View Compilation Error

0_0_18218650_20349.cpp: In function 'int main()':
0_0_18218650_20349.cpp:10:19: error: 'fork' was not declared in this scope
     if((pid1=fork())<0)
                   ^
0_0_18218650_20349.cpp:14:15: error: 'exit' was not declared in this scope
         exit(1);        
               ^
0_0_18218650_20349.cpp:16:19: error: 'fork' was not declared in this scope
     if((pid1=fork())==0)
                   ^
0_0_18218650_20349.cpp:23:19: error: 'fork' was not declared in this scope
     if((pid2=fork())<0)
                   ^
0_0_18218650_20349.cpp:27:15: error: 'exit' was not declared in this scope
         exit(1);
               ^
0_0_18218650_20349.cpp:29:19: error: 'fork' was not declared in this scope
     if((pid2=fork())==0)
                   ^
0_0_18218650_20349.cpp:37:14: error: 'wait' was not declared in this scope
         wait();    //系统调用，等待子进程完成
              ^
0_0_18218650_20349.cpp:42:11: error: 'exit' was not declared in this scope
     exit(0);
           ^