poll, ppoll — wait for some event on a file descriptor
#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); |
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
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>:
POLLINThere is data to read.
POLLPRIThere 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).
POLLOUTWriting now will not block.
POLLRDHUP(since Linux 2.6.17)Stream socket peer closed connection, or shut down writing half of connection. The
_GNU_SOURCEfeature test macro must be defined in order to obtain this definition.POLLERRError condition (output only).
POLLHUPHang up (output only).
POLLNVALInvalid request:
fdnot open (output only).
When compiling with _XOPEN_SOURCE defined, one also has the
following, which convey no further information beyond the
bits listed above:
POLLRDNORMEquivalent to
POLLIN.POLLRDBANDPriority band data can be read (generally unused on Linux).
POLLWRNORMEquivalent to
POLLOUT.POLLWRBANDPriority data may be written.
Linux also knows about, but does not use POLLMSG.
The relationship between poll() and ppoll() is analogous to the relationship
between select(2) and pselect(2): like pselect(2), 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 = poll(&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.
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.
An invalid file descriptor was given in one of the sets.
The array given as argument was not contained in the calling program's address space.
A signal occurred before any requested event.
The nfds
value exceeds the RLIMIT_NOFILE value.
There was no space to allocate file descriptor tables.
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.
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(2) 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.
Some implementations define the non-standard constant
INFTIM with the value −1
for use as a timeout.
This constant is not provided in glibc.
select(2), select_tut(2), feature_test_macros(7)
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