1. 使用fgets统计给定文件的行数

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main(int argc, char const *argv[])
{
    if (argc != 2)
    {
        puts("input file error");
        puts("usage:./a.out filename");
        return -1;
    }

    FILE* fp = NULL;
    if ((fp = fopen(argv[1], "r")) == NULL)
    {
        perror("fopen error");
        return -1;
    }

    int count = 0;
    char buf[5];
    while (1)
    {
        fgets(buf, sizeof(buf), fp);

        if (strlen(buf) == 0)
        {
            break;
        }

        printf("%s", buf);
        for (int i = 0; i < 5; i++)
        {
            if (buf[i] == '\n')
            {
                count++;
            }
        }

        memset(buf, 0, sizeof(buf));
    }

    printf("%d\n", count);

    return 0;
}

2. 使用fputs和fgets完成两个文件的拷贝

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#define MAXSIZE 10

FILE* open_file(char name[], char method[])
{
    FILE* fp = NULL;
    if ((fp = fopen(name, method)) == NULL)
    {
        return NULL;
    }

    return fp;
}

int main(int argc, char const *argv[])
{
    if (argc != 3)
    {
        puts("input file error");
        puts("usage:./a.out srcfile destfile");
        return -1;
    }

    FILE* fp = open_file(argv[1], "r");
    if (NULL == fp)
    {
        perror("fopen error");
        return -1;
    }

    FILE* fp1 = open_file(argv[2], "w");
    if (NULL == fp1)
    {
        perror("fopen error");
        return -1;
    }

    char buf[MAXSIZE];
    while (1)
    {
        fgets(buf, sizeof(buf), fp);
        if (strlen(buf) == 0)
        {
            break;
        }

        fputs(buf, fp1);

        memset(buf, 0, sizeof(buf));
    }

    fclose(fp);
    fclose(fp1);

    return 0;
}

3. 完成登录注册功能

做个小菜单，功能1是注册功能，输入注册账户和注册密码，将账户和密码写入文件中

功能2是登录功能，提示输入登陆账户和登录密码，并用其遍历文件中的每一组账户和密码，如果匹配成功，则登录成功，如果全部不匹配则提示登陆失败

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#define MAXSIZE 100

FILE* open_file(char name[], char mode[])
{
    FILE* fp = NULL;
    if ((fp = fopen(name, mode)) == NULL)
    {
        return NULL;
    }

    return fp;
}

void read_and_save(FILE* src, FILE* dest)
{
    char buf[MAXSIZE];
    fgets(buf, sizeof(buf), src);
    fputs(buf, dest);
}

void sign_up()
{
    FILE* fp = open_file("test.txt", "a+");
    if (NULL == fp)
    {
        perror("fopen error");
        return;
    }

    printf("请输入用户名：");
    read_and_save(stdin, fp);

    printf("请输入密码：");
    read_and_save(stdin, fp);

    fclose(fp);

    puts("注册成功！");
    puts("");
}

void login()
{
    FILE* fp = open_file("test.txt", "a+");
    if (NULL == fp)
    {
        perror("fopen error");
        return;
    }

    char username[MAXSIZE], passwd[MAXSIZE];
    printf("请输入用户名：");
    fgets(username, sizeof(username), stdin);

    printf("请输入密码：");
    fgets(passwd, sizeof(passwd), stdin);

    char buf1[MAXSIZE], buf2[MAXSIZE];
    while (1)
    {
        fgets(buf1, sizeof(buf1), fp);
        if (strlen(buf1) == 0)
        {
            puts("登录失败！");
            puts("");
            break;
        }
        fgets(buf2, sizeof(buf2), fp);

        if (strcmp(username, buf1) == 0 && strcmp(passwd, buf2) == 0)
        {
            puts("登录成功！");
            puts("");
            break;
        }

        memset(buf1, 0, sizeof(buf1));
        memset(buf2, 0, sizeof(buf2));
    }

    fclose(fp);
}

int main(int argc, char const *argv[])
{
    int flag = 1;
    while (flag)
    {
        printf("====================\n");
        printf("------ 登录界面 -----\n");
        printf("-------1、注册-------\n");
        printf("-------2、登录-------\n");
        printf("-------0、退出-------\n");
        printf("====================\n");
        printf("请输入选项：");

        int menu;
        scanf("%d", &menu);
        getchar();

        switch (menu)
        {
        case 1:
            sign_up();
            break;
        case 2:
            login();
            break;
        case 0:
            flag = 0;
            break;
        default:
            break;
        }

    }

    return 0;
}