学生管理system的代码找错！

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int num=0;
int mid=0;
struct info
{
    char id[20];
    char name[20];
    int c;
    int sql;
    int count;
}stu[100];
void login()
{

}


void fun(x[],char s[])
{
    int i,o;
    for(i=0;i<num;i++)
       {
           if(strcmp(x,s)==0)
           {
               printf("%s\n",stu[i]);
               printf("1.修改学号 2.修改姓名 3.修改C 4.修改SQL ");
               scanf("%d",&o);
               switch(o)
               {
               case 1:
                printf("修改为:");
                scanf("%d",&stu[i].id);
        
                   break;
               case 2:
                printf("修改为:");
                scanf("%d",&stu[i].name);
            
                   break;
               case 3:
                printf("修改为:");
                scanf("%d",&stu[i].c);
                   break;
               case 4:
                printf("修改为:");
                scanf("%d",&stu[i].sql);
            
                   break;
               default:
                   printf("莫瞎搞！");
                   break;
               }
                   printf("学号%s 姓名%s C成绩%d SQL成绩%d 总成绩%d\n",stu[i].id,stu[i].name,stu[i].c,stu[i].sql,stu[i].count);
              
           }
       }
}
input(int num)
{
    int i=num;
    printf("输入要录入的学生数量：",i+1);
    scanf("%d",&num);
    do{
        printf("请输入第%d个学生的学号：",i+1);
        fflush(stdin);
        gets(stu[i].id);
        printf("请输入第%d个学生的姓名：",i+1);
        fflush(stdin);
        gets(stu[i].name);
        printf("请输入第%d个学生的c成绩：",i+1);
        scanf("%d",&stu[i].c);
        printf("请输入第%d个学生的sql成绩：",i+1);
        scanf("%d",&stu[i].sql);
        stu[i].count=stu[i].c+stu[i].sql;
        i++;
    }while(i<num);
    return num;
}
insert(int num)
{
    int i=num;
    printf("输入要插入的数量:");
    scanf("%d",&mid);
    num+=mid;
    do
    {
        printf("请输入第%d个学生的学号：",i+1);
        fflush(stdin);
        gets(stu[i].id);
        printf("请输入第%d个学生的姓名：",i+1);
        fflush(stdin);
        gets(stu[i].name);
        printf("请输入第%d个学生的c成绩：",i+1);
        scanf("%d",&stu[i].c);
        printf("请输入第%d个学生的sql成绩：",i+1);
        scanf("%d",&stu[i].sql);
        stu[i].count=stu[i].c+stu[i].sql;
        i++;
    }while(i<num);

    return num;   
}


chang(int num)
{
    int o,i=0;
    char oid[20],oname[20];
   printf("1.使用学号  2.使用姓名\n");
   scanf("%d",&o);
   switch(o)
   {
   case 1:
       printf("输入学号：");
       scanf("%s",&oid);
       fun(stu[i].id,oid);
       break;
   case 2:
       printf("输入姓名：");
       scanf("%s",&oname);
       fun(stu[i].name,oname);

       break;
   default:
       printf("莫瞎搞！");
       break;
   }


   return num;
}



range(int num)
{
    int i,j;
    int temp;
    
    for(i=0;i<num;i++)
    {
        for(j=0;j<num-1-i;j++)
        {
            if(stu[j].count>stu[j+1].count)
            {
                temp=stu[j];
                stu[j]=stu[j+1];
                stu[j+1]=temp;
            }

        }
    }
    for(i=0;i<num;i++)
    {
     printf("第%d名 学号%s 姓名%s C成绩%d SQL成绩%d 总成绩%d\n",i+1,stu[i].id,stu[i].name,stu[i].c,stu[i].sql,stu[i].count);
    }
    return num;
}


void fun1(char s[])
{
   int i;
   for(i=0;i<num;i++)
   {
       if(strcmp(stu[i]))
       {
       }
   }
}

del(int num)
{
    int o,i,o1;
    char oid[20],oname[20];
    printf("1.使用学号\n2.使用姓名\n");
    scanf("%d",&o);

       switch(o)
       {
       case 1:
           printf("输入学号：");
           scanf("%s",&oid);
            fun1(oid);
           break;
       case 2:
           printf("输入姓名：");
           scanf("%s",&oname);
           fun1(oname);
           
           break;
       default:
              printf("莫瞎搞！");
           break;
       }
    return num;
}








void main()
{
    int n;
    login();

   do
   {
    printf("1.输入\n");
    printf("2.插入\n");
    printf("3.修改\n");
    printf("4.排列\n");
    printf("5.删除\n");
    printf("6.退出\n");
    printf("请选择：");
    scanf("%d",&n);

    switch(n)
    {
    case 1:
        num=input(num);
          break;
    case 2:
        num=insert(num);
        break;
    case 3:
        num=chang(num);
        break;
    case 4:
        num=range(num);
        break;
    case 5:
        num=del(num);
        break;
    case 6:
        exit(1);
    }
   }while(1);
}
----------------解决方案--------------------------------------------------------
莫瞎搞。
呵呵、。
代码太长，偶头疼，
----------------解决方案--------------------------------------------------------
你的错误提示是什么，请发上来。。
----------------解决方案--------------------------------------------------------