Date: Wed, 9 Apr 2003 15:49:14 +0400 From: Yar Tikhiy <yar@freebsd.org> To: Bruce Evans <bde@zeta.org.au> Cc: arch@freebsd.org Subject: Re: termios & non-blocking I/O Message-ID: <20030409114914.GB63770@comp.chem.msu.su> In-Reply-To: <20030409044301.J628@gamplex.bde.org> References: <20030408164614.GA7236@comp.chem.msu.su> <20030409044301.J628@gamplex.bde.org>
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On Wed, Apr 09, 2003 at 05:33:28AM +1000, Bruce Evans wrote: > On Tue, 8 Apr 2003, Yar Tikhiy wrote: > > > ... > > Let's consider a non-blocking file descriptor that correspons to a > > terminal in raw mode. Let's also assume read(2) is issued on it > > when there is no data to read. > > > > If for this terminal MIN > 0 and TIME == 0, read(2) will return -1 > > and set errno to EAGAIN. > > > > OTOH, if MIN == 0 and TIME > 0, read(2) will return 0. > > > > While not in disagreement with POSIX[1], such a behaviour has at > > least one unwelcome consequence: If a program has been compiled > > with ``-pthread'', the TIME counter won't work on terminal descriptors > > that are in blocking mode from the program's point of view -- read(2) > > will instantly return 0 on them. That is because the following > > scenario will happen: > > The spec is not very clear, but I think it means to say to return > -1/EAGAIN (it says that first). As far as I can understand, the spec says both 0 and -1/EAGAIN are valid return values. So it's up to us to decide which way is better. > ... > > 1) libc_r sets non-blocking mode on a descriptor as soon as a device > > is opened (that is how i/o in user-land threads work); > > 2) the program sets the TIME counter through tcsetattr(3); > > 3) the program issues read(2), which ends up in the actual read() > > syscall, which in turn returns 0 to libc_r (assuming there is no > > data to read); > > 4) libc_r thinks this is the EOF indicator, so it instantly returns > > 0 to the program; > > 5) the program breaks. > > > > Notice, that MIN works right with libc_r since read() syscall will > > return -1/EAGAIN, which is correctly understood by libc_r: it will > > block the current thread until there is data to read. > > Does it keep the fd in non-blocking mode and wait for the data using > select() or similar? Yes, libc_r does read() on the fd and if the read() returns -1/EAGAIN, it does poll() on the fd (as can be seen with ktrace.) > This wouldn't work so well for TIME because > select() doesn't really understand MIN/TIME; in particular, TIME > has no effect on select() in the MIN == 0 && TIME > 0 case. I > think libc_r would have to duplicate most of the kernel's MIN/TIME > stuff to fake things properly. It correctly doesn't go near this. Hmmm, it is a serious complication. Shame on me that I have overlooked it. But that doesn't seem to keep us from making the MIN and TIME cases consistent WRT the return values for non-blocking fd's, does it? -- Yar
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