View Compilation Error

0_0_27732459_25867.cpp: In function 'char add(char*, char*)':
0_0_27732459_25867.cpp:7:29: error: 'strlen' was not declared in this scope
      int up=0, lena=strlen(a)-1, lenb=strlen(b)-1, id=0;
                             ^
0_0_27732459_25867.cpp:9:21: error: 'lenb' was not declared in this scope
      while(lena>=0||lenb>=0)
                     ^
0_0_27732459_25867.cpp:16:14: error: 'id' was not declared in this scope
          ans[id++] = k + '0';
              ^
0_0_27732459_25867.cpp:19:20: error: 'id' was not declared in this scope
      if(up==1) ans[id++] = '1';
                    ^
0_0_27732459_25867.cpp:20:10: error: 'id' was not declared in this scope
      ans[id] = '\0';
          ^
0_0_27732459_25867.cpp:21:16: error: 'strrev' was not declared in this scope
      strrev(ans);
                ^
0_0_27732459_25867.cpp:22:18: error: 'strcpy' was not declared in this scope
      strcpy(c,ans);
                  ^