POLL(2) | System Calls Manual | POLL(2) |
poll
, ppoll
— synchronous I/O multiplexing
#include
<poll.h>
int
poll
(struct
pollfd *fds, nfds_t
nfds, int
timeout);
int
ppoll
(struct
pollfd *fds, nfds_t
nfds, const struct
timespec *timeout, const
sigset_t *mask);
poll
()
provides a mechanism for multiplexing I/O across a set of file descriptors.
It is similar in function to select(2).
Unlike select(2), however, it is possible
to only pass in data corresponding to the file descriptors for which events
are wanted. This makes poll
() more efficient than
select(2) in most cases.
The arguments are as follows:
struct pollfd { int fd; short events; short revents; };
The fd member is an open file descriptor. If fd is -1, the pollfd structure is considered unused, and revents will be cleared.
The events and revents members are bitmasks of conditions to monitor and conditions found, respectively.
poll
()
will return immediately. If this value is INFTIM
(-1), poll
() will block indefinitely until a
condition is found.The calling process sets the
events bitmask and
poll
() sets
the revents bitmask. Each call to
poll
() resets the revents
bitmask for accuracy. The condition flags in the bitmasks are defined
as:
POLLIN
POLLRDNORM
POLLRDBAND
POLLNORM
POLLRDNORM
. This flag is provided for
source code compatibility with older programs and should not be used in
new code.POLLPRI
POLLOUT
POLLWRNORM
POLLOUT
.POLLWRBAND
POLLERR
POLLHUP
POLLOUT
are mutually-exclusive; a descriptor can
never be writable if a hangup has occurred. However, this event and
POLLIN
, POLLRDNORM
,
POLLRDBAND
, or POLLPRI
are
not mutually-exclusive. This flag is only valid in the
revents bitmask; it is ignored in the
events member.POLLNVAL
The significance and semantics of normal, priority, and
high-priority data are device-specific. For example, on
OpenBSD, the POLLPRI
and
POLLRDBAND
flags may be used to detect when
out-of-band socket data may be read without blocking.
The
ppoll
()
function is similar to poll
() except that it
specifies the timeout using a timespec structure, and a null pointer is used
to specify an indefinite timeout instead of INFTIM
.
Also, if mask is a non-null pointer,
ppoll
() atomically sets the calling thread's signal
mask to the signal set pointed to by mask for the
duration of the function call. In this case, the original signal mask will
be restored before ppoll
() returns.
Upon error, poll
() and
ppoll
() return -1 and set the global variable
errno to indicate the error. If the timeout interval
was reached before any events occurred, they return 0. Otherwise, they
return the number of pollfd structures for which
revents is non-zero.
Care must be taken when converting code from
select(2) to
poll
() as they have slightly different semantics.
The first semantic difference is that, unlike
select(2), poll
()
has a way of indicating that one or more file descriptors is invalid by
setting a flag in the revents field of corresponding
entry of fds, whereas
select(2) returns an error (-1) if any of
the descriptors with bits set in the fd_set are
invalid. The second difference is that on EOF there is no guarantee that
POLLIN
will be set in revents,
the caller must also check for POLLHUP
. This differs
from select(2) where EOF is considered as
a read event.
Consider the following usage of select(2) that implements a read from the standard input with a 60 second time out:
struct timeval timeout; fd_set readfds; char buf[BUFSIZ]; int nready; timeout.tv_sec = 60; timeout.tv_usec = 0; FD_ZERO(&readfds); FD_SET(STDIN_FILENO, &readfds); nready = select(STDIN_FILENO + 1, &readfds, NULL, NULL, &timeout); if (nready == -1) err(1, "select"); if (nready == 0) errx(1, "time out"); if (FD_ISSET(STDIN_FILENO, &readfds)) { if (read(STDIN_FILENO, buf, sizeof(buf)) == -1) err(1, "read"); }
This can be converted to
poll
() as
follows:
struct pollfd pfd[1]; char buf[BUFSIZ]; int nready; pfd[0].fd = STDIN_FILENO; pfd[0].events = POLLIN; nready = poll(pfd, 1, 60 * 1000); if (nready == -1) err(1, "poll"); if (nready == 0) errx(1, "time out"); if ((pfd[0].revents & (POLLERR|POLLNVAL))) errx(1, "bad fd %d", pfd[0].fd); if ((pfd[0].revents & (POLLIN|POLLHUP))) { if (read(STDIN_FILENO, buf, sizeof(buf)) == -1) err(1, "read"); }
poll
() and ppoll
()
will fail if:
clock_gettime(2), getrlimit(2), read(2), select(2), write(2)
The poll
() function is compliant with the
IEEE Std 1003.1-2008 (“POSIX.1”)
specification. The ppoll
() function is a Linux
extension.
A poll
() system call appeared in
AT&T System V Release 3 UNIX. The
ppoll
() function appeared in
OpenBSD 5.4.
The POLLWRBAND
flag is accepted but
ignored by the kernel.
Because OpenBSD does not implement
STREAMS, there is no distinction between some of the fields in the
events and revents bitmasks. As
a result, the POLLIN
,
POLLNORM
, and POLLRDNORM
flags are equivalent. Similarly, the POLLPRI
and
POLLRDBAND
flags are also equivalent.
Internally to the kernel, poll
() and
ppoll
() work poorly if multiple processes wait on
the same file descriptor.
August 18, 2018 | OpenBSD-current |