Compilare: user.c: In function ‘isoperator’: user.c:28:3: warning: array subscript has type ‘char’ [-Wchar-subscripts] return pass[c]; ^ user.c: In function ‘main’: user.c:271:9: warning: ignoring return value of ‘fscanf’, declared with attribute warn_unused_result [-Wunused-result] fscanf(f, "%s", s); ^ user.c:291:10: warning: ignoring return value of ‘freopen’, declared with attribute warn_unused_result [-Wunused-result] freopen("expr.out", "w", stdout); ^