#include <sys/eventfd.h>
int eventfd(unsigned int initval, int flags);
eventfd() creates an "eventfd object" that can be used as an event wait/notify mechanism by user-space applications, and by the kernel to
notify user-space applications of events. The object contains an unsigned 64-bit integer (uint64_t) counter that is maintained by the ker‐
nel. This counter is initialized with the value specified in the argument initval.
The following values may be bitwise ORed in flags to change the behaviour of eventfd():
EFD_CLOEXEC (since Linux 2.6.27)
Set the close-on-exec (FD_CLOEXEC) flag on the new file descriptor. See the description of the O_CLOEXEC flag in open(2) for rea‐
sons why this may be useful.
EFD_NONBLOCK (since Linux 2.6.27)
Set the O_NONBLOCK file status flag on the new open file description. Using this flag saves extra calls to fcntl(2) to achieve the
same result.
EFD_SEMAPHORE (since Linux 2.6.30)
Provide semaphore-like semantics for reads from the new file descriptor. See below.
In Linux up to version 2.6.26, the flags argument is unused, and must be specified as zero.
As its return value, eventfd() returns a new file descriptor that can be used to refer to the eventfd object. The following operations can
be performed on the file descriptor:
read(2)
Each successful read(2) returns an 8-byte integer. A read(2) will fail with the error if the size of the supplied buffer is
less than 8 bytes.
The value returned by read(2) is in host byte order, i.e., the native byte order for integers on the host machine.
The semantics of read(2) depend on whether the eventfd counter currently has a nonzero value and whether the EFD_SEMAPHORE flag was
specified when creating the eventfd file descriptor:
* If EFD_SEMAPHORE was not specified and the eventfd counter has a nonzero value, then a read(2) returns 8 bytes containing that
value, and the counter's value is reset to zero.
如果没有指定EFD_SEMAPHORE，同时eventfd计数器的值非零，那么read将会读出计数器的值，该值的长度为8个字节。
* If EFD_SEMAPHORE was specified and the eventfd counter has a nonzero value, then a read(2) returns 8 bytes containing the value
1, and the counter's value is decremented by 1.
如果指定了EFD_SEMAPHORE，同时eventfd计数器的值非零，那么read读出的值只为1，并且eventfd计数器的值减1.
综上所述，没有指定EFD_SEMAPHORE，read一次读出计数器的值，计数器的值归零。如果指定了EFD_SEMAPHORE，read一次只返回1，计数器只减1.
那么问题来了，一般EFD_SEMAPHORE用在什么场景呢?
* If the eventfd counter is zero at the time of the call to read(2), then the call either blocks until the counter becomes nonzero
(at which time, the read(2) proceeds as described above) or fails with the error EAGAIN if the file descriptor has been made non‐
blocking.
当计数器的值为0时，调用read函数，将会阻塞，直到计数器的值非零。如果文件描述符设置为非阻塞，那么会失败并返回EAGAIN.
write(2)
A write(2) call adds the 8-byte integer value supplied in its buffer to the counter. The maximum value that may be stored in the
counter is the largest unsigned 64-bit value minus 1 (i.e., 0xfffffffffffffffe). If the addition would cause the counter's value to
exceed the maximum, then the write(2) either blocks until a read(2) is performed on the file descriptor, or fails with the error
EAGAIN if the file descriptor has been made nonblocking.
如果计数器的值超过最大值，write函数将会阻塞，直到read函数被调用。如果fd设置为非阻塞，那么失败并返回EAGAIN.
A write(2) will fail with the error EINVAL if the size of the supplied buffer is less than 8 bytes, or if an attempt is made to
write the value 0xffffffffffffffff.
poll(2), select(2) (and similar)
The returned file descriptor supports poll(2) (and analogously epoll(7)) and select(2), as follows:
* The file descriptor is readable (the select(2) readfds argument; the poll(2) POLLIN flag) if the counter has a value greater than
0.
* The file descriptor is writable (the select(2) writefds argument; the poll(2) POLLOUT flag) if it is possible to write a value of
at least "1" without blocking.
* If an overflow of the counter value was detected, then select(2) indicates the file descriptor as being both readable and
writable, and poll(2) returns a POLLERR event. As noted above, write(2) can never overflow the counter. However an overflow can
occur if 2^64 eventfd "signal posts" were performed by the KAIO subsystem (theoretically possible, but practically unlikely). If
an overflow has occurred, then read(2) will return that maximum uint64_t value (i.e., 0xffffffffffffffff).
The eventfd file descriptor also supports the other file-descriptor multiplexing APIs: pselect(2) and ppoll(2).
close(2)
When the file descriptor is no longer required it should be closed. When all file descriptors associated with the same eventfd
object have been closed, the resources for object are freed by the kernel.
A copy of the file descriptor created by eventfd() is inherited by the child produced by fork(2). The duplicate file descriptor is associ‐
ated with the same eventfd object. File descriptors created by eventfd() are preserved across execve(2), unless the close-on-exec flag has
been set.
RETURN VALUE
On success, eventfd() returns a new eventfd file descriptor. On error, -1 is returned and errno is set to indicate the error.
ERRORS
EINVAL An unsupported value was specified in flags.
EMFILE The per-process limit on open file descriptors has been reached.
ENFILE The system-wide limit on the total number of open files has been reached.
ENODEV Could not mount (internal) anonymous inode device.
ENOMEM There was insufficient memory to create a new eventfd file descriptor.
VERSIONS
eventfd() is available on Linux since kernel 2.6.22. Working support is provided in glibc since version 2.8. The eventfd2() system call
(see NOTES) is available on Linux since kernel 2.6.27. Since version 2.9, the glibc eventfd() wrapper will employ the eventfd2() system
call, if it is supported by the kernel.
NOTES
Applications can use an eventfd file descriptor instead of a pipe (see pipe(2)) in all cases where a pipe is used simply to signal events.
The kernel overhead of an eventfd file descriptor is much lower than that of a pipe, and only one file descriptor is required (versus the
two required for a pipe).
When used in the kernel, an eventfd file descriptor can provide a bridge from kernel to user space, allowing, for example, functionalities
like KAIO (kernel AIO) to signal to a file descriptor that some operation is complete.
A key point about an eventfd file descriptor is that it can be monitored just like any other file descriptor using select(2), poll(2), or
epoll(7). This means that an application can simultaneously monitor the readiness of "traditional" files and the readiness of other kernel
mechanisms that support the eventfd interface. (Without the eventfd() interface, these mechanisms could not be multiplexed via select(2),
poll(2), or epoll(7).)