1EPOLL_WAIT(2) Linux Programmer's Manual EPOLL_WAIT(2)
2
6 epoll_wait, epoll_pwait, epoll_pwait2 - wait for an I/O event on an
7 epoll file 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 int epoll_pwait2(int epfd, struct epoll_event *events,
18 int maxevents, const struct timespec *timeout,
19 const sigset_t *sigmask);
20
22 The epoll_wait() system call waits for events on the epoll(7) instance
23 referred to by the file descriptor epfd. The buffer pointed to by
24 events is used to return information from the ready list about file de‐
25 scriptors in the interest list that have some events available. Up to
26 maxevents are returned by epoll_wait(). The maxevents argument must be
27 greater than zero.
28 The timeout argument specifies the number of milliseconds that
30 epoll_wait() will block. Time is measured against the CLOCK_MONOTONIC
31 clock.
32 A call to epoll_wait() will block until either:
34 • a file descriptor delivers an event;
36 • the call is interrupted by a signal handler; or
38 • the timeout expires.
40 Note that the timeout interval will be rounded up to the system clock
42 granularity, and kernel scheduling delays mean that the blocking inter‐
43 val may overrun by a small amount. Specifying a timeout of -1 causes
44 epoll_wait() to block indefinitely, while specifying a timeout equal to
45 zero cause epoll_wait() to return immediately, even if no events are
46 available.
47 The struct epoll_event is defined as:
49 typedef union epoll_data {
51 void *ptr;
52 int fd;
53 uint32_t u32;
54 uint64_t u64;
55 } epoll_data_t;
56 struct epoll_event {
58 uint32_t events; /* Epoll events */
59 epoll_data_t data; /* User data variable */
60 };
61 The data field of each returned epoll_event structure contains the same
63 data as was specified in the most recent call to epoll_ctl(2)
64 (EPOLL_CTL_ADD, EPOLL_CTL_MOD) for the corresponding open file descrip‐
65 tor.
66 The events field is a bit mask that indicates the events that have oc‐
68 curred for the corresponding open file description. See epoll_ctl(2)
69 for a list of the bits that may appear in this mask.
70 epoll_pwait()
72 The relationship between epoll_wait() and epoll_pwait() is analogous to
73 the relationship between select(2) and pselect(2): like pselect(2),
74 epoll_pwait() allows an application to safely wait until either a file
75 descriptor becomes ready or until a signal is caught.
76 The following epoll_pwait() call:
78 ready = epoll_pwait(epfd, &events, maxevents, timeout, &sigmask);
80 is equivalent to atomically executing the following calls:
82 sigset_t origmask;
84 pthread_sigmask(SIG_SETMASK, &sigmask, &origmask);
86 ready = epoll_wait(epfd, &events, maxevents, timeout);
87 pthread_sigmask(SIG_SETMASK, &origmask, NULL);
88 The sigmask argument may be specified as NULL, in which case
90 epoll_pwait() is equivalent to epoll_wait().
91 epoll_pwait2()
93 The epoll_pwait2() system call is equivalent to epoll_pwait() except
94 for the timeout argument. It takes an argument of type timespec to be
95 able to specify nanosecond resolution timeout. This argument functions
96 the same as in pselect(2) and ppoll(2). If timeout is NULL, then
97 epoll_pwait2() can block indefinitely.
98
100 On success, epoll_wait() returns the number of file descriptors ready
101 for the requested I/O, or zero if no file descriptor became ready dur‐
102 ing the requested timeout milliseconds. On failure, epoll_wait() re‐
103 turns -1 and errno is set to indicate the error.
104
106 EBADF epfd is not a valid file descriptor.
107 EFAULT The memory area pointed to by events is not accessible with
109 write permissions.
110 EINTR The call was interrupted by a signal handler before either (1)
112 any of the requested events occurred or (2) the timeout expired;
113 see signal(7).
114 EINVAL epfd is not an epoll file descriptor, or maxevents is less than
116 or equal to zero.
117
119 epoll_wait() was added to the kernel in version 2.6. Library support
120 is provided in glibc starting with version 2.3.2.
121 epoll_pwait() was added to Linux in kernel 2.6.19. Library support is
123 provided in glibc starting with version 2.6.
124 epoll_pwait2() was added to Linux in kernel 5.11.
126
128 epoll_wait(), epoll_pwait(), and epoll_pwait2() are Linux-specific.
129
131 While one thread is blocked in a call to epoll_wait(), it is possible
132 for another thread to add a file descriptor to the waited-upon epoll
133 instance. If the new file descriptor becomes ready, it will cause the
134 epoll_wait() call to unblock.
135 If more than maxevents file descriptors are ready when epoll_wait() is
137 called, then successive epoll_wait() calls will round robin through the
138 set of ready file descriptors. This behavior helps avoid starvation
139 scenarios, where a process fails to notice that additional file de‐
140 scriptors are ready because it focuses on a set of file descriptors
141 that are already known to be ready.
142 Note that it is possible to call epoll_wait() on an epoll instance
144 whose interest list is currently empty (or whose interest list becomes
145 empty because file descriptors are closed or removed from the interest
146 in another thread). The call will block until some file descriptor is
147 later added to the interest list (in another thread) and that file de‐
148 scriptor becomes ready.
149 C library/kernel differences
151 The raw epoll_pwait() and epoll_pwait2() system calls have a sixth ar‐
152 gument, size_t sigsetsize, which specifies the size in bytes of the
153 sigmask argument. The glibc epoll_pwait() wrapper function specifies
154 this argument as a fixed value (equal to sizeof(sigset_t)).
155
157 In kernels before 2.6.37, a timeout value larger than approximately
158 LONG_MAX / HZ milliseconds is treated as -1 (i.e., infinity). Thus,
159 for example, on a system where sizeof(long) is 4 and the kernel HZ
160 value is 1000, this means that timeouts greater than 35.79 minutes are
161 treated as infinity.
162
164 epoll_create(2), epoll_ctl(2), epoll(7)
165
167 This page is part of release 5.12 of the Linux man-pages project. A
168 description of the project, information about reporting bugs, and the
169 latest version of this page, can be found at
170 https://www.kernel.org/doc/man-pages/.
171Linux 2021-03-22 EPOLL_WAIT(2)