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# poll() - Unix, Linux System Call

# 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);

# 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 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.
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).
POLLOUT
| Writing now will not block.
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.
POLLERR
| Error condition (output only).
POLLHUP
| Hang up (output only).
POLLNVAL
| Invalid request: fd not open (output only).

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).
POLLWRNORM
| Equivalent to POLLOUT.
POLLWRBAND
| Priority data may be written.

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);

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 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.
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 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

- select (2)

- select_tut (2)

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