fgets读入一行字符串测试注意点

安全的用法是：如果你一行至多有x个字符要读入，就用char s[x + 2];fgets(s, x + 2, stdin);这样保证读入的字符串不会被截断(截断是很危险的，不仅影响这一次的读入，还会影响后续的fgets的执行)，如果输入K个字符，则s[0..K-1]是输入的字符串，s[K]='\n'，s[K+1]='\0'，调用printf("%s", s);将打印这一行所有的字符，外加一个换行符

如果有类似scanf("%d", &n);后面紧跟着fgets(s, x + 2, stdin);——这个fgets将不会要求用户输入，而是直接读如输入缓冲区残留的'\n'

如果有耐心，可以读读下面的注释，也可以自己用gdb调试，测试各种情况

#include <stdio.h>
int main()
{
    char s[4];
    int n;
    //scanf("%d", &n);
    //fgets(s, 4, stdin);
    //
    //--in this case, you only need to input a number, s[0] = '\n', s[1] = '\0'

    //scanf("%d\n", &n);
    //fgets(s, 4, stdin);
    //
    //--in this case, you need to input 2 lines, a number, a string
    //if you input:
    //2
    //ab
    //s will be "ab\n"--s[3] = '\0'

    //if you input:
    //2
    //abc
    //s will be "abc"--s[3] = '\0'

    //if you input:
    //2
    //abcd
    //s will be "abc"--s[3] = '\0'
    
    //so, how to deal with that?
    //if there are at most x chars, you can use fgets(s, x + 2, stdin)
    //this will ensure there is '\n' and '\0'

    fgets(s,4,stdin);
    fgets(s,4,stdin);
    //if you input:
    //ab
    //--it will prompt you to enter another string

    //if you input:
    //abc
    //--you don't have the chance to input another string,
    //the later fgets will read remaining bytes from input buffer
    //so, the final s will be "\n",s[0]='\n',s[1]='\0',s[2]='c',s[3]='\0'
    
    //if you input:
    //abcd
    //finally you get s = "d\n",s[0]='d',s[1]='\n',s[2]='\0',s[3]='\0'
    return 0;
}