fence.cpp: In function 'int main()': fence.cpp:27: warning: ignoring return value of 'FILE* freopen(const char*, const char*, FILE*)', declared with attribute warn_unused_result fence.cpp:28: warning: ignoring return value of 'FILE* freopen(const char*, const char*, FILE*)', declared with attribute warn_unused_result fence.cpp:30: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result fence.cpp:33: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result