POLL (2)
wait for some event on a file descriptor
SYNOPSIS
#include <sys/poll.h>
int poll(struct pollfd * ufds , unsigned int nfds , int timeout );
DESCRIPTION
poll
is a variation on the theme of
select .
It specifies an array of
nfds
structures of type
struct pollfd {
int fd; /* file descriptor */
short events; /* requested events */
short revents; /* returned events */
};
and a
timeout
in milliseconds. A negative value means infinite timeout.
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 parameter, filled by the kernel with the events that
actually occurred, either of the type requested, or of one of the
types
POLLERR
or
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, the kernel waits for
timeout
milliseconds for one of these events to occur.
The following possible bits in these masks 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 */
#define POLLHUP 0x0010 /* Hung up */
#define POLLNVAL 0x0020 /* Invalid request: fd not open */
In <asm/poll.h> also the values
POLLRDNORM ,
POLLRDBAND ,
POLLWRNORM ,
POLLWRBAND
and
POLLMSG
are defined.
RETURN VALUE
On success, a positive number is returned, where the number returned
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 have been selected. On error, -1 is returned, and
errno
is set appropriately.
ERRORS
ENOMEM
There was no space to allocate file descriptor tables.
EFAULT
The array given as argument was not contained in the calling program's
address space.
EINTR
A signal occurred before any requested event.
CONFORMING TO
AVAILABILITY
The poll() systemcall 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 syscall).
SEE ALSO
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