gument.</pre>
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<pre>The caller of start_async() must close the returned FDs after it
has completed reading from/writing from them.</pre>
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<p>Specify a file descriptor &gt; 0 to be used by the function:</p>
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<pre>.in: The FD must be readable; it becomes the function's in.
.out: The FD must be writable; it becomes the function's out.</pre>
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