foo.cpp: In function 'void highprecision(char*, char*)':
foo.cpp:23:21: error: 'strlen' was not declared in this scope
  int lena = strlen(a);
                     ^
foo.cpp:27:35: error: 'memset' was not declared in this scope
  memset(p, 0, maxlen * sizeof(int));
                                   ^
foo.cpp: In function 'int* addprecision(char*, char*)':
foo.cpp:54:21: error: 'strlen' was not declared in this scope
  int lena = strlen(a);
                     ^
foo.cpp:58:30: error: 'memset' was not declared in this scope
  memset(arra, 0, sizeof(arra));
                              ^
foo.cpp:57:30: warning: address of local variable 'arrc' returned [-Wreturn-local-addr]
  int arra[maxn], arrb[maxn], arrc[maxn];
                              ^~~~