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POLL(2) Linux Programmer's Manual POLL(2) NAME poll, ppoll - wait for some event on a file descriptor SYNOPSIS #include <poll.h> int poll(struct pollfd *fds, nfds_t nfds, int timeout); #define _GNU_SOURCE /* See feature_test_macros(7) */ #include <signal.h> #include <poll.h> int ppoll(struct pollfd *fds, nfds_t nfds, const struct timespec *timeout_ts, const sigset_t *sigmask); DESCRIPTION poll() performs a similar task to select(2): it waits for one of a set of file descriptors to become ready to perform I/O. The set of file descriptors to be monitored is specified in the fds ar- gument, 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. If this field is negative, then the corresponding events field is ignored and the revents field returns zero. (This provides an easy way of ignoring a file descriptor for a single poll() call: simply negate the fd field. Note, however, that this technique can't be used to ignore file de- scriptor 0.) The field events is an input parameter, a bit mask specifying the events the application is interested in for the file descriptor fd. This field may be specified as zero, in which case the only events that can be returned in revents are POLLHUP, POLLERR, and POLLNVAL (see be- low). The field revents is an output parameter, filled by the kernel with the events that actually occurred. The bits returned in revents can in- clude 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 oc- curs. The timeout argument specifies the number of milliseconds that poll() should block waiting for a file descriptor to become ready. The call will block until either: * a file descriptor becomes ready; * the call is interrupted by a signal handler; or * the timeout expires. Note that the timeout interval will be rounded up to the system clock granularity, and kernel scheduling delays mean that the blocking inter- val may overrun by a small amount. Specifying a negative value in timeout means an infinite timeout. Specifying a timeout of zero causes poll() to return immediately, even if no file descriptors are ready. The bits that may be set/returned in events and revents are defined in _poll.h_: POLLIN There is data to read. POLLPRI There is urgent data to read (e.g., out-of-band data on TCP socket; pseudoterminal master in packet mode has seen state change in slave). POLLOUT Writing is now possible, though a write larger that the available space in a socket or pipe will still block (un- less O_NONBLOCK is set). POLLRDHUP (since Linux 2.6.17) Stream socket peer closed connection, or shut down writ- ing half of connection. The _GNU_SOURCE feature test macro must be defined (before including any header files) in order to obtain this definition. POLLERR Error condition (output only). POLLHUP Hang up (output only). POLLNVAL Invalid request: fd not open (output only). When compiling with _XOPEN_SOURCE defined, one also has the following, which convey no further information beyond the bits listed above: POLLRDNORM Equivalent to POLLIN. POLLRDBAND Priority band data can be read (generally unused on Linux). POLLWRNORM Equivalent to POLLOUT. POLLWRBAND Priority data may be written. Linux also knows about, but does not use POLLMSG. ppoll() The relationship between poll() and ppoll() is analogous to the rela- tionship between select(2) and pselect(2): like pselect(2), ppoll() al- lows an application to safely wait until either a file descriptor be- comes ready or until a signal is caught. 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 ma- nipulation 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 indefi- nitely. RETURN VALUE On success, a positive number is returned; this is the number of struc- tures which have nonzero revents fields (in other words, those descrip- tors with events or errors reported). A value of 0 indicates that the call timed out and no file descriptors were ready. On error, -1 is re- turned, and errno is set appropriately. ERRORS EFAULT The array given as argument was not contained in the calling program's address space. EINTR A signal occurred before any requested event; see signal(7). EINVAL The nfds value exceeds the RLIMIT_NOFILE value. ENOMEM There was no space to allocate file descriptor tables. VERSIONS The poll() system call was introduced in Linux 2.1.23. On older ker- nels that lack this system call, the glibc (and the old Linux libc) poll() wrapper function provides emulation using select(2). The ppoll() system call was added to Linux in kernel 2.6.16. The ppoll() library call was added in glibc 2.4. CONFORMING TO poll() conforms to POSIX.1-2001. ppoll() is Linux-specific. NOTES Some implementations define the nonstandard constant INFTIM with the value -1 for use as a timeout for poll(). This constant is not pro- vided in glibc. For a discussion of what may happen if a file descriptor being moni- tored by poll() is closed in another thread, see select(2). C library/kernel ABI differences The Linux ppoll() system call modifies its timeout_ts argument. How- ever, the glibc wrapper function hides this behavior by using a local variable for the timeout argument that is passed to the system call. Thus, the glibc ppoll() function does not modify its timeout_ts argu- ment. The raw ppoll() system call has a fifth argument, size_t sigsetsize, which specifies the size in bytes of the sigmask argument. The glibc ppoll() wrapper function specifies this argument as a fixed value (equal to sizeof(sigset_t)). BUGS See the discussion of spurious readiness notifications under the BUGS section of select(2). SEE ALSO restart_syscall(2), select(2), select_tut(2), time(7) COLOPHON This page is part of release 3.74 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at http://www.kernel.org/doc/man-pages/. Linux 2014-09-06 POLL(2)
NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | CONFORMING TO | NOTES | BUGS | SEE ALSO | COLOPHON
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