POLL

NAME

SYNOPSIS

#include <poll.h>

int poll(struct pollfd *fds, nfds_t nfds, int timeout);

#define _GNU_SOURCE
#include <poll.h>

int ppoll(struct pollfd *fds, nfds_t nfds, 
        const struct timespec *timeout, const sigset_t *sigmask);

DESCRIPTION

The set of file descriptors to be monitored is specified in the fds argument, which is an array of nfds structures of the following form:

struct pollfd {
    int   fd;         /* file descriptor */
    short events;     /* requested events */
    short revents;    /* returned events */
};

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>:

POLLIN

There is data to read.

POLLPRI

There is urgent data to read (e.g., out-of-band data on TCP socket; pseudo-terminal master in packet mode has seen state change in slave).

POLLOUT

Writing now will not block.

POLLRDHUP (since Linux 2.6.17)

Stream socket peer closed connection, or shut down writing half of connection. The _GNU_SOURCE feature test macro must be defined 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()

Other than the difference in the timeout argument, the following ppoll() call:

    ready = ppoll(&fds, nfds, timeout, &sigmask);

    sigset_t origmask;

    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 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 is specified as NULL, then ppoll() can block indefinitely.  

RETURN VALUE

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 ppoll() system call was added to Linux in kernel 2.6.16. The ppoll() library call was added in glibc 2.4.  

CONFORMING TO

NOTES

Linux Notes

BUGS

SEE ALSO

COLOPHON