diff --git a/notes/Socket.md b/notes/Socket.md
index 56cbad75..d179b387 100644
--- a/notes/Socket.md
+++ b/notes/Socket.md
@@ -48,15 +48,15 @@ Unix 有五种 I/O 模型：
ssize_t recvfrom(int sockfd, void *buf, size_t len, int flags, struct sockaddr *src_addr, socklen_t *addrlen);
```
-
+
### 非阻塞式 I/O
-应用进程执行系统调用之后，内核返回一个错误码。应用进程可以继续执行，但是需要不断的执行系统调用来获知 I/O 是否完成，这种方式称为轮询（polling）。
+应用进程执行系统调用之后，如果数据还没有准备好，内核立即返回错误码 EAGAIN（也写作 EWOULDBLOCK）。应用进程可以继续执行，但是需要不断的执行系统调用来获知 I/O 是否完成，这种方式称为轮询（polling）。
由于 CPU 要处理更多的系统调用，因此这种模型的 CPU 利用率比较低。
-
+
### I/O 复用
@@ -66,7 +66,7 @@ ssize_t recvfrom(int sockfd, void *buf, size_t len, int flags, struct sockaddr *
如果一个 Web 服务器没有 I/O 复用，那么每一个 Socket 连接都需要创建一个线程去处理。如果同时有几万个连接，那么就需要创建相同数量的线程。相比于多进程和多线程技术，I/O 复用不需要进程线程创建和切换的开销，系统开销更小。
-
+
### 信号驱动 I/O
@@ -74,7 +74,7 @@ ssize_t recvfrom(int sockfd, void *buf, size_t len, int flags, struct sockaddr *
相比于非阻塞式 I/O 的轮询方式，信号驱动 I/O 的 CPU 利用率更高。
-
+
### 异步 I/O
@@ -82,7 +82,7 @@ ssize_t recvfrom(int sockfd, void *buf, size_t len, int flags, struct sockaddr *
异步 I/O 与信号驱动 I/O 的区别在于，异步 I/O 的信号是通知应用进程 I/O 完成，而信号驱动 I/O 的信号是通知应用进程可以开始 I/O。
-
+
### 五大 I/O 模型比较
@@ -93,7 +93,7 @@ ssize_t recvfrom(int sockfd, void *buf, size_t len, int flags, struct sockaddr *
非阻塞式 I/O 、信号驱动 I/O 和异步 I/O 在第一阶段不会阻塞。
-
+
## 二、I/O 复用
@@ -138,17 +138,21 @@ tv.tv_usec = 0;
int ret = select( largest_sock + 1, &fd_in, &fd_out, NULL, &tv );
// Check if select actually succeed
-if ( ret == -1 )
+if ( ret == -1 ) {
// report error and abort
-else if ( ret == 0 )
+}
+else if ( ret == 0 ) {
// timeout; no event detected
+}
else
{
- if ( FD_ISSET( sock1, &fd_in ) )
+ if ( FD_ISSET( sock1, &fd_in ) ) {
// input event on sock1
+ }
- if ( FD_ISSET( sock2, &fd_out ) )
+ if ( FD_ISSET( sock2, &fd_out ) ) {
// output event on sock2
+ }
}
```
@@ -184,12 +188,14 @@ fds[1].fd = sock2;
fds[1].events = POLLOUT;
// Wait 10 seconds
-int ret = poll( &fds, 2, 10000 );
+int ret = poll( fds, 2, 10000 );
// Check if poll actually succeed
-if ( ret == -1 )
+if ( ret == -1 ) {
// report error and abort
-else if ( ret == 0 )
+}
+else if ( ret == 0 ) {
// timeout; no event detected
+}
else
{
// If we detect the event, zero it out so we can reuse the structure
@@ -234,7 +240,7 @@ epoll_ctl() 用于向内核注册新的描述符或者是改变某个文件描
从上面的描述可以看出，epoll 只需要将描述符从进程缓冲区向内核缓冲区拷贝一次，并且进程不需要通过轮询来获得事件完成的描述符。
-
+
epoll 仅适用于 Linux OS。
@@ -247,8 +253,9 @@ epoll 对多线程编程更加友好，一个线程调用了 epoll_wait() 另一
// The function argument is ignored (it was not before, but now it is), so put your favorite number here
int pollingfd = epoll_create( 0xCAFE );
-if ( pollingfd < 0 )
- // report error
+if ( pollingfd < 0 ) {
+ // report error
+}
// Initialize the epoll structure in case more members are added in future
struct epoll_event ev = { 0 };
@@ -261,8 +268,9 @@ ev.data.ptr = pConnection1;
ev.events = EPOLLIN | EPOLLONESHOT;
// Add the descriptor into the monitoring list. We can do it even if another thread is
// waiting in epoll_wait - the descriptor will be properly added
-if ( epoll_ctl( epollfd, EPOLL_CTL_ADD, pConnection1->getSocket(), &ev ) != 0 )
+if ( epoll_ctl( pollingfd, EPOLL_CTL_ADD, pConnection1->getSocket(), &ev ) != 0 ) {
// report error
+}
// Wait for up to 20 events (assuming we have added maybe 200 sockets before that it may happen)
struct epoll_event pevents[ 20 ];
@@ -270,10 +278,12 @@ struct epoll_event pevents[ 20 ];
// Wait for 10 seconds, and retrieve less than 20 epoll_event and store them into epoll_event array
int ready = epoll_wait( pollingfd, pevents, 20, 10000 );
// Check if epoll actually succeed
-if ( ret == -1 )
+if ( ready == -1 ) {
// report error and abort
-else if ( ret == 0 )
+}
+else if ( ready == 0 ) {
// timeout; no event detected
+}
else
{
// Check if any events detected
@@ -337,3 +347,4 @@ poll 没有最大描述符数量的限制，如果平台支持并且对实时性
- [poll vs select vs event-based](https://daniel.haxx.se/docs/poll-vs-select.html)
- [select / poll / epoll: practical difference for system architects](http://www.ulduzsoft.com/2014/01/select-poll-epoll-practical-difference-for-system-architects/)
- [Browse the source code of userspace/glibc/sysdeps/unix/sysv/linux/ online](https://code.woboq.org/userspace/glibc/sysdeps/unix/sysv/linux/)
+- [epoll(7) — Linux manual page](https://man7.org/linux/man-pages/man7/epoll.7.html)
diff --git a/notes/pics/io-async.png b/notes/pics/io-async.png
new file mode 100644
index 00000000..728ab3a7
Binary files /dev/null and b/notes/pics/io-async.png differ
diff --git a/notes/pics/io-blocking.png b/notes/pics/io-blocking.png
new file mode 100644
index 00000000..d2d0f503
Binary files /dev/null and b/notes/pics/io-blocking.png differ
diff --git a/notes/pics/io-compare.png b/notes/pics/io-compare.png
new file mode 100644
index 00000000..470432bc
Binary files /dev/null and b/notes/pics/io-compare.png differ
diff --git a/notes/pics/io-multiplex.png b/notes/pics/io-multiplex.png
new file mode 100644
index 00000000..f00a310b
Binary files /dev/null and b/notes/pics/io-multiplex.png differ
diff --git a/notes/pics/io-nonblocking.png b/notes/pics/io-nonblocking.png
new file mode 100644
index 00000000..1c65449e
Binary files /dev/null and b/notes/pics/io-nonblocking.png differ
diff --git a/notes/pics/io-signal.png b/notes/pics/io-signal.png
new file mode 100644
index 00000000..b700a40b
Binary files /dev/null and b/notes/pics/io-signal.png differ
