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Linux系统调用

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<TITLE>Syscall specifications of Linux - select</TITLE>
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<P>
<H3>SYNOPSIS</H3>
<P>
<CODE>int select(int <VAR>numfds</VAR>, fd_set *<VAR>readfds</VAR>, fd_set
*<VAR>writefds</VAR>, fd_set *<VAR>exceptfds</VAR>, struct timeval *<VAR>timeout</VAR>);</CODE>
<P>
Macro utilities:
<P>
<CODE>FD_CLR(int <VAR>fd</VAR>, fd_set *<VAR>set</VAR>);</CODE>
<P>
<CODE>FD_ISSET(int <VAR>fd</VAR>, fd_set *<VAR>set</VAR>);</CODE>
<P>
<CODE>FD_SET(int <VAR>fd</VAR>, fd_set *<VAR>set</VAR>);</CODE>
<P>
<CODE>FD_ZERO(fd_set *<VAR>set</VAR>);</CODE>
<P>
<H3>PARAMETERS</H3>
<P>
<VAR>numfds</VAR>: [in] the number of file descriptors to watch.
<P>
<VAR>readfds</VAR>: [in out] on entry, the file descriptor to watch for
readability. On exit, the file descriptors that are readable.
<P>
<VAR>writefds</VAR>: [in out] on entry, the file descriptors to watch for
writability. On exit, the file descriptors that are writable.
<P>
<VAR>exceptfds</VAR>: [in out] on entry, the file descriptor to watch
exceptions. On exit, the file descriptors have exceptions raised.
<P>
<VAR>timeout</VAR>: [in out] on entry, the timeout value. On return, contains
the remaining time.
<P>
<H3>DESCRIPTION</H3>
<P>
Makes the calling task sleep until some conditions on the file
descriptors become true or until a timeout value expires. There are
three conditions for which files may be tested:
<P>
<UL>
<LI>with <VAR>readfds</VAR>: Can the file be read?
<LI>with <VAR>writefds</VAR>: Can the file be written?
<LI>with <VAR>exceptfds</VAR>: Is there any exception pending on the file?
</UL>
<P>
Whenever one of the conditions on one the specified files is true, the
call returns.
<P>
There are four utilities provided for manipulation of the file
descriptor sets:
<P>
<DL COMPACT>
<DT><CODE>FD_SET</CODE>
<DD>sets the specified file descriptor flag.
<P>
<DT><CODE>FD_CLR</CODE>
<DD>clears the specified file descriptor flag.
<P>
<DT><CODE>FD_ZERO</CODE>
<DD>clears the set.
<P>
<DT><CODE>FD_ISSET</CODE>
<DD>returns true if the file descriptor flag is set.
</DL>
<P>
<H3>RETURN VALUE</H3>
<P>
On success, returns zero. On error, returns -1 and sets <CODE>errno</CODE> to
one of the following value:
<P>
<UL>
<LI><CODE>EBADF</CODE>, <CODE>EINTR</CODE>, <CODE>EINVAL</CODE> or <CODE>ENOMEM</CODE>
</UL>
<P>
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