void add();  //预约人数增加
void ck1();  //到窗口1进行业务
void  ck2();  //到窗口2进行业务
void  ck3();   //到窗口3进行业务
void left();   //还在等待中的客户人数
void  sum();   //总共服务客户人数
#include
void printft(){
    printf("=========排队系统===========\n");
        printf("|                          |\n");
        printf("|         有人预约:(1)      |\n");
        printf("|                          |\n");
        printf("|         到窗口1 :(2)      |\n");
        printf("|                          |\n");
        printf("|         到窗口2 :(3)      |\n");
        printf("|                          |\n");
        printf("|         到窗口3 :(4)      |\n");
        printf("|                          |\n");
        printf("|       还在排队人数:(5)    |\n");
        printf("|                          |\n");
        printf("|       总共顾客人数:(6)    |\n");
        printf("|                          |\n");
        printf("============================\n");
}

void  add(int s){                     //增加等候人数
    s=s+1;
    printf("现在有%d人正在等候",s);
}

void  left(int s){                      //删除一个人
    s=s-1;
        printf("现在还有%d人正在等候",s);
}


void ck1(int i,int a){                         //到窗口1
    printf("请%d 号到窗口1进行业务处理",i);
    a=a+1;
}


void  ck2(int i,int b){                                //到窗口2
        printf("请%d 号到窗口2进行业务处理",i);
        b=b+1;
}


void  ck3(int i,int c){                                //到窗口3
        printf("请%d 号到窗口3进行业务处理",i);
        c=c+1;
}

void  sum(int n){                              //总共处理的人数
    printf("今天总共有%d人进行了业务处理",n);
}

#include

main(){
    int s=0;
    int i=1;
    int n=0;
    int a=0,b=0,c=0;
    char k;
    int j=0;
    printf("\033[2J");
        printft();
do
        {
        scanf("%c",&k);
        if( k== '1')            add();  
        else if(k == '2')       ck1();
        else if( k== '3')       ck2();
        else if(k == '4')       ck3();
        else if(k == '5')       left();
        else if(k == '6')       sum();
       else if(k=='0')           j=1;
        else  printf("输入错误\n");
        }while(j==1);
        printf("您已退出系统!\n");
                                                                                                                                               
}


错误是：122.obj : error LNK2001: unresolved external symbol "void __cdecl sum(void)" (?sum@@YAXXZ)
122.obj : error LNK2001: unresolved external symbol "void __cdecl left(void)" (?left@@YAXXZ)
122.obj : error LNK2001: unresolved external symbol "void __cdecl ck3(void)" (?ck3@@YAXXZ)
122.obj : error LNK2001: unresolved external symbol "void __cdecl ck2(void)" (?ck2@@YAXXZ)
122.obj : error LNK2001: unresolved external symbol "void __cdecl ck1(void)" (?ck1@@YAXXZ)
122.obj : error LNK2001: unresolved external symbol "void __cdecl add(void)" (?add@@YAXXZ)