On Fri, Mar 21, 2014 at 3:03 PM, Peter Hurley <peter@xxxxxxxxxxxxxxxxxx> 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...) Cheers, Michael -- Michael Kerrisk Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/ Linux/UNIX System Programming Training: http://man7.org/training/ -- To unsubscribe from this list: send the line "unsubscribe linux-serial" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
