diff --git a/day4/bbs_bob.c b/day4/bbs_bob.c
new file mode 100644
index 0000000..220ed97
--- /dev/null
+++ b/day4/bbs_bob.c
@@ -0,0 +1,92 @@
+#include <stdio.h>
+#include <unistd.h>
+#include <fcntl.h>
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <sys/wait.h>
+#include <errno.h>
+#include <string.h>
+
+// 单机聊天工具 (bob方)
+int main(int argc, char const *argv[])
+{
+// 使用方法 gcc xx.c -D CREATE_FIFO 会在运行时执行预处理区段
+#ifdef CREATE_FIFO
+    mkfifo("fifo_bob_to_lucy", 0644);
+    mkfifo("fifo_lucy_to_bob", 0644);
+#endif
+
+    if (mkfifo("fifo_bob_to_lucy", 0644) != -1)
+    {
+        if (errno != EEXIST)
+        {
+            perror("fifo_bob_to_lucy");
+            return 1;
+        }
+    }
+    if (mkfifo("fifo_lucy_to_bob", 0644) != -1)
+    {
+        if (errno != EEXIST)
+        {
+            perror("fifo_lucy_to_bob");
+            return 1;
+        }
+    }
+
+    int i = 0;
+    for (; i < 2; i++)
+    {
+        int pid = fork();
+        if (pid == 0)
+            break;
+    }
+    if (i == 0)
+    {
+        // 第一个子进程
+        // 从键盘输入数据并发送给对方 (bob -> lucy)
+        int fd = open("fifo_bob_to_lucy", O_WRONLY); // 只写消息
+        while (1)
+        {
+            char buf[128] = {};
+            int len = read(STDIN_FILENO, buf, 128);
+            buf[len - 1] = '\0';
+            write(fd, buf, len);
+            if (strcmp(buf, "bye") == 0)
+            {
+                break;
+            }
+        }
+        close(fd);
+        _exit(0);
+    }
+    else if (i == 1)
+    {
+        // 第二个进程，读数据 (lucy -> bob)
+        int fd = open("fifo_lucy_to_bob", O_RDONLY);
+        while (1)
+        {
+            char buf[128] = "";
+            int len = read(fd, buf, 128);
+            if (len <= 0)
+                break;
+            printf("接收 Lucy: %s\n", buf);
+        }
+        close(fd);
+        _exit(0);
+    }
+    else
+    {
+        // 主进程
+        while (1)
+        {
+            // int status;
+            int pid_ = waitpid(0, NULL, WNOHANG);
+            if (pid_ == -1)
+            {
+                // 所有子进程都已经退出
+                break;
+            }
+        }
+    }
+    return 0;
+}
diff --git a/day4/bbs_lucy.c b/day4/bbs_lucy.c
new file mode 100644
index 0000000..cbe740d
--- /dev/null
+++ b/day4/bbs_lucy.c
@@ -0,0 +1,92 @@
+#include <stdio.h>
+#include <unistd.h>
+#include <fcntl.h>
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <sys/wait.h>
+#include <errno.h>
+#include <string.h>
+
+// 单机聊天工具 (lucy方)
+int main(int argc, char const *argv[])
+{
+// 使用方法 gcc xx.c -D CREATE_FIFO 会在运行时执行预处理区段
+#ifdef CREATE_FIFO
+    mkfifo("fifo_bob_to_lucy", 0644);
+    mkfifo("fifo_lucy_to_bob", 0644);
+#endif
+
+    if (mkfifo("fifo_bob_to_lucy", 0644) != -1)
+    {
+        if (errno != EEXIST)
+        {
+            perror("fifo_bob_to_lucy");
+            return 1;
+        }
+    }
+    if (mkfifo("fifo_lucy_to_bob", 0644) != -1)
+    {
+        if (errno != EEXIST)
+        {
+            perror("fifo_lucy_to_bob");
+            return 1;
+        }
+    }
+
+    int i = 0;
+    for (; i < 2; i++)
+    {
+        int pid = fork();
+        if (pid == 0)
+            break;
+    }
+    if (i == 0)
+    {
+        // 第一个子进程
+        // 从键盘输入数据并发送给对方 (lucy -> bob)
+        int fd = open("fifo_lucy_to_bob", O_WRONLY); // 只写消息
+        while (1)
+        {
+            char buf[128] = {};
+            int len = read(STDIN_FILENO, buf, 128);
+            buf[len - 1] = '\0';
+            write(fd, buf, len);
+            if (strcmp(buf, "bye") == 0)
+            {
+                break;
+            }
+        }
+        close(fd);
+        _exit(0);
+    }
+    else if (i == 1)
+    {
+        // 第二个进程，读数据 (bob -> lucy)
+        int fd = open("fifo_bob_to_lucy", O_RDONLY);
+        while (1)
+        {
+            char buf[128] = "";
+            int len = read(fd, buf, 128);
+            if (len <= 0)
+                break;
+            printf("接收 Bob: %s\n", buf);
+        }
+        close(fd);
+        _exit(0);
+    }
+    else
+    {
+        // 主进程
+        while (1)
+        {
+            // int status;
+            int pid_ = waitpid(0, NULL, WNOHANG);
+            if (pid_ == -1)
+            {
+                // 所有子进程都已经退出
+                break;
+            }
+        }
+    }
+    return 0;
+}
diff --git a/day4/demo1.c b/day4/demo1.c
new file mode 100644
index 0000000..c32c384
--- /dev/null
+++ b/day4/demo1.c
@@ -0,0 +1,38 @@
+#include <stdio.h>
+#include <unistd.h>
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <sys/wait.h>
+#include <fcntl.h>
+
+int main(int argc, char const *argv[])
+{
+    int fd[2];
+    if (pipe(fd) == -1)
+    {
+        perror("pipe");
+        return 1;
+    }
+
+    pid_t pid = fork();
+    if (pid == 0)
+    {
+        close(fd[1]);
+        dup2(fd[0], 0); // 0 -> fd[0] ,标准输入设备改为管道
+        execlp("grep", "grep", "fish", NULL);
+        close(fd[0]);
+        _exit(0);
+    }
+
+    else if (pid > 0) // 父进程
+    {
+        close(fd[0]); // 关闭读
+        // 将命令执行结果写入管道中
+        dup2(fd[1], 1); // 1 -> fd[1] ,标准输出设备改为管道
+        execlp("ps", "ps", "-A", NULL);
+        close(fd[1]);
+        // wait(pid);
+    }
+
+    return 0;
+}
diff --git a/day4/demo2.c b/day4/demo2.c
new file mode 100644
index 0000000..67d9f8c
--- /dev/null
+++ b/day4/demo2.c
@@ -0,0 +1,39 @@
+#include <stdio.h>
+#include <unistd.h>
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <sys/wait.h>
+#include <fcntl.h>
+
+int main(int argc, char const *argv[])
+{
+    int fd[2];
+    if (pipe(fd) == -1)
+    {
+        perror("pipe");
+        return 1;
+    }
+
+    pid_t pid = fork();
+    if (pid == 0)
+    {
+        close(fd[0]); // 关闭读
+        // 重定向标准输出到管道
+        dup2(fd[1], 1);
+        execlp("expr", "expr", "4", "+", "5", NULL);
+        close(fd[1]);
+        _exit(0);
+    }
+
+    else if (pid > 0) // 父进程
+    {
+        close(fd[1]); // 关闭写
+        char buf[10] = {};
+        int len = read(fd[0], buf, 10);
+        buf[len - 1] = '\0';
+        printf("读取子进程发送的数据: %s\n", buf);
+        close(fd[0]);
+    }
+
+    return 0;
+}
diff --git a/day4/fifo_read.c b/day4/fifo_read.c
new file mode 100644
index 0000000..a998839
--- /dev/null
+++ b/day4/fifo_read.c
@@ -0,0 +1,35 @@
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <stdio.h>
+#include <unistd.h>
+#include <fcntl.h>
+#include <stdio.h>
+#include <errno.h>
+
+int main()
+{
+    // 创建 fifo 的有名（文件名）管道 if (mkfifo("myfifo", 0755) != 0)
+    if (mkfifo("myfifo", 0755) != 0)
+    {
+        if (errno != EEXIST)
+        {
+            perror("mkfifo");
+            return 1;
+        }
+    }
+
+    printf("准备以只读方式打开 myfifo\n");
+    int fd = open("myfifo", O_RDONLY);
+    printf("open myfifo fd = %d\n", fd);
+    int n = 1;
+    while (1)
+    {
+        char buf[32] = "";
+        int len = read(fd, buf, 32); // 阻塞（写进程运行期间），非阻塞（没有写进程运行）
+        buf[len - 1] = '\0';
+        printf("第%d次读取的数据(%d Bytes): %s\n", n++, len, buf);
+        sleep(1);
+    }
+    close(fd);
+    return 0;
+}
\ No newline at end of file
diff --git a/day4/fifo_read_2.c b/day4/fifo_read_2.c
new file mode 100644
index 0000000..b80f726
--- /dev/null
+++ b/day4/fifo_read_2.c
@@ -0,0 +1,35 @@
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <stdio.h>
+#include <unistd.h>
+#include <fcntl.h>
+#include <stdio.h>
+#include <errno.h>
+
+int main()
+{
+    // 创建 fifo 的有名（文件名）管道 if (mkfifo("myfifo", 0755) != 0)
+    if (mkfifo("myfifo", 0755) != 0)
+    {
+        if (errno != EEXIST)
+        {
+            perror("mkfifo");
+            return 1;
+        }
+    }
+
+    printf("准备以只读方式打开 myfifo\n");
+    int fd = open("myfifo", O_RDONLY | O_NONBLOCK); // 非阻塞
+    printf("open myfifo fd = %d\n", fd);
+    int n = 1;
+    while (1)
+    {
+        char buf[32] = "";
+        int len = read(fd, buf, 32); // 阻塞（写进程运行期间），非阻塞（没有写进程运行）
+        buf[len - 1] = '\0';
+        printf("第%d次读取的数据(%d Bytes): %s\n", n++, len, buf);
+        sleep(1);
+    }
+    close(fd);
+    return 0;
+}
\ No newline at end of file
diff --git a/day4/fifo_write.c b/day4/fifo_write.c
new file mode 100644
index 0000000..b0db717
--- /dev/null
+++ b/day4/fifo_write.c
@@ -0,0 +1,30 @@
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <stdio.h>
+#include <unistd.h>
+#include <fcntl.h>
+#include <stdio.h>
+#include <errno.h>
+
+int main()
+{
+    // 创建 fifo 的有名（文件名）管道
+    if (mkfifo("myfifo", 0755) != 0)
+    {
+        if (errno != EEXIST)
+        {
+
+            perror("mkfifo");
+            return 1;
+        }
+    }
+
+    printf("准备以只写方式打开 myfifo\n");
+    int fd = open("myfifo", O_WRONLY);
+    printf("open myfifo fd = %d\n", fd);
+
+    sleep(3); // 阻塞 5 秒
+
+    close(fd);
+    return 0;
+}
\ No newline at end of file
diff --git a/day4/fifo_write_2.c b/day4/fifo_write_2.c
new file mode 100644
index 0000000..403bec9
--- /dev/null
+++ b/day4/fifo_write_2.c
@@ -0,0 +1,38 @@
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <stdio.h>
+#include <unistd.h>
+#include <fcntl.h>
+#include <stdio.h>
+#include <errno.h>
+
+int main()
+{
+    // 创建 fifo 的有名（文件名）管道
+    if (mkfifo("myfifo", 0755) != 0)
+    {
+        // 管道文件已经存在，代码将继续执行后面的部分，其他错误报错
+        // 如果errno等于EEXIST，则表示管道文件已经存在，代码将继续执行后面的部分
+        if (errno != EEXIST)
+        {
+
+            perror("mkfifo");
+            return 1;
+        }
+    }
+
+    printf("准备以只写方式打开 myfifo\n");
+    int fd = open("myfifo", O_WRONLY | O_NONBLOCK);
+    if (fd == -1)
+    {
+        perror("open");
+        return 1;
+    }
+
+    printf("open myfifo fd = %d\n", fd);
+
+    sleep(3); // 阻塞 5 秒
+
+    close(fd);
+    return 0;
+}
\ No newline at end of file
diff --git a/day4/file1.c b/day4/file1.c
new file mode 100644
index 0000000..d0436d9
--- /dev/null
+++ b/day4/file1.c
@@ -0,0 +1,14 @@
+#include <stdio.h>
+#include <unistd.h>
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <fcntl.h>
+
+int main()
+{
+    close(1); // 关闭标准输出描述符
+    int fd = open("a.txt", O_WRONLY | O_CREAT | O_APPEND, 0644);
+    printf("hello fd\n"); // fd 描述符会变为 1 ,printf 回答引导 fd 对应文件中
+
+    return 0;
+}
\ No newline at end of file
diff --git a/day4/file2.c b/day4/file2.c
new file mode 100644
index 0000000..d37a42e
--- /dev/null
+++ b/day4/file2.c
@@ -0,0 +1,20 @@
+#include <stdio.h>
+#include <unistd.h>
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <fcntl.h>
+
+int main()
+{
+    int fd = open("a.txt", O_WRONLY | O_CREAT | O_APPEND, 0644);
+    printf("新打开的hello fd\n"); // fd 描述符会变为 1 ,printf 回答引导 fd 对应文件中
+    close(1);                     // 关闭标准输出（文件描述符为1 ）
+    int newfd = dup(fd);          // 1->a.txt，会分配当前最小可用的文件描述符(这个时候标准输出1的描述符空闲)，即拿到文件描述符1
+    // 下面的打印(写入)会重定向到 a.txt 文件中
+    printf("newfd = %d\n", newfd);
+    printf("hahah\n");
+    close(fd);
+    printf("yes\n");
+
+    return 0;
+}
\ No newline at end of file
diff --git a/day4/file3_dup2.c b/day4/file3_dup2.c
new file mode 100644
index 0000000..ba41d29
--- /dev/null
+++ b/day4/file3_dup2.c
@@ -0,0 +1,20 @@
+#include <stdio.h>
+#include <unistd.h>
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <fcntl.h>
+
+int main()
+{
+    int fd = open("a.txt", O_WRONLY | O_CREAT | O_APPEND, 0644);
+    printf("新打开的hello fd\n"); // fd 描述符会变为 1 ,printf 回答引导 fd 对应文件中
+    // dup2 会自动关闭 newfd，即关闭 1 标准输出
+    int newfd = dup2(fd, 1); // 1->a.txt, 会将新的 fd 描述符值为 1
+    close(fd);
+    // 下面的打印(写入)会重定向到 a.txt 文件中
+    printf("newfd = %d\n", newfd);
+    printf("no\n");
+    printf("good\n");
+
+    return 0;
+}
\ No newline at end of file
diff --git a/day4/file4_0.c b/day4/file4_0.c
new file mode 100644
index 0000000..bf4179e
--- /dev/null
+++ b/day4/file4_0.c
@@ -0,0 +1,21 @@
+// 配合file4使用
+#include <stdio.h>
+#include <unistd.h>
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <fcntl.h>
+
+int main()
+{
+    // 设置 1 的 close_on_exec 打开状态(默认为关闭)
+    int flags;
+    flags = fcntl(1, F_GETFD);
+    flags |= FD_CLOEXEC;
+    // flags &= ~FD_CLOEXEC; // 关闭 FD_CLOEXEC 标志位
+    fcntl(1, F_SETFD, flags);
+
+    execl("./file4_1", "file4_1", NULL); // 当 file4_1 存在时，会切换到file4_1运行，不会继续执行本程序后续行内容
+
+    printf("file4_0"); // 当file4_1不存在时才会执行到这里
+    return 0;
+}
\ No newline at end of file
diff --git a/day4/file4_1.c b/day4/file4_1.c
new file mode 100644
index 0000000..4df5d1c
--- /dev/null
+++ b/day4/file4_1.c
@@ -0,0 +1,11 @@
+#include <stdio.h>
+#include <unistd.h>
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <fcntl.h>
+
+int main()
+{
+    printf("hi, file4_1");
+    return 0;
+}
\ No newline at end of file
diff --git a/day4/pipe1.c b/day4/pipe1.c
new file mode 100644
index 0000000..d46bd7d
--- /dev/null
+++ b/day4/pipe1.c
@@ -0,0 +1,45 @@
+#include <stdio.h>
+#include <unistd.h>
+#include <sys/types.h>
+#include <string.h>
+
+int main()
+{
+    int fd[2];
+    int f = pipe(fd);
+    if (f == -1)
+    {
+        perror("pipe");
+        return 1;
+    }
+
+    int pid = fork();
+    if (pid == 0)
+    {
+        // 子进程: 读
+        // 关闭写通道
+        close(fd[1]);
+
+        char buf[64] = {};
+        int len = read(fd[0], buf, 64); // 可能会阻塞，数据为空时会阻塞
+
+        printf("%d 子进程读取到: %s\n", getpid(), buf);
+        // 关闭使用完之后，记得关闭
+        close(fd[0]); // 关闭读管道
+        _exit(0);
+    }
+    else if (pid > 0)
+    {
+        // 父进程，写
+        // 关闭读通道
+        close(fd[0]);
+        printf("父进程(%d)等待3秒向子进程(%d)写数据\n", getpid(), pid);
+        sleep(3);
+        char buf[64] = "hello,flykhan";
+        write(fd[1], buf, strlen(buf));
+        // 关闭写通道
+        close(fd[1]);
+    }
+
+    return 0;
+}
\ No newline at end of file
diff --git a/day4/pipe3.c b/day4/pipe3.c
new file mode 100644
index 0000000..1df2031
--- /dev/null
+++ b/day4/pipe3.c
@@ -0,0 +1,7 @@
+#include <stdio.h>
+#include <unistd.h>
+#include <sys/types.h>
+
+int main()
+{
+}
\ No newline at end of file
diff --git a/day4/pipe4.c b/day4/pipe4.c
new file mode 100644
index 0000000..28ec58f
--- /dev/null
+++ b/day4/pipe4.c
@@ -0,0 +1,53 @@
+#include <stdio.h>
+#include <unistd.h>
+#include <sys/types.h>
+#include <string.h>
+
+int main()
+{
+    int fd[2];
+    int f = pipe(fd);
+    if (f == -1)
+    {
+        perror("pipe");
+        return 1;
+    }
+
+    int pid = fork();
+    if (pid == 0)
+    {
+        // 子进程: 读
+        // 关闭写通道
+        close(fd[1]);
+        for (int i = 0; i < 3; i++)
+        {
+            char buf[1024] = {};
+            int len = read(fd[0], buf, 1024); // 可能会阻塞，数据为空时会阻塞
+            buf[len - 1] = '\0';
+
+            printf("%d 子进程读取到: %s ,数据大小为 %d\n", getpid(), buf, len);
+        }
+        // 关闭使用完之后，记得关闭
+        close(fd[0]); // 关闭读管道
+        _exit(0);
+    }
+    else if (pid > 0)
+    {
+        // 父进程，写
+        // 关闭读通道
+        close(fd[0]);
+
+        // 从键盘读取数据写入到管道中
+        while (1)
+        {
+            char buf[32] = {""};
+            int len = read(0, buf, 32);
+            write(fd[1], buf, len);
+        }
+
+        // 关闭写通道
+        close(fd[1]);
+    }
+
+    return 0;
+}
\ No newline at end of file
diff --git a/day4/pipe5.c b/day4/pipe5.c
new file mode 100644
index 0000000..4b85b8e
--- /dev/null
+++ b/day4/pipe5.c
@@ -0,0 +1,55 @@
+#include <stdio.h>
+#include <unistd.h>
+#include <sys/types.h>
+#include <string.h>
+
+int main()
+{
+    int fd[2];
+    int f = pipe(fd);
+    if (f == -1)
+    {
+        perror("pipe");
+        return 1;
+    }
+
+    int pid = fork();
+    if (pid > 0)
+    {
+        // 父进程: 读
+        // 关闭写通道
+        close(fd[1]);
+        for (int i = 0; i < 3; i++)
+        {
+            char buf[1024] = {};
+            int len = read(fd[0], buf, 1024); // 可能会阻塞，数据为空时会阻塞
+            buf[len - 1] = '\0';
+
+            printf("%d 父进程读取到: %s ,数据大小为 %d\n", getpid(), buf, len);
+        }
+        // 关闭使用完之后，记得关闭
+        close(fd[0]); // 关闭读管道
+        _exit(0);
+    }
+    else if (pid == 0)
+    {
+        // 子进程，写
+        // 关闭读通道
+        close(fd[0]);
+
+        // 从键盘读取数据写入到管道中
+        while (1)
+        {
+            char buf[32] = {""};
+            // int len = read(0, buf, 32);
+            scanf("%s", buf);
+            // buf[strlen(buf) - 1] = '\0';
+            write(fd[1], buf, strlen(buf) + 1);
+        }
+
+        // 关闭写通道
+        close(fd[1]);
+    }
+
+    return 0;
+}
\ No newline at end of file