[英]libevent - event_base_loop() should it get events repeatly?
Here is a simple program using libevent
on linux, it tracks the stdout
fd, when it's writable, the callback will print some info to stdout
. 这是一个在Linux上使用libevent
的简单程序,它跟踪stdout
fd,当可写时,回调会将一些信息打印到stdout
。
hello_libevent.c: hello_libevent.c:
// libevent hello
#include <stdio.h>
#include <string.h>
#include <errno.h>
#include <unistd.h>
#include <event2/event.h>
#include <event2/thread.h>
void event_callback(evutil_socket_t fd, short events, void *arg) {
if(events | EV_WRITE) {
write(fd, "hello\n", 7);
}
sleep(1);
}
int libevent_test() {
int opr;
// enable pthread
if(evthread_use_pthreads() == -1) {
printf("error while evthread_use_pthreads(): %s\n", strerror(errno));
return -1;
}
// create event_base
struct event_base* eb;
if((eb = event_base_new()) == NULL) {
printf("error while event_base_new(): %s\n", strerror(errno));
return -1;
}
// create event
int fd_stdout = fileno(stdout);
struct event* event_stdout;
event_stdout = event_new(eb, fd_stdout, EV_WRITE, &event_callback, NULL);
// add event as pending
struct timeval timeout = {10, 0};
if(event_add(event_stdout, &timeout) == -1) {
printf("error while event_add(): %s\n", strerror(errno));
return -1;
}
// dispatch
if((opr = event_base_loop(eb, EVLOOP_NONBLOCK)) == -1) {
printf("error while event_base_dispatch(): %s\n", strerror(errno));
return -1;
} else if(opr == 1) {
printf("no more events\n");
} else {
printf("exit normally\n");
}
// free event
event_free(event_stdout);
return 0;
}
int main(int argc, char * argv[]) {
return libevent_test();
}
Compile: 编译:
gcc -Wall hello_libevent.c -levent -levent_pthreads gcc -Wall hello_libevent.c -levent -levent_pthreads
Execution result: 执行结果:
hello
no more events
event_base_loop
within a loop, while it's already a loop
? 在已经是loop
,是否有必要在循环中调用event_base_loop
? From http://www.wangafu.net/~nickm/libevent-book/Ref3_eventloop.html it looks like you can call event_base_loop
or event_base_dispatch
from within a loop. 从http://www.wangafu.net/~nickm/libevent-book/Ref3_eventloop.html看来,您可以在循环内调用event_base_loop
或event_base_dispatch
。
while (1) { /* This schedules an exit ten seconds from now. */ event_base_loopexit(base, &ten_sec);`` event_base_dispatch(base); puts("Tick"); }
The main purpose of events is to inform some busy thread about some event that happened elsewhere. 事件的主要目的是通知繁忙的线程有关其他地方发生的某些事件。 So, this looks logical. 因此,这看起来合乎逻辑。
I think the event flag EV_PERSIST
mentioned in event.h File Reference might help. 我认为event.h文件参考中提到的事件标志EV_PERSIST
可能会有所帮助。
Persistent event: won't get removed automatically when activated. 持续事件:激活后不会自动删除。
When a persistent event with a timeout becomes activated, its timeout is reset to 0. 当具有超时的持续事件被激活时,其超时将重置为0。
Instead of 代替
//...
event_stdout = event_new(eb, fd_stdout, EV_WRITE, &event_callback, NULL);
//...
you can pass this flag to event_new 您可以将此标志传递给event_new
//...
event_stdout = event_new(eb, fd_stdout, EV_WRITE|EV_PERSIST, &event_callback, NULL);
//...
and the other parts of code remain unchanged. 而代码的其他部分则保持不变。 At this time you create and add an event once and there's no need to call event_base_loop within a loop. 此时,您只需创建并添加一个事件,就无需在循环内调用event_base_loop。 The compiled program just keeps printing lines of "hello" until it is terminated. 编译后的程序仅保留打印“ hello”的行,直到终止为止。
By the way, I noticed that changing 顺便说一句,我注意到变化
write(fd, "hello\n", 7);
into 进入
write(fd, "hello\n", 6);
eliminates the leading character '\\0' of each line. 消除每行的开头字符“ \\ 0”。
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