Compilare: user.c: In function ‘read’: user.c:37:13: warning: ignoring return value of ‘freopen’, declared with attribute warn_unused_result [-Wunused-result] freopen(filename, "r", stdin); ^ user.c:39:11: warning: ignoring return value of ‘scanf’, declared with attribute warn_unused_result [-Wunused-result] scanf("%d %d", &N, &M); ^ user.c:43:15: warning: ignoring return value of ‘scanf’, declared with attribute warn_unused_result [-Wunused-result] scanf("%d %d", &i, &j); ^ user.c: In function ‘write’: user.c:87:13: warning: ignoring return value of ‘freopen’, declared with attribute warn_unused_result [-Wunused-result] freopen(filename, "w", stdout); ^