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poll(2) - poll, poll - wait for some event on a file descriptor - man 2 poll

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POLL(2)                    Linux Programmer's Manual                   POLL(2)

       poll - wait for some event on a file(1,n) descriptor

       #include <sys/poll.h>

       int poll(struct pollfd *ufds, unsigned int nfds, int timeout(1,3x,3x cbreak));

       poll  is  a variation on the theme of select(2,7,2 select_tut).  It specifies an array of
       nfds structures of type
               struct pollfd {
                       int fd;           /* file(1,n) descriptor */
                       short events;     /* requested events */
                       short revents;    /* returned events */
       and a timeout(1,3x,3x cbreak) in(1,8) milliseconds. A negative value means infinite timeout.
       The  field  fd  contains a file(1,n) descriptor for an open(2,3,n) file.  The field
       events is an input parameter,  a  bitmask  specifying  the  events  the
       application  is  interested in.  The field revents is an output parame-
       ter, 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(1,8)  the  events  field,
       and  will be set(7,n,1 builtins) in(1,8) the revents field whenever the corresponding condi-
       tion is true.)  If none of the events  requested  (and  no  error(8,n))  has
       occurred  for any of the file(1,n) descriptors, the kernel waits for timeout(1,3x,3x cbreak)
       milliseconds for one of these events to occur.  The following  possible
       bits in(1,8) these masks are defined in(1,8) <sys/poll.h>
           #define POLLIN      0x0001    /* There is data to read(2,n,1 builtins) */
           #define POLLPRI     0x0002    /* There is urgent data to read(2,n,1 builtins) */
           #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(2,3,n) */
       When compiling XPG4.2 source one also has
       #ifdef _XOPEN_SOURCE
           #define POLLRDNORM  0x0040    /* Normal data may be read(2,n,1 builtins) */
           #define POLLRDBAND  0x0080    /* Priority data may be read(2,n,1 builtins) */
           #define POLLWRNORM  0x0100    /* Writing now will not block */
           #define POLLWRBAND  0x0200    /* Priority data may be written */
       Finally, Linux knows about
       #ifdef _GNU_SOURCE
           #define POLLMSG     0x0400

       On success, a positive number is returned, where the number returned is
       the number of structures which have non-zero revents fields  (in(1,8)  other
       words, those descriptors with events or errors reported).  A value of 0
       indicates that the call timed out and no  file(1,n)  descriptors  have  been
       selected. On error(8,n), -1 is returned, and errno is set(7,n,1 builtins) appropriately.

       EBADF  An invalid file(1,n) descriptor was given in(1,8) one of the sets.

       EFAULT The  array  given  as  argument was not contained in(1,8) the calling
              program's address space.

       EINTR  A signal(2,7) occurred before any requested event.

       EINVAL The nfds value exceeds the RLIMIT_NOFILE value.

       ENOMEM There was no space to allocate file(1,n) descriptor tables.

       See the BUGS section of select(2,7,2 select_tut)(2).


       The poll() systemcall was  introduced  in(1,8)  Linux  2.1.23.   The  poll()
       library  call  was  introduced  in(1,8)  libc 5.4.28 (and provides emulation
       using select(2,7,2 select_tut) if(3,n) your kernel does not have a poll syscall).

       select(2,7,2 select_tut)(2), select_tut(2)

Linux 2.1.23                      1997-12-07                           POLL(2)

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