April 2024 | ||||||
Mo | Tu | We | Th | Fr | Sa | Su |
1 | 2 | 3 | 4 | 5 | 6 | 7 |
8 | 9 | 10 | 11 | 12 | 13 | 14 |
15 | 16 | 17 | 18 | 19 | 20 | 21 |
22 | 23 | 24 | 25 | 26 | 27 | 28 |
29 | 30 | 1 | 2 | 3 | 4 | 5 |
6 | 7 | 8 | 9 | 10 | 11 | 12 |
#include <poll.h> int poll(struct pollfd *fds, nfds_t nfds, int timeout); #define _GNU_SOURCE /* See feature_test_macros(7) */ #include <poll.h> int ppoll(struct pollfd *fds, nfds_t nfds, const struct timespec *timeout_ts, const sigset_t *sigmask);
The set of file descriptors to be monitored is specified in the fds argument, which is an array of structures of the following form:
struct pollfd { int fd; /* file descriptor */ short events; /* requested events */ short revents; /* returned events */ };
The caller should specify the number of items in the fds array in nfds.
The field fd contains a file descriptor for an open file.
The field events is an input parameter, a bit mask specifying the events the application is interested in.
The field revents is an output parameter, filled by the kernel with the events that actually occurred. The bits returned in revents can include any of those specified in events, or one of the values POLLERR, POLLHUP, or POLLNVAL. (These three bits are meaningless in the events field, and will be set in the revents field whenever the corresponding condition is true.)
If none of the events requested (and no error) has occurred for any of the file descriptors, then poll() blocks until one of the events occurs.
The timeout argument specifies an upper limit on the time for which poll() will block, in milliseconds. Specifying a negative value in timeout means an infinite timeout.
The bits that may be set/returned in events and revents are defined in <poll.h>:
When compiling with _XOPEN_SOURCE defined, one also has the following, which convey no further information beyond the bits listed above:
Linux also knows about, but does not use POLLMSG.
Other than the difference in the precision of the timeout argument, the following ppoll() call:
ready = ppoll(&fds, nfds, timeout_ts, &sigmask);is equivalent to atomically executing the following calls:
sigset_t origmask; int timeout; timeout = (timeout_ts == NULL) ? -1 : (timeout_ts.tv_sec * 1000 + timeout_ts.tv_nsec / 1000000); sigprocmask(SIG_SETMASK, &sigmask, &origmask); ready = poll(&fds, nfds, timeout); sigprocmask(SIG_SETMASK, &origmask, NULL);
See the description of pselect(2) for an explanation of why ppoll() is necessary.
If the sigmask argument is specified as NULL, then no signal mask manipulation is performed (and thus ppoll() differs from poll() only in the precision of the timeout argument).
The timeout_ts argument specifies an upper limit on the amount of time that ppoll() will block. This argument is a pointer to a structure of the following form:
struct timespec { long tv_sec; /* seconds */ long tv_nsec; /* nanoseconds */ };
If timeout_ts is specified as NULL, then ppoll() can block indefinitely.
The ppoll() system call was added to Linux in kernel 2.6.16. The ppoll() library call was added in glibc 2.4.