foo.cc: In function 'int main()':
foo.cc:16:22: warning: ignoring return value of 'FILE* freopen(const char*, const char*, FILE*)', declared with attribute warn_unused_result [-Wunused-result]
 #define fr(x) freopen(x, "r", stdin)
               ~~~~~~~^~~~~~~~~~~~~~~
foo.cc:77:3: note: in expansion of macro 'fr'
   fr("data/data.in");
   ^~
foo.cc:17:22: warning: ignoring return value of 'FILE* freopen(const char*, const char*, FILE*)', declared with attribute warn_unused_result [-Wunused-result]
 #define fw(x) freopen(x, "w", stdout)
               ~~~~~~~^~~~~~~~~~~~~~~~
foo.cc:78:3: note: in expansion of macro 'fw'
   fw("data/data.out");
   ^~

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