1EPOLL_WAIT(2) Linux Programmer's Manual EPOLL_WAIT(2)
2
6 epoll_wait, epoll_pwait - wait for an I/O event on an epoll file
7 descriptor
8
10 #include <sys/epoll.h>
11 int epoll_wait(int epfd, struct epoll_event *events,
13 int maxevents, int timeout);
14 int epoll_pwait(int epfd, struct epoll_event *events,
15 int maxevents, int timeout,
16 const sigset_t *sigmask);
17
19 The epoll_wait() system call waits for events on the epoll(7) instance
20 referred to by the file descriptor epfd. The memory area pointed to by
21 events will contain the events that will be available for the caller.
22 Up to maxevents are returned by epoll_wait(). The maxevents argument
23 must be greater than zero.
24 The timeout argument specifies the minimum number of milliseconds that
26 epoll_wait() will block. (This interval will be rounded up to the sys‐
27 tem clock granularity, and kernel scheduling delays mean that the
28 blocking interval may overrun by a small amount.) Specifying a timeout
29 of -1 causes epoll_wait() to block indefinitely, while specifying a
30 timeout equal to zero cause epoll_wait() to return immediately, even if
31 no events are available.
32 The struct epoll_event is defined as :
34 typedef union epoll_data {
36 void *ptr;
37 int fd;
38 uint32_t u32;
39 uint64_t u64;
40 } epoll_data_t;
41 struct epoll_event {
43 uint32_t events; /* Epoll events */
44 epoll_data_t data; /* User data variable */
45 };
46 The data of each returned structure will contain the same data the user
48 set with an epoll_ctl(2) (EPOLL_CTL_ADD,EPOLL_CTL_MOD) while the events
49 member will contain the returned event bit field.
50 epoll_pwait()
52 The relationship between epoll_wait() and epoll_pwait() is analogous to
53 the relationship between select(2) and pselect(2): like pselect(2),
54 epoll_pwait() allows an application to safely wait until either a file
55 descriptor becomes ready or until a signal is caught.
56 The following epoll_pwait() call:
58 ready = epoll_pwait(epfd, &events, maxevents, timeout, &sigmask);
60 is equivalent to atomically executing the following calls:
62 sigset_t origmask;
64 sigprocmask(SIG_SETMASK, &sigmask, &origmask);
66 ready = epoll_wait(epfd, &events, maxevents, timeout);
67 sigprocmask(SIG_SETMASK, &origmask, NULL);
68 The sigmask argument may be specified as NULL, in which case
70 epoll_pwait() is equivalent to epoll_wait().
71
73 When successful, epoll_wait() returns the number of file descriptors
74 ready for the requested I/O, or zero if no file descriptor became ready
75 during the requested timeout milliseconds. When an error occurs,
76 epoll_wait() returns -1 and errno is set appropriately.
77
79 EBADF epfd is not a valid file descriptor.
80 EFAULT The memory area pointed to by events is not accessible with
82 write permissions.
83 EINTR The call was interrupted by a signal handler before either any
85 of the requested events occurred or the timeout expired; see
86 signal(7).
87 EINVAL epfd is not an epoll file descriptor, or maxevents is less than
89 or equal to zero.
90
92 epoll_wait() was added to the kernel in version 2.6. Library support
93 is provided in glibc starting with version 2.3.2.
94 epoll_pwait() was added to Linux in kernel 2.6.19. Library support is
96 provided in glibc starting with version 2.6.
97
99 epoll_wait() is Linux-specific.
100
102 While one thread is blocked in a call to epoll_pwait(), it is possible
103 for another thread to add a file descriptor to the waited-upon epoll
104 instance. If the new file descriptor becomes ready, it will cause the
105 epoll_wait() call to unblock.
106 For a discussion of what may happen if a file descriptor in an epoll
108 instance being monitored by epoll_wait() is closed in another thread,
109 see select(2).
110
112 In kernels before 2.6.37, a timeout value larger than approximately
113 LONG_MAX / HZ milliseconds is treated as -1 (i.e., infinity). Thus,
114 for example, on a system where the sizeof(long) is 4 and the kernel HZ
115 value is 1000, this means that timeouts greater than 35.79 minutes are
116 treated as infinity.
117
119 epoll_create(2), epoll_ctl(2), epoll(7)
120
122 This page is part of release 3.53 of the Linux man-pages project. A
123 description of the project, and information about reporting bugs, can
124 be found at http://www.kernel.org/doc/man-pages/.
125Linux 2012-08-17 EPOLL_WAIT(2)