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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
#include <poll.h>
int ppoll(struct pollfd *fds, nfds_t nfds,
const struct timespec *timeout, 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 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 fd contains a file descriptor for an open file.
The field events is an input parameter, a bitmask specifying the events the appli-
cation 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()
The relationship between poll() and ppoll() is analogous to the relationship
between select() and pselect(): like pselect(), ppoll() allows an application to
safely wait until either a file descriptor becomes ready or until a signal is
caught.
Other than the difference in the timeout argument, the following ppoll() call:
ready = ppoll(&fds, nfds, timeout, &sigmask);
is equivalent to atomically executing the following calls:
sigset_t origmask;
sigprocmask(SIG_SETMASK, &sigmask, &origmask);
ready = ppoll(&fds, nfds, timeout);
sigprocmask(SIG_SETMASK, &origmask, NULL);
See the description of pselect(2) for an explanation of why ppoll() is necessary.
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
On success, a positive number is returned; this is the number of structures which
have non-zero revents fields (in other words, those descriptors 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 returned, and errno is set appropriately.
ERRORS
EBADF An invalid file descriptor was given in one of the sets.
EFAULT The array given as argument was not contained in the calling program's
address space.
EINTR A signal occurred before any requested event.
EINVAL The nfds value exceeds the RLIMIT_NOFILE value.
ENOMEM There was no space to allocate file descriptor tables.
LINUX NOTES
The Linux ppoll() system call modifies its timeout argument. However, the glibc
wrapper function hides this behaviour 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 argument.
BUGS
See the discussion of spurious readiness notifications under the BUGS section of
select(2).
CONFORMING TO
poll() conforms to POSIX.1-2001. ppoll() is Linux specific.
VERSIONS
The poll() system call was introduced in Linux 2.1.23. The poll() library call was
introduced in libc 5.4.28 (and provides emulation using select() if your kernel
does not have a poll() system call).
The ppoll() system call was added to Linux in kernel 2.6.16. The ppoll() library
call was added in glibc 2.4.
NOTES
Some implementations define the non-standard constant INFTIM with the value -1 for
use as a timeout. This constant is not provided in glibc.
SEE ALSO
select(2), select_tut(2), feature_test_macros(7)
Linux 2.6.16 2006-03-13 POLL(2)
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