为什么注册了第一次就不能注册第二次了 ？而且登录的时候不管登录什么都能成功？

Question

#include<stdio.h>
#include<stdlib.h>
#include<string.h>
struct log
{
char name[10];
char key[10];
struct log * next;
};
int main()
{
int m,n=1;
FILE * pf;
struct log * head=NULL, * p=NULL,* tail=NULL,* pMove=NULL;
char i[10],j[10];
while(n!=0)
{

    printf("-------学籍信息管理系统-教师端------\n");
    printf("-------------1.注册-----------------\n");
    printf("-------------2.登录-----------------\n");
    scanf("%d",&m);
    if(m==1)
    {
        printf("请新建用户名:");
        scanf("%s",i);
        if ((pf = fopen("m.txt", "r")) == NULL)
        {
            pf = fopen("m.txt", "w");
        }
        while(1)
        { 
            if (fscanf(pf,"%s%s\n",i,j) != 2) break;
            p = (struct log*)malloc(sizeof(struct log));
            p->next = NULL;
            strcpy(p->name,i);strcpy(p->key,j);
            if (head == NULL) 
                head = p;
            else 
                tail->next = p;
            tail = p;
        }
        fclose(pf);
        pMove=head;
        while(pMove!=NULL&&strcmp(pMove->name,i))
        {
            pMove=pMove->next;
        }
        if(pMove==NULL)
        {
            printf("设 置 密 码:");
            scanf("%s",j);
            if ((pf=fopen("m.txt","a"))==NULL)
            {
                printf("文件打开失败！");
                return;
            }
            printf("注册成功！\n");
            fprintf(pf,"%s %s",i,j);
            fclose(pf);
        }
        else if(strcmp(pMove->name,i)==0)
        {
            printf("该用户名已经被注册,请重新注册!\n");return;
        }
        else
        {
            printf("设 置 密 码:");
            scanf("%s",j);
            if ((pf=fopen("m.txt","a"))==NULL)
            {
                printf("文件打开失败！");
                return;
            }
            fprintf(pf,"%s %s",i,j);
            fclose(pf);
        }
    }
    else if(m==2)
    {
        printf("用户名:");
        scanf("%s",i);
        if ((pf = fopen("m.txt", "r")) == NULL)
        {
            printf("文件打开失败！");return;
        }
        while(1)
        { 
            if (fscanf(pf,"%s%s\n",i,j) != 2) break;
            p = (struct log*)malloc(sizeof(struct log));
            p->next = NULL;
            strcpy(p->name,i);strcpy(p->key,j);
            if (head == NULL) 
                head = p;
            else 
                tail->next = p;
            tail = p;
        }
        fclose(pf);
        pMove=head;
        while(pMove!=NULL&&strcmp(pMove->name,i))
        {
            pMove=pMove->next;
        }
        if(pMove!=NULL)
        {
            printf("密 码:");
            scanf("%s",j);
            pMove=head;
            while(pMove!=NULL&&strcmp(pMove->key,j))
            {
                pMove=pMove->next;
            }
            printf("登录成功！\n");
        }
        else
            printf("用户名不存在！\n");
    }
    else
        printf("输入错误！请重新输入！\n");
}

}

Answer 1

修改如下，供参考:

#include<stdio.h>
#include<stdlib.h>
#include<string.h>
struct log
{
    char name[10];
    char key[10];
    struct log* next;
};
int main()
{
    int m, n = 1;
    FILE* pf;
    struct log* head = NULL, * p = NULL, * tail = NULL, * pMove = NULL;
    char i[10], j[10];
    while (n != 0)
    {
        printf("-------学籍信息管理系统-教师端------\n");
        printf("-------------1.注册-----------------\n");
        printf("-------------2.登录-----------------\n");
        scanf("%d", &m);
        if (m == 1)
        {
            if ((pf = fopen("m.txt", "r")) == NULL)
            {
                
                if ((pf = fopen("m.txt", "a")) == NULL)
                {
                    printf("文件打开失败！");
                }
                else {
                    printf("请新建用户名:");
                    scanf("%s", i);
                    printf("设 置 密 码:");
                    scanf("%s", j);
                    fprintf(pf, "%s %s\n", i, j);
                    fclose(pf);
                    printf("用户注册成功！\n");
                }                
            }
            else{
                head = NULL, p = NULL, tail = NULL, pMove = NULL;
                while (1)
                {
                    if (fscanf(pf, "%s %s\n", i, j) != 2) break;
                    p = (struct log*)malloc(sizeof(struct log));
                    p->next = NULL;
                    strcpy(p->name, i); strcpy(p->key, j);
                    if (head == NULL)
                        head = p;
                    else
                        tail->next = p;
                    tail = p;
                }
                fclose(pf);
                do {
                    printf("请新建用户名:");
                    scanf("%s", i);
                    pMove = head;
                    while (pMove != NULL && strcmp(pMove->name, i) != 0)
                    {
                        pMove = pMove->next;
                    }
                    if (pMove == NULL)
                    {
                        printf("设 置 密 码:");
                        scanf("%s", j);
                        pf = fopen("m.txt", "a");
                        fprintf(pf, "%s %s\n", i, j);
                        fclose(pf);
                        printf("用户注册成功！\n");
                    }
                    else
                        printf("该用户名已经被注册,请重新输入!\n");
                } while (pMove != NULL);
                    
            }
        }
        else if (m == 2)
        {
            if ((pf = fopen("m.txt", "r")) == NULL)
            {
                printf("无注册用户，请先注册用户。\n"); //return -1;
            }
            else{
                head = NULL, p = NULL, tail = NULL, pMove = NULL;
                while (1)
                {
                    if (fscanf(pf, "%s %s\n", i, j) != 2) break;
                    p = (struct log*)malloc(sizeof(struct log));
                    p->next = NULL;
                    strcpy(p->name, i); strcpy(p->key, j);
                    if (head == NULL)
                        head = p;
                    else
                        tail->next = p;
                    tail = p;
                }
                fclose(pf);
                printf("用户名:");
                scanf("%s", i);
                pMove = head;
                while (pMove != NULL && strcmp(pMove->name, i) != 0)
                {
                    pMove = pMove->next;
                }
                if (pMove != NULL)
                {
                    printf("密 码:");
                    scanf("%s", j);
                    if (strcmp(pMove->key, j) == 0)
                    {
                        printf("登录成功！\n");
                        break;
                    }
                    else {
                        printf("密码错误!\n");
                    }
                }
                else
                    printf("用户名不存在！\n");
            }
        }
        else
            printf("输入错误！请重新输入！\n");
    }
    return 0;
}