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

NAME

     epoll_wait,  epoll_pwait  -  wait  for  an  I/O  event on an epoll file
     descriptor

SYNOPSIS

     #include <sys/epoll.h>

     int epoll_wait(int epfd, struct epoll_event *events,
                    int maxevents, int timeout);
     int epoll_pwait(int epfd, struct epoll_event *events,
                    int maxevents, int timeout,
                    const sigset_t *sigmask);

DESCRIPTION

     The epoll_wait() system call waits for events on the epoll(7)  instance
     referred to by the file descriptor epfd.  The memory area pointed to by
     events will contain the events that will be available for  the  caller.
     Up  to  maxevents are returned by epoll_wait().  The maxevents argument
     must be greater than zero.

     The  timeout  argument  specifies  the  number  of  milliseconds   that
     epoll_wait()  will block.  Time is measured against the CLOCK_MONOTONIC
     clock.  The call will block until either:

a file descriptor delivers an event;

the call is interrupted by a signal handler; or

the timeout expires.

     Note that the timeout interval will be rounded up to the  system  clock
     granularity, and kernel scheduling delays mean that the blocking inter-
     val may overrun by a small amount.  Specifying a timeout of  -1  causes
     epoll_wait() to block indefinitely, while specifying a timeout equal to
     zero cause epoll_wait() to return immediately, even if  no  events  are
     available.

     The struct epoll_event is defined as:

         typedef union epoll_data {
             void    *ptr;
             int      fd;
             uint32_t u32;
             uint64_t u64; } epoll_data_t;

         struct epoll_event {
             uint32_t     events;    /* Epoll events */
             epoll_data_t data;      /* User data variable */ };

     The data field of each returned structure contains the same data as was
     specified in the  most  recent  call  to  epoll_ctl(2)  (EPOLL_CTL_ADD,
     EPOLL_CTL_MOD) for the corresponding open file description.  The events
     field contains the returned event bit field.

 epoll_pwait()
     The relationship between epoll_wait() and epoll_pwait() is analogous to
     the  relationship  between  select(2)  and pselect(2): like pselect(2),
     epoll_pwait() allows an application to safely wait until either a  file
     descriptor becomes ready or until a signal is caught.

     The following epoll_pwait() call:

         ready = epoll_pwait(epfd, &events, maxevents, timeout, &sigmask);

     is equivalent to atomically executing the following calls:

         sigset_t origmask;

         pthread_sigmask(SIG_SETMASK,    &sigmask,   &origmask);   ready   =
         epoll_wait(epfd,   &events,   maxevents,   timeout);   pthread_sig-
         mask(SIG_SETMASK, &origmask, NULL);

     The   sigmask  argument  may  be  specified  as  NULL,  in  which  case
     epoll_pwait() is equivalent to epoll_wait().

RETURN VALUE

     When successful, epoll_wait() returns the number  of  file  descriptors
     ready for the requested I/O, or zero if no file descriptor became ready
     during the requested  timeout  milliseconds.   When  an  error  occurs,
     epoll_wait() returns -1 and errno is set appropriately.

ERRORS

     EBADF  epfd is not a valid file descriptor.

     EFAULT The  memory  area  pointed  to  by events is not accessible with
            write permissions.

     EINTR  The call was interrupted by a signal handler before  either  (1)
            any of the requested events occurred or (2) the timeout expired;
            see signal(7).

     EINVAL epfd is not an epoll file descriptor, or maxevents is less  than
            or equal to zero.

VERSIONS

     epoll_wait()  was  added to the kernel in version 2.6.  Library support
     is provided in glibc starting with version 2.3.2.

     epoll_pwait() was added to Linux in kernel 2.6.19.  Library support  is
     provided in glibc starting with version 2.6.

CONFORMING TO

     epoll_wait() is Linux-specific.

NOTES

     While  one thread is blocked in a call to epoll_pwait(), it is possible
     for another thread to add a file descriptor to  the  waited-upon  epoll
     instance.   If the new file descriptor becomes ready, it will cause the
     epoll_wait() call to unblock.

     For a discussion of what may happen if a file descriptor  in  an  epoll
     instance  being  monitored by epoll_wait() is closed in another thread,
     see select(2).

BUGS

     In kernels before 2.6.37, a timeout  value  larger  than  approximately
     LONG_MAX  /  HZ  milliseconds is treated as -1 (i.e., infinity).  Thus,
     for example, on a system where sizeof(long) is  4  and  the  kernel  HZ
     value  is 1000, this means that timeouts greater than 35.79 minutes are
     treated as infinity.

 C library/kernel differences
     The raw epoll_pwait() system call has a sixth argument, size_t  sigset-
     size,  which  specifies the size in bytes of the sigmask argument.  The
     glibc epoll_pwait() wrapper function specifies this argument as a fixed
     value (equal to sizeof(sigset_t)).