Messages in this thread | | | Date | Fri, 21 Mar 2014 10:51:31 -0400 | From | Peter Hurley <> | Subject | Re: man termios |
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On 03/21/2014 10:17 AM, Michael Kerrisk (man-pages) wrote: > On Fri, Mar 21, 2014 at 3:03 PM, Peter Hurley <peter@hurleysoftware.com> wrote: >> On 03/21/2014 09:15 AM, Michael Kerrisk (man-pages) wrote: >>> >>> On 03/21/2014 12:21 PM, Peter Hurley wrote: >>>> >>>> On 03/21/2014 06:45 AM, Michael Kerrisk (man-pages) wrote: >> >> >>>>>> Finally, if the 'count' parameter is less than MIN, read() may return >>>>>> before >>>>>> MIN bytes have been received, if 'count' bytes have been received. >>>>> >>>>> >>>>> Yes. But it's not clear to me here: do you mean that something in the >>>>> man page (or in TLPI) needs fixing? >>>> >>>> >>>> Well, what I mean here is that read() may also _not_ return until MIN >>>> bytes have >>>> been received, even if 'count' bytes have been received. >>> >>> >>> Ahh -- I see what you mean. And, it looks like there is a point here where >>> Linux >>> differs from POSIX and (at least) Solaris. See the current man-page text >>> below, >>> in particular the MIN>0, TIME>0 case. I've also attached a simple test >>> program >>> that I used, below. >>> >>> In noncanonical mode input is available immediately (without the >>> user having to type a line-delimiter character), no input pro‐ >>> cessing is performed, and line editing is disabled. The settings >>> of MIN (c_cc[VMIN]) and TIME (c_cc[VTIME]) determine the circum‐ >>> stances in which a read(2) completes; there are four distinct >>> cases: >>> >>> MIN == 0; TIME == 0: >>> If data is available, read(2) returns immediately, with >>> the lesser of the number of bytes available, or the number >>> of bytes requested. If no data is available, read(2) >>> returns 0. >>> >>> MIN > 0; TIME == 0: >>> read(2) blocks until MIN bytes are available, and returns >>> up to the number of bytes requested. >>> >>> MIN == 0; TIME > 0: >>> TIME specifies the limit for a timer in tenths of a sec‐ >>> ond. The timer is started when read(2) is called. >>> read(2) returns either when at least one byte of data is >>> available, or when the timer expires. If the timer >>> expires without any input becoming available, read(2) >>> returns 0. If data is already available at the time of >>> the call to read() the call behaves as though the data was >>> received immediately after the call. >>> >>> MIN > 0; TIME > 0: >>> TIME specifies the limit for a timer in tenths of a sec‐ >>> ond. Once an initial byte of input becomes available, the >>> timer is restarted after each further byte is received. >>> read(2) returns when any of the following conditions is >>> met: >>> >>> * MIN bytes have been received. >>> >>> * The interbyte timer expires. >>> >>> * The number of bytes requested by read(2) has been >>> received. (POSIX does not specify this termination >>> condition, and on some other implementations read() >>> does not return in this case.) >>> >>> Because the timer is started only after the initial byte >>> becomes available, at least one byte will be read. If >>> data is already available at the time of the call to >>> read() the call behaves as though the data was received >>> immediately after the call. >>> >>> POSIX does not specify whether the setting of the O_NONBLOCK file >>> status flag takes precedence over the MIN and TIME settings. If >>> O_NONBLOCK is set, a read() in noncanonical mode may return imme‐ >>> diately, regardless of the setting of MIN or TIME. Furthermore, >>> if no data is available, POSIX permits a read() in noncanonical >>> mode to return either 0, or -1 with errno set to EAGAIN. >> >> >> All looks good. > > Peter, do you agree that Linux appears to differ from POSIX here? (Not > sure if you tried my test program to verify...)
I did run the test program to validate that it's observed behavior is that implemented by Linux, with which I'm very familiar. I don't have a test setup for other *nixes.
I would be interested to know the results of
./noncanonical 0 5 3 0 hello
and
./noncanonical 0 5 3 2 hel
on other platforms.
With respect to POSIX compliance, it's hard to say. I'm not sure the spec contemplates the degenerate case where max bytes < MIN. And specifically with regard to terminal i/o behavior, POSIX is essentially ex post facto, and is really documenting existing behavior.
Other than the degenerate case of max bytes < MIN, is there any other variation between Solaris and Linux in non-canonical mode?
Regards, Peter Hurley
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