diff --git a/Chapter3/C3-输入输出专题.md b/Chapter3/C3-输入输出专题.md
index bf850d2..4e4ffa8 100644
--- a/Chapter3/C3-输入输出专题.md
+++ b/Chapter3/C3-输入输出专题.md
@@ -207,6 +207,8 @@ int main()
 ```c
 int scanf(const char *format, 地址表);
 format:抑制符*
+%s非常危险，因为不知道储存空间的大小
+放在循环结构中时要注意能否接收到正常内容
 ```
 
 
@@ -303,5 +305,173 @@ int main()
 
 ## 字符输入输出函数
 
-S
+```c
+int getchar(void);// 返回int型是考虑出错的情况
+int putchar(int c); 
+
+#include <stdio.h>
+#include <stdlib.h>
+
+int main()
+{
+    int ch;
+    ch = getchar();
+    putchar(ch);
+
+    exit(0);
+}
+```
+
+## 字符串输入输出函数
+
+```c
+char *gets(char *s);
+// 从标准输入读到内容放到s指针指向的缓冲区
+// 停止条件：换行或者EOF
+// 尾部会追加'\0'
+
+int puts(const char *s);
+// 将s指针缓冲区内容往外写
+// 以换行符中止
+
+#include <stdio.h>
+#include <stdlib.h>
+
+#define STRSIZE 32
+
+int main()
+{
+    char str[STRSIZE];
+    gets(str);
+    puts(str);
+
+    exit(0);
+}
+```
+
+`gets`非常危险，不会检查是否越界。使用的话用`fgets`来代替。
+
+```c
+char *fgets(char *s, int size, FILE *stream);// 最多读到'size-1'个
+```
+
+`getline`是`GNU libc`的方言，可以动态内存保证读取一整行。
+
+
+
+## 练习专题
+
+### 夸脱水
+
+> 一个水分子3.0e-23g，一夸脱水约950g。
+>
+> 变成，输入水的夸脱数，输出其包含的水分子质量。
+
+```c
+static void water(void)
+{
+    float num;
+    float sum;
+
+    printf("Please input for num:");
+    scanf("%f", &num);
+
+    sum = num * KQ / WEIGHT;
+
+    printf("Total = %e\n", sum);
+
+    return;
+}
+```
+
+### 三角形面积
+
+> 输入三角形的三边长，求三角形面积。
+
+```c
+static void area(void)
+{
+    float a, b, c;
+    float s, area;
+
+    printf("Please input:");
+    scanf("%f%f%f", &a, &b, &c);
+    // 对scanf做好校验是好习惯
+    if (a + b <= c || b + c <= a || a + c <= b)
+    {
+        fprintf(stderr, "EINVAL!\n");
+        exit(0);
+    }
+
+    printf("a = %f\n", a);
+    printf("b = %f\n", b);
+    printf("c = %f\n", c);
+
+    // s = 1 / 2 * (a + b + c);
+    s = 1.0 / 2 * (a + b + c);
+    printf("s = %f\n", s);
+    // s=0.000000
+    // 1 / 2是int型计算，得到的是商0余1
+
+    area = sqrt(s * (s - a) * (s - b) * (s - c));
+    printf("area = %f\n", area);
+}
+
+```
+
+编译需要`-lm`。`makefile`如下，千万要注意`makefile`要使用**制表符**缩进，而非空格。
+
+```makefile
+CC=cc
+CFLAGS=
+LDFLAGS=-lm
+TARGET=test
+SRC=test.c
+OBJ=$(SRC:.c=.o)
+
+$(TARGET): $(OBJ)
+	$(CC) -o $@ $^ $(LDFLAGS)
+
+%.o: %.c
+	$(CC) $(CFLAGS) -c $< -o $@
+
+clean:
+	rm -f $(TARGET) $(OBJ)
+```
+
+### 求根公式
+
+> ax^2^ + bx + c = 0，假设b^2^ - 4ac >0
+>
+> x~1~ = (-b + sqrt( b*b - 4ac )) / 2a
+>
+> x~2~ = (-b - sqrt( b*b - 4ac )) / 2a
+
+```C
+void root(void)
+{
+    float a, b, c;
+    float disc;
+    float p, q;
+    float x1, x2;
+
+    printf("Please input :\n");
+    scanf("%f%f%f", &a, &b, &c);
+
+    if (b * b - 4 * a * c < 0)
+    {
+        fprintf(stderr, "EINVAL\n");
+        exit(1);
+    }
+    disc = b * b - 4 * a * c;
+    p = -b / 2 * a;
+    q = sqrt(disc) / 2 * a;
+
+    x1 = p + q;
+    x2 = p - q;
+
+    printf("x1 = %f\n", x1);
+    printf("x2 = %f\n", x2);
+}
+```
 
diff --git a/Chapter3/IO/getchar_putchar.c b/Chapter3/IO/getchar_putchar.c
new file mode 100644
index 0000000..dfd8865
--- /dev/null
+++ b/Chapter3/IO/getchar_putchar.c
@@ -0,0 +1,11 @@
+#include <stdio.h>
+#include <stdlib.h>
+
+int main()
+{
+    int ch;
+    ch = getchar();
+    putchar(ch);
+
+    exit(0);
+}
\ No newline at end of file
diff --git a/Chapter3/IO/gets_puts.c b/Chapter3/IO/gets_puts.c
new file mode 100644
index 0000000..3baea8e
--- /dev/null
+++ b/Chapter3/IO/gets_puts.c
@@ -0,0 +1,13 @@
+#include <stdio.h>
+#include <stdlib.h>
+
+#define STRSIZE 32
+
+int main()
+{
+    char str[STRSIZE];
+    gets(str);
+    puts(str);
+
+    exit(0);
+}
\ No newline at end of file
diff --git a/Chapter3/IO/test.c b/Chapter3/IO/test.c
new file mode 100644
index 0000000..6bdcb5c
--- /dev/null
+++ b/Chapter3/IO/test.c
@@ -0,0 +1,91 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <math.h>
+
+#define WEIGHT 3.0e-23
+#define KQ 950
+
+static void water(void)
+{
+    float num;
+    float sum;
+
+    printf("Please input for num:");
+    scanf("%f", &num);
+
+    if (num <= 0)
+    {
+        fprintf(stderr, "Input Error!\n");
+        exit(1);
+    }
+    sum = num * KQ / WEIGHT;
+
+    printf("Total = %e\n", sum);
+
+    return;
+}
+
+static void area(void)
+{
+    float a, b, c;
+    float s, area;
+
+    printf("Please input:");
+    scanf("%f%f%f", &a, &b, &c);
+    // 对scanf做好校验是好习惯
+    if (a + b <= c || b + c <= a || a + c <= b)
+    {
+        fprintf(stderr, "EINVAL!\n");
+        exit(0);
+    }
+
+    printf("a = %f\n", a);
+    printf("b = %f\n", b);
+    printf("c = %f\n", c);
+
+    // s = 1 / 2 * (a + b + c);
+    s = 1.0 / 2 * (a + b + c);
+    printf("s = %f\n", s);
+    // s=0.000000
+    // 1 / 2是int型计算，得到的是商0余1
+
+    area = sqrt(s * (s - a) * (s - b) * (s - c));
+    printf("area = %f\n", area);
+}
+
+void root(void)
+{
+    float a, b, c;
+    float disc;
+    float p, q;
+    float x1, x2;
+
+    printf("Please input :\n");
+    scanf("%f%f%f", &a, &b, &c);
+
+    if (b * b - 4 * a * c < 0)
+    {
+        fprintf(stderr, "EINVAL\n");
+        exit(1);
+    }
+    disc = b * b - 4 * a * c;
+    p = -b / 2 * a;
+    q = sqrt(disc) / 2 * a;
+
+    x1 = p + q;
+    x2 = p - q;
+
+    printf("x1 = %f\n", x1);
+    printf("x2 = %f\n", x2);
+}
+
+int main()
+{
+    // water();
+
+    // area();
+
+    root();
+
+    exit(0);
+}
\ No newline at end of file