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           #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);


           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.  The call will block until either:
           *  a file descriptor delivers an event;
           *  the call is interrupted by a signal handler; or
           *  the timout 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
           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 of each returned structure will contain the same data the user
           set  with  an  epoll_ctl(2)  (EPOLL_CTL_ADD,  EPOLL_CTL_MOD)  while the
           events member will contain the returned event bit field.
               ready = epoll_wait(epfd, &events, maxevents, timeout);
               sigprocmask(SIG_SETMASK, &origmask, NULL);
           The   sigmask  argument  may  be  specified  as  NULL,  in  which  case
           epoll_pwait() is equivalent to epoll_wait().


           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.


           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.


           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.


           epoll_wait() is Linux-specific.


           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).


           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 the sizeof(long) is 4 and the kernel  HZ
           value  is 1000, this means that timeouts greater than 35.79 minutes are
           treated as infinity.

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