent's file descriptor table, but the file description is not copied. Entry number 3 in both processes' file tables points to the same <code>struct file</code>.</p> <p>The parent process waits while the child process writes. The child's write causes the first half of <code>"hello world\n"</code> to be stored in the file, and advances the file offset by 6. The file offset is in the <code>struct file</code>!</p> <p>The child exits, the parent's <code>wait()</code> finishes, and the parent writes, using fd 3 which is still associated with the same file description that had its file offset updated by the child's <code>write()</code>. So the second half of the message is stored <em>after</em> the first part, not overwriting it as it would have done if the parent had a file offset of zero, which would be the case if the file description was not shared.</p> <p>Finally the parent exits, and the kernel sees that the <code>struct file</code> is no longer in use and frees it.</p>
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    1. POCan anyone explain a simple description regarding 'file descriptor' after fork()?
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    1. USAlan Curry
    1. USAlan Curry
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    1. POCan anyone explain a simple description regarding 'file descriptor' after fork()?
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    1. COAlan, the explanation is great! Thanks for taking the time and explaining it in such detail for a newbie.
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