PR_Poll
=======
Detects when I/O is ready for a set of socket file descriptors.
Syntax
------
.. code::
#include <prio.h>
PRInt32 PR_Poll(
PRPollDesc *pds,
PRIntn npds,
PRIntervalTime timeout);
Parameters
~~~~~~~~~~
The function has the following parameters:
``pds``
A pointer to the first element of an array of ``PRPollDesc``
structures.
``npds``
The number of elements in the ``pds`` array. If this parameter is
zero, :ref:`PR_Poll` is equivalent to :ref:`PR_Sleep` with a timeout.
``timeout``
Amount of time the call will block waiting for I/O to become ready.
If this time expires without any I/O becoming ready, :ref:`PR_Poll`
returns zero.
Returns
~~~~~~~
The function returns one of these values:
- If successful, the function returns a positive number indicating the
number of ``PRPollDesc`` structures in ``pds`` that have events.
- The value 0 indicates the function timed out.
- The value -1 indicates the function failed. The reason for the
failure can be obtained by calling :ref:`PR_GetError`.
Description
~~~~~~~~~~~
This function returns as soon as I/O is ready on one or more of the
underlying socket objects. A count of the number of ready descriptors is
returned unless a timeout occurs, in which case zero is returned.
The ``in_flags`` field of the ``PRPollDesc`` data structure should be
set to the I/O events (readable, writable, exception, or some
combination) that the caller is interested in. On successful return, the
``out_flags`` field of the ``PRPollDesc`` data structure is set to
indicate what kind of I/O is ready on the respective descriptor.
:ref:`PR_Poll` uses the ``out_flags`` fields as scratch variables during
the call. If :ref:`PR_Poll` returns 0 or -1, the ``out_flags`` fields do
not contain meaningful values and must not be used.
The ``PRPollDesc`` structure is defined as follows:
.. code::
struct PRPollDesc {
PRFileDesc* fd;
PRInt16 in_flags;
PRInt16 out_flags;
};
typedef struct PRPollDesc PRPollDesc;
The structure has the following fields:
``fd``
A pointer to a :ref:`PRFileDesc` object representing a socket or a
pollable event. This field can be set to ``NULL`` to indicate to
:ref:`PR_Poll` that this ``PRFileDesc object`` should be ignored.
.. note::
On Unix, the ``fd`` field can be set to a pointer to any
:ref:`PRFileDesc` object, including one representing a file or a
pipe. Cross-platform applications should only set the ``fd`` field
to a pointer to a :ref:`PRFileDesc` object representing a socket or a
pollable event because on Windows the ``select`` function can only
be used with sockets.
``in_flags``
A bitwise ``OR`` of the following bit flags:
- :ref:`PR_POLL_READ`: ``fd`` is readable.
- :ref:`PR_POLL_WRITE`: ``fd`` is writable.
- :ref:`PR_POLL_EXCEPT`: ``fd`` has an exception condition.
``out_flags``
A bitwise ``OR`` of the following bit flags:
- :ref:`PR_POLL_READ`
- :ref:`PR_POLL_WRITE`
- :ref:`PR_POLL_EXCEPT`
- :ref:`PR_POLL_ERR`: ``fd`` has an error.
- :ref:`PR_POLL_NVAL`: ``fd`` is bad.
Note that the ``PR_POLL_ERR`` and ``PR_POLL_NVAL`` flags are used only
in ``out_flags``. The ``PR_POLL_ERR`` and ``PR_POLL_NVAL`` events are
always reported by :ref:`PR_Poll`.
