Compilare: user.cpp: In function ‘unsigned int sieve(unsigned int)’: user.cpp:33:19: warning: operation on ‘i’ may be undefined [-Wsequence-point] ((p[++i >> 5] & (1 << (i & 31))) == 0); ^~~ user.cpp:33:19: warning: operation on ‘i’ may be undefined [-Wsequence-point] user.cpp:33:19: warning: operation on ‘i’ may be undefined [-Wsequence-point] user.cpp:7:29: warning: unused variable ‘k’ [-Wunused-variable] register unsigned i, j, k, nr = 1 + (n > 2); ^ user.cpp: In function ‘int main()’: user.cpp:41:35: 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:42:37: 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:44:32: warning: ignoring return value of ‘int scanf(const char*, ...)’, declared with attribute warn_unused_result [-Wunused-result] unsigned N; scanf("%d", &N); ^