Compilare: user.c: In function ‘eratosthenes’: user.c:4:2: warning: implicit declaration of function ‘malloc’ [-Wimplicit-function-declaration] int *a = malloc(2000005 * sizeof(int)); ^ user.c:4:11: warning: incompatible implicit declaration of built-in function ‘malloc’ [enabled by default] int *a = malloc(2000005 * sizeof(int)); ^ user.c:15:2: warning: implicit declaration of function ‘free’ [-Wimplicit-function-declaration] free(a); ^ user.c:15:2: warning: incompatible implicit declaration of built-in function ‘free’ [enabled by default] user.c: In function ‘main’: user.c:22:9: warning: ignoring return value of ‘freopen’, declared with attribute warn_unused_result [-Wunused-result] freopen("ciur.in", "r", stdin); ^ user.c:23:9: warning: ignoring return value of ‘freopen’, declared with attribute warn_unused_result [-Wunused-result] freopen("ciur.out", "w", stdout); ^ user.c:24:7: warning: ignoring return value of ‘scanf’, declared with attribute warn_unused_result [-Wunused-result] scanf("%d", &n); ^