View Source: poll(2) - Waikato Linux Users Group

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!!NAME
poll - wait for some event on a file descriptor

!!SYNOPSIS
 __#include <sys/poll.h>__

 __int poll(struct pollfd *__''ufds''__, unsigned int__ ''nfds''__, int__ ''timeout''__);__

!!DESCRIPTION
poll(2) is a variation on the theme of __select__. It specifies an array of ''nfds'' structures of type

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

and a ''timeout'' in milliseconds. A negative value means infinite timeout. 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, either of the type requested, or of one of the types __POLLERR__ or __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, the kernel waits for ''timeout'' milliseconds for one of these events to occur. The following possible bits in these masks are defined in '<sys/poll.h>'

 #define POLLIN      0x0001    /* There is data to read */
 #define POLLPRI     0x0002    /* There is urgent data to read */
 #define POLLOUT     0x0004    /* Writing now will not block */
 #define POLLERR     0x0008    /* Error condition */
 #define POLLHUP     0x0010    /* Hung up */
 #define POLLNVAL    0x0020    /* Invalid request: fd not open */

In POLLRDNORM__, __POLLRDBAND__, __POLLWRNORM__, __POLLWRBAND__ and __POLLMSG__ are defined.

!!RETURN VALUE
On success, a positive number is returned, where the number returned 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 have been selected. On error, -1 is returned, and ''errno'' is set
appropriately.

!!ERRORS
;[EBADF]: An invalid file descriptor was given in one of the sets.
;[ENOMEM]: There was no space to allocate file descriptor tables.
;[EFAULT]: The array given as argument was not contained in the calling program's address space.
;[EINTR]: A signal occurred before any requested event.

!!CONFORMING TO
XPG4-UNIX.

!!AVAILABILITY
The poll() systemcall 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 syscall).

!!SEE ALSO
select(2)

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Last edited on Sunday, March 16, 2003 6:35:01 pm by PerryLorier

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