diff --git a/day12/d3.c b/day12/d3.c
new file mode 100644
index 0000000..a56fe0d
--- /dev/null
+++ b/day12/d3.c
@@ -0,0 +1,15 @@
+// 格式化输出字符串 sprintf(char *buf, const char *format, ...)
+// 函数返回输出到 buf 字符串，并返回 buf 的字符串长度
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+int main()
+{
+    char buf[20] = "";
+    // ret 返回字符串 长度
+    // %0nd :  n为输出长度，不足n位的前面位置补0
+    int ret = sprintf(buf, "%d 年 %02d 月 %d 日", 2023, 7, 18);
+    printf("ret = %d,%s\n", ret, buf);
+    return 0;
+}
\ No newline at end of file
diff --git a/day12/d3_2.c b/day12/d3_2.c
new file mode 100644
index 0000000..6b0d30d
--- /dev/null
+++ b/day12/d3_2.c
@@ -0,0 +1,12 @@
+// sprintf() 实现字符串与字符串拼接，字符串与数字拼接，数字与数字拼接
+#include <stdio.h>
+#include <stdlib.h>
+
+int main()
+{
+    char buf1[20] = "hello";
+    char buf2[20] = "world";
+    sprintf(buf1, "%s %s", buf1, buf2);
+    printf("%s", buf1);
+    return 0;
+}
\ No newline at end of file
diff --git a/day12/d4.c b/day12/d4.c
new file mode 100644
index 0000000..db092f1
--- /dev/null
+++ b/day12/d4.c
@@ -0,0 +1,18 @@
+// 从字符串获取 sscanf(const char *buf, const char *format, ...);
+// 从 buf 字符串按 format 格式提取数据并赋值给后面的变量地址
+#include <stdio.h>
+#include <stdlib.h>
+
+int main()
+{
+    char *content = "disen,17791692095,610039018";
+    char name[10] = ""; // 换行符占最后一位
+    long phone;
+    char qq[10] = "";
+
+    sscanf(content, "%5s,%ld,%9s", name, &phone, qq);
+    // sscanf(content, "%5s,%ld,%9s", name, &phone, qq);
+    printf("name: %s\nphone: %ld\nqq: %s\n", name, phone, qq);
+
+    return 0;
+}
\ No newline at end of file
diff --git a/day12/d5.c b/day12/d5.c
new file mode 100644
index 0000000..bfa8844
--- /dev/null
+++ b/day12/d5.c
@@ -0,0 +1,18 @@
+// sscanf() 高级用法: 跳过内容
+#include <stdio.h>
+#include <stdlib.h>
+
+int main()
+{
+    char *content = "disen,17791692095,610039018";
+    char first = 0;
+    char phone[12] = "";
+    char qq[12] = "";
+    sscanf(content, "%c", &first);
+    printf("%c\n", first);
+    // 跳过前6个字符，然后读取11个字符，放入 phone 中
+    sscanf(content, "%*6s%11s", phone);
+    printf("%s\n", phone);
+    sscanf(content, "%*18s%11s", qq);
+    printf("%s\n", qq);
+}
\ No newline at end of file
diff --git a/day12/d5_2.c b/day12/d5_2.c
new file mode 100644
index 0000000..cbc9562
--- /dev/null
+++ b/day12/d5_2.c
@@ -0,0 +1,16 @@
+// sscanf() 高级用法: 跳过空格
+// 遇到空格则跳过
+#include <stdio.h>
+#include <stdlib.h>
+
+int main()
+{
+    char *content = "disen 17791692095,610039018";
+    char phone[12] = "";
+    // *s 表示跳过空格, 11s 表示读取11个字符
+    sscanf(content, "%*s%11s", phone);
+    // sscanf(content, "%*[a-z]%11s", phone);
+    printf("%s\n", phone);
+
+    return 0;
+}
\ No newline at end of file
diff --git a/day12/d5_3.c b/day12/d5_3.c
new file mode 100644
index 0000000..13cac27
--- /dev/null
+++ b/day12/d5_3.c
@@ -0,0 +1,18 @@
+// 提取内容 bc#def@ghi 中的 # 两边的内容
+// 都是贪婪匹配, 会一直读取到 # 为止
+#include <stdio.h>
+#include <stdlib.h>
+
+int main()
+{
+    char *content = "bc#def@ghi";
+    char buf1[20] = "";
+    char buf2[20] = "";
+    // %[^#] 表示读取到 # 为止的内容, %[^@] 表示读取到 @ 为止的内容
+    sscanf(content, "%[a-z]#%[a-z@]", buf1, buf2);
+
+    printf("%s\n", buf1);
+    printf("%s\n", buf2);
+
+    return 0;
+}
\ No newline at end of file
diff --git a/day12/d5_4_test.c b/day12/d5_4_test.c
new file mode 100644
index 0000000..f440e4b
--- /dev/null
+++ b/day12/d5_4_test.c
@@ -0,0 +1,18 @@
+#include <stdio.h>
+#include <stdlib.h>
+
+int main()
+{
+    char *content = "disen:17791692095\njack:18799101234\nlucy:19088675432";
+    char phone[3][12];
+    // * 用来表示跳过内容
+    sscanf(content, "%*[^:]:%s%*[^:]:%s%*[^:]:%s", phone[0], phone[1], phone[2]);
+    // sscanf(content, "%*[^0-9]%[0-9]%*[^0-9]%[0-9]%*[^0-9]%[0-9]", phone[0], phone[1], phone[2]);
+    // sscanf(content, "%*[^0-9]%s%*[^0-9]%s%*[^0-9]%s", phone[0], phone[1], phone[2]);
+    // sscanf(content, "%*[a-z:\n]%s%*[a-z:\n]%s%*[a-z:\n]%s", phone[0], phone[1], phone[2]);
+
+    for (int i = 0; i < 3; i++)
+        printf("%s\n", phone[i]); // 循环打印
+
+    return 0;
+}
\ No newline at end of file
diff --git a/day12/d6.c b/day12/d6.c
new file mode 100644
index 0000000..1bb4a48
--- /dev/null
+++ b/day12/d6.c
@@ -0,0 +1,14 @@
+// const 修饰变量，表示只读变量
+#include <stdio.h>
+
+int main()
+{
+    const int m = 10;
+    // m = 20; // 编译时报错: 因为 m 是只读变量
+
+    int *p = &m; // 地址可以赋值给指针变量
+    *p = 20;     // 可以通过指针变量修改只读变量的值
+
+    printf("%d\n", m);
+    return 0;
+}
\ No newline at end of file
diff --git a/day12/d6_2.c b/day12/d6_2.c
new file mode 100644
index 0000000..4830e28
--- /dev/null
+++ b/day12/d6_2.c
@@ -0,0 +1,14 @@
+#include <stdio.h>
+
+int main()
+{
+    const int m = 10;
+
+    const int *p = &m; // 地址可以赋值给指针变量
+
+    int n = 50;
+    p = &n; // 可以修改指针变量的指向,指针从指向 m 变成指向 n
+
+    printf("%d\n", *p);
+    return 0;
+}
\ No newline at end of file
diff --git a/day12/d7.c b/day12/d7.c
new file mode 100644
index 0000000..b711184
--- /dev/null
+++ b/day12/d7.c
@@ -0,0 +1,14 @@
+// const 修饰指针变量 p, 使得 p 变成只读变量, 不能指向新的地址
+// 指针变量是只读 (不能指向新的地址)
+#include <stdio.h>
+
+int main()
+{
+    const int n = 10;
+    int *const p = &n; // 必须初始化, 不等于 const int *p = &n;
+    *p = 20;           // 可以修改指针变量指向的内容
+    printf("%d\n", *p);
+    int m = 100;
+    // p = &m; // 编译时报错: 因为 p 是只读变量
+    return 0;
+}
\ No newline at end of file
diff --git a/day12/d8.c b/day12/d8.c
new file mode 100644
index 0000000..c24e32a
--- /dev/null
+++ b/day12/d8.c
@@ -0,0 +1,16 @@
+// const 修饰 * 和指针变量
+
+#include <stdio.h>
+#include <stdlib.h>
+
+int main()
+{
+    int m = 100;
+    const int *const p = &m; // 第一个 const 修饰 *p, 第二个 const 修饰 p
+    *p += 10;                // 编译时报错: 因为 *p 是只读变量
+    int n = 90;
+    p = &n; // 编译时报错: 因为 p 是只读变量
+    printf("%d\n", *p);
+
+    return 0;
+}
\ No newline at end of file
diff --git a/day12/d8_test1.c b/day12/d8_test1.c
new file mode 100644
index 0000000..6afd449
--- /dev/null
+++ b/day12/d8_test1.c
@@ -0,0 +1,13 @@
+#include <stdio.h>
+
+int main()
+{
+    char m[30] = "lucy, 666";
+    char *const p = m; // 必须初始化, 不等于 const char *p = m;
+    p[0] = 'd';
+    *(p + 1) = 'o';
+    printf("%s\n", p);
+    // p = "jack"; // 编译时报错: 因为 p 是只读变量
+
+    return 0;
+}
\ No newline at end of file