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| author | Michael Kerrisk <mtk.manpages@gmail.com> | 2019-07-29 14:01:36 +0200 |
|---|---|---|
| committer | Michael Kerrisk <mtk.manpages@gmail.com> | 2019-07-30 08:25:27 +0200 |
| commit | 0bdda5d08e82cfb05422ef0c9994d752fc9494ff (patch) | |
| tree | ab91ff0eecff341a1bf55880e3ce847c9ff16d9b | |
| parent | 40ca38806ddd6b0108cccc08db9ec6deca7161ba (diff) | |
| download | man-pages-0bdda5d08e82cfb05422ef0c9994d752fc9494ff.tar.gz | |
poll.2: Note that poll() equivalent code for ppoll() is not quite equivalent
As reported by Alan Stern:
Here are two extracts from the man page for ppoll(2):
Specifying a negative value in timeout means an infinite
timeout.
Other than the difference in the precision of the timeout
argument, the following ppoll() call:
ready = ppoll(&fds, nfds, tmo_p, &sigmask);
is equivalent to atomically executing the following calls:
sigset_t origmask;
int timeout;
timeout = (tmo_p == NULL) ? -1 :
(tmo_p->tv_sec * 1000 + tmo_p->tv_nsec / 1000000);
pthread_sigmask(SIG_SETMASK, &sigmask, &origmask);
ready = poll(&fds, nfds, timeout);
pthread_sigmask(SIG_SETMASK, &origmask, NULL);
But if tmo_p->tv_sec is negative, the ppoll() call is not
equivalent to the corresponding poll() call. The kernel rejects
negative values of tv_sec with an EINVAL error; it does not
interpret the value as meaning an infinite timeout.
(Yes, the kernel interprets tmo_p == NULL as an infinite timeout,
but the man page is still wrong for the case tmo_p->tv_sec < 0.)
Suggested fix: Following the end of the second extract above, add:
except that negative time values in tmo_p are not
interpreted as an infinite timeout.
Also, in the ERRORS section, change the text for EINVAL to:
EINVAL The nfds value exceeds the RLIMIT_NOFILE value or
*tmo_p contains an invalid (negative) time value.
Reported-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
| -rw-r--r-- | man2/poll.2 | 19 |
1 files changed, 18 insertions, 1 deletions
diff --git a/man2/poll.2 b/man2/poll.2 index 0b023e0a5f..2799f56ac7 100644 --- a/man2/poll.2 +++ b/man2/poll.2 @@ -271,7 +271,7 @@ ready = ppoll(&fds, nfds, tmo_p, &sigmask); .EE .in .PP -is equivalent to +is nearly equivalent to .I atomically executing the following calls: .PP @@ -288,6 +288,17 @@ pthread_sigmask(SIG_SETMASK, &origmask, NULL); .EE .in .PP +The above code segment is described as +.I nearly +equivalent because whereas a negative +.I timeout +value for +.BR poll () +is interpreted as an infinite timeout, a negative value expressed in +.IR *tmo_p +results in an error from +.BR ppoll (). +.PP See the description of .BR pselect (2) for an explanation of why @@ -354,6 +365,12 @@ value exceeds the .B RLIMIT_NOFILE value. .TP +.B EINVAL +.RB ( ppoll ()) +The timeout value expressed in +.IR *ip +is invalid (negative). +.TP .B ENOMEM There was no space to allocate file descriptor tables. .SH VERSIONS |