#include "stdio.h"
#include "stdlib.h"
#include "time.h"

int f1(int x,int y,int z);
main()
{
    int x,y,z;
    srand(time(NULL));
    x=rand()%100;
    y=rand()%100;
    z=x+y;
    int f1(x,y,z);
}
int f1(int x,int y,int z)
{
   int i,m;

   for(i=1;i<=3;i++)
     {
     printf("%d+%d=?\n",x,y);
     scanf("%d",&m);

      if(m==z)
            {
           printf("Right!");
            break;
         }
      else if(m!=z)
                {
           printf("NOT corect! try again!");
            }
      }
    printf("not correct ,you have tried three times! Test over!");
    return 0;
}

int f1(int x,int y,int z)
调用函数时不用 int
f1(int x,int y,int z）；

那什么情况下可以定义函数类型

函数声名和函数定义的时候要说明函数类型

root@~ #cat 1.c
#include "stdio.h"
#include "time.h"

int f1(int x,int y,int z);
int main (void) {
    int x,y,z;
    srand((unsigned)time(NULL));
    x=rand()%100;
    y=rand()%100;
    z=x+y;
        f1(x,y,z);
}
int f1(int x,int y,int z) {

   int i,m;

        for(i=1;i<=3;i++) {
                printf("%i+%i=?\n",x,y);
                scanf("%d",&m);
                if(m==z) {
                        printf("Right!\n");
                        return 0;
                }
       if(m!=z) {
                        printf("NOT corect! try again!\n");
                }
        }
        printf ("not correct,you have tried three times ! Test over!\n");
}
root@~ #

root@~ #./1
76+56=?
1
NOT corect! try again!
76+56=?
1
NOT corect! try again!
76+56=?
1
NOT corect! try again!
not correct,you have tried three times ! Test over!
root@~ #./1
32+86=?
1
NOT corect! try again!
32+86=?
1
NOT corect! try again!
32+86=?
118
Right!
root@~ #