Compilare: user.cpp: In function ‘int main()’: user.cpp:27:35: warning: ignoring return value of ‘FILE* freopen(const char*, const char*, FILE*)’, declared with attribute warn_unused_result [-Wunused-result] freopen("prefix.in","r",stdin); ^ user.cpp:28:37: warning: ignoring return value of ‘FILE* freopen(const char*, const char*, FILE*)’, declared with attribute warn_unused_result [-Wunused-result] freopen("prefix.out","w",stdout); ^ user.cpp:29:19: warning: ignoring return value of ‘int scanf(const char*, ...)’, declared with attribute warn_unused_result [-Wunused-result] scanf("%d",&N); ^ user.cpp:30:16: warning: ignoring return value of ‘char* gets(char*)’, declared with attribute warn_unused_result [-Wunused-result] gets(s + 1); ^ user.cpp:32:20: warning: ignoring return value of ‘char* gets(char*)’, declared with attribute warn_unused_result [-Wunused-result] gets(s + 1); ^ /tmp/ccrjDVu8.o: In function `main': user.cpp:(.text.startup+0x67): warning: the `gets' function is dangerous and should not be used.