poll() - Unix, Linux System Call
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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);
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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 */
};
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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>:
Tag | Description |
POLLIN |
There is data to read.
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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).
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POLLOUT | |
Writing now will not block.
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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.
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POLLERR | |
Error condition (output only).
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POLLHUP | |
Hang up (output only).
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POLLNVAL | |
Invalid request:
fd not open (output only).
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When compiling with
_XOPEN_SOURCE defined, one also has the following,
which convey no further information beyond the bits listed above:
Tag | Description |
POLLRDNORM | |
Equivalent to
POLLIN. |
POLLRDBAND | |
Priority band data can be read (generally unused on Linux).
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POLLWRNORM | |
Equivalent to
POLLOUT. |
POLLWRBAND | |
Priority data may be written.
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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);
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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);
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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 */
};
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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
Tag | Description |
EBADF |
An invalid file descriptor was given in one of the sets.
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EFAULT |
The array given as argument was not contained in the calling programs
address space.
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EINTR |
A signal occurred before any requested event.
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EINVAL |
The
nfds value exceeds the RLIMIT_NOFILE value.
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ENOMEM |
There was no space to allocate file descriptor tables.
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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
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