View Compilation Error

0_0_17979513_30290.cpp: In function 'int main()':
0_0_17979513_30290.cpp:12:22: error: 'strlen' was not declared in this scope
   int i,len=strlen(s1);
                      ^
0_0_17979513_30290.cpp:13:15: error: 'strcpy' was not declared in this scope
   strcpy(s3,s1);
               ^
0_0_17979513_30290.cpp:16:17: error: 'strstr' was not declared in this scope
   p=strstr(s3,s2);
                 ^
0_0_17979513_30290.cpp:19:25: error: 'memset' was not declared in this scope
   memset(s1,0,sizeof(s1));
                         ^