| IO::Poll allows you to monitor handles for a richer set of conditions than those made available by IO::Select. In addition to watching a handle for incoming data and the ability to accept outgoing data without blocking, IO::Poll allows you to watch handles for two levels of incoming "priority data," for end-of-file conditions, and for several different types of error. Each condition is known as an "event." Each event is designated by one of the constants summarized in Table 16.1. They are divided into constants that can be added to bitmasks sent to poll() using the mask() method, and constants that are returned from poll() via the handles() method. Table 16.1. IO::Poll Mask Constants | | TO poll() | FROM poll() | Description | | Input conditions | | | | | POLLIN | X | X | normal or priority data readable | | POLLRDNORM | X | X | normal data readable | | POLLRDBAND | X | X | priority data readable | | POLLPRI | X | X | high priority data readable | | Output conditions | | | | | POLLOUT | X | X | normal or priority data writable | | POLLWRNORM | X | X | normal data writable | | POLLWRBAND | X | X | priority data writable | | Error conditions | | | | | POLLHUP | | X | hangup has occurred | | POLLNVAL | | X | handle is not open | | POLLERR | | X | error | The following list explains the significance of each event in more detail. POLLIN The handle has data for reading, and sysread () will not block. In the case of a listening socket, POLLIN detects the presence of an incoming connection and accept() will not block. What happens at an end of file varies somewhat among operating systems and is discussed later. POLLRDNORM Like POLLIN , but applies only to normal (nonpriority) data. POLLRDBAND Priority data is available for reading. An attempt to read out-of- band data (Chapter 17) will succeed. POLLPRI "High priority" data is available for reading. High priority data is a historical relic and should not be used for TCP/IP programming. POLLOUT The handle can accept at least 1 byte of data for writing (as modified by the value of the socket's send buffer low water mark, as described in Chapter 12). syswrite() does not block as long as its length does not exceed this value. This event does not distinguish between normal and priority data. POLLWRNORM The handle can accept at least 1 byte of normal (nonpriority) data. POLLWRBAND The handle can accept at least 1 byte of out-of-band data (Chapter 17). POLLERR An error occurred on the handle, such as a PIPE error. For sockets, you may be able to recover the actual error number by calling sockopt() with the SO_ERROR option (Chapter 13). POLLNVAL The handle is invalid. For example, it is closed. POLLHUP In the case of pipes and sockets, the remote process closed the connection. For normal files, this event doesn't apply. There are subtle differences in the behavior of POLLIN and POLLHUP among operating systems and among different types of I/O handles. On many systems, poll() returns POLLIN on a readable handle if an end of file occurs. As you recall, for regular filehandles, this occurs when the end of the file is read. For sockets, this occurs when the peer closes its end of the connection. Unfortunately, this behavior is not universal. On some, if not all, Linux systems, POLLIN is not set when a socket is closed. Instead, you must check for a POLLHUP event. However, POLLHUP is relevant only to sockets and pipes, and does not apply to ordinary filehandles; this makes program logic a bit convoluted. The most reasonable strategy is to recover the handles that may be readable by calling handles with the bitmask POLLINPOLLHUPPOLLERR . Pass each handle to sysread() , and let the return value tell you what state the handle was in. Similarly, it is easiest to check for handles that are writable using the bitmask POLLOUTPOLLERR . The subsequent call to syswrite() will indicate whether the handle is open or has an error. | 
