Manpages - poll.2

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 /* See feature_test_macros(7) */
  #include <poll.h>

  int ppoll(struct pollfd *fds, nfds_t nfds,
   const struct timespec *tmo_p, 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 Linux-specific *epoll*(7) API performs a similar task, but offers features beyond those found in *poll*().

The set of file descriptors to be monitored is specified in the fds argument, which is an array of structures of the following form:

  struct pollfd {
      int   fd;         /* file descriptor */
      short events;     /* requested events */
      short revents;    /* returned events */
  };

The caller should specify the number of items in the fds array in nfds.

The field fd contains a file descriptor for an open file. If this field is negative, then the corresponding events field is ignored and the revents field returns zero. (This provides an easy way of ignoring a file descriptor for a single *poll*() call: simply negate the fd field. Note, however, that this technique can’t be used to ignore file descriptor 0.)

The field events is an input parameter, a bit mask specifying the events the application is interested in for the file descriptor fd. This field may be specified as zero, in which case the only events that can be returned in revents are POLLHUP, POLLERR, and POLLNVAL (see below).

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 the number of milliseconds that *poll*() should block waiting for a file descriptor to become ready. The call will block until either:

• a file descriptor becomes ready;
• the call is interrupted by a signal handler; or
• the timeout expires.

Note that the timeout interval will be rounded up to the system clock granularity, and kernel scheduling delays mean that the blocking interval may overrun by a small amount. Specifying a negative value in timeout means an infinite timeout. Specifying a timeout of zero causes *poll*() to return immediately, even if no file descriptors are ready.

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 some exceptional condition on the file descriptor. Possibilities include:

• There is out-of-band data on a TCP socket (see *tcp*(7)).
• A pseudoterminal master in packet mode has seen a state change on the slave (see *ioctl_tty*(2)).
• A cgroup.events file has been modified (see *cgroups*(7)).

POLLOUT

Writing is now possible, though a write larger than the available space in a socket or pipe will still block (unless O_NONBLOCK is set).

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 (before including any header files) in order to obtain this definition.

POLLERR

Error condition (only returned in revents; ignored in events). This bit is also set for a file descriptor referring to the write end of a pipe when the read end has been closed.

POLLHUP

Hang up (only returned in revents; ignored in events). Note that when reading from a channel such as a pipe or a stream socket, this event merely indicates that the peer closed its end of the channel. Subsequent reads from the channel will return 0 (end of file) only after all outstanding data in the channel has been consumed.

POLLNVAL

Invalid request: fd not open (only returned in revents; ignored in events).

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.

On success, *poll*() returns a nonnegative value which is the number of elements in the pollfds whose revents fields have been set to a nonzero value (indicating an event or an error). A return value of zero indicates that the system call timed out before any file descriptors became read.

On error, -1 is returned, and errno is set to indicate the error.

EFAULT

fds points outside the process’s accessible address space. The array given as argument was not contained in the calling program’s address space.

EINTR

A signal occurred before any requested event; see *signal*(7).

EINVAL

The nfds value exceeds the RLIMIT_NOFILE value.

EINVAL

(*ppoll*()) The timeout value expressed in *ip is invalid (negative).

ENOMEM

Unable to allocate memory for kernel data structures.

The *poll*() system call was introduced in Linux 2.1.23. On older kernels that lack this system call, the glibc *poll*() wrapper function provides emulation using *select*(2).

The *ppoll*() system call was added to Linux in kernel 2.6.16. The *ppoll*() library call was added in glibc 2.4.

*poll*() conforms to POSIX.1-2001 and POSIX.1-2008. *ppoll*() is Linux-specific.

The operation of poll*() and *ppoll*() is not affected by the *O_NONBLOCK flag.

On some other UNIX systems, poll*() can fail with the error *EAGAIN if the system fails to allocate kernel-internal resources, rather than ENOMEM as Linux does. POSIX permits this behavior. Portable programs may wish to check for EAGAIN and loop, just as with EINTR.

Some implementations define the nonstandard constant INFTIM with the value -1 for use as a timeout for *poll*(). This constant is not provided in glibc.

For a discussion of what may happen if a file descriptor being monitored by *poll*() is closed in another thread, see *select*(2).

See the discussion of spurious readiness notifications under the BUGS section of *select*(2).

The program below opens each of the files named in its command-line arguments and monitors the resulting file descriptors for readiness to read (POLLIN). The program loops, repeatedly using *poll*() to monitor the file descriptors, printing the number of ready file descriptors on return. For each ready file descriptor, the program:

• displays the returned revents field in a human-readable form;
• if the file descriptor is readable, reads some data from it, and displays that data on standard output; and
• if the file descriptors was not readable, but some other event occurred (presumably POLLHUP), closes the file descriptor.

Suppose we run the program in one terminal, asking it to open a FIFO:

  $ mkfifo myfifo
  $ ./poll_input myfifo

In a second terminal window, we then open the FIFO for writing, write some data to it, and close the FIFO:

  $ echo aaaaabbbbbccccc > myfifo

In the terminal where we are running the program, we would then see:

  Opened "myfifo" on fd 3
  About to poll()
  Ready: 1
    fd=3; events: POLLIN POLLHUP
      read 10 bytes: aaaaabbbbb
  About to poll()
  Ready: 1
    fd=3; events: POLLIN POLLHUP
      read 6 bytes: ccccc

  About to poll()
  Ready: 1
    fd=3; events: POLLHUP
      closing fd 3
  All file descriptors closed; bye

In the above output, we see that *poll*() returned three times:

• On the first return, the bits returned in the revents field were POLLIN, indicating that the file descriptor is readable, and POLLHUP, indicating that the other end of the FIFO has been closed. The program then consumed some of the available input.
• The second return from poll*() also indicated *POLLIN and POLLHUP; the program then consumed the last of the available input.
• On the final return, poll*() indicated only *POLLHUP on the FIFO, at which point the file descriptor was closed and the program terminated.

*restart_syscall*(2), *select*(2), *select_tut*(2), *epoll*(7), *time*(7)

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