Compilare: user.cpp: In function ‘unsigned int sieve(unsigned int)’: user.cpp:9:18: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(i = 2; i <= n; ++i) ~~^~~~ user.cpp:15:30: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(j = i + i; j <= n; j += i) ~~^~~~ 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("ciur.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("ciur.out", "w", stdout); ^ user.cpp:30:32: warning: ignoring return value of ‘int scanf(const char*, ...)’, declared with attribute warn_unused_result [-Wunused-result] unsigned N; scanf("%d", &N); ^