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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 <sys/poll.h>
int poll(struct pollfd *fds, nfds_t nfds, int timeout);
#define _GNU_SOURCE
#include <sys/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
application is interested in. The field revents is an output parame-
ter, 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
bits that may be set/returned in events and revents are defined in
<sys/poll.h>:
#define POLLIN 0x0001 /* There is data to read */
#define POLLPRI 0x0002 /* There is urgent data to read */
#define POLLOUT 0x0004 /* Writing now will not block */
#define POLLERR 0x0008 /* Error condition (output only) */
#define POLLHUP 0x0010 /* Hung up (output only) */
#define POLLNVAL 0x0020 /* Invalid request: fd not open
(output only) */
When compiling XPG4.2 source one also has:
#ifdef _XOPEN_SOURCE
#define POLLRDNORM 0x0040 /* Normal data may be read */
#define POLLRDBAND 0x0080 /* Priority data may be read */
#define POLLWRNORM 0x0100 /* Writing now will not block */
#define POLLWRBAND 0x0200 /* Priority data may be written */
#endif
Finally, Linux knows about:
#ifdef _GNU_SOURCE
#define POLLMSG 0x0400
#endif
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.
ppoll()
The relationship between poll() and ppoll() is analogous to the rela-
tionship 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 struc-
tures 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 vari-
able 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)
Linux 2.6.16 2006-03-13 POLL(2)
Man(1) output converted with
man2html and wrapped by fishsponge
This page was generated on Tue Feb 13 02:17:35 GMT 2007
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