Compilare: user.c: In function ‘main’: user.c:13:5: warning: implicit declaration of function ‘sqrt’ [-Wimplicit-function-declaration] for(i=2; i<=sqrt(N); i++) ^ user.c:13:17: warning: incompatible implicit declaration of built-in function ‘sqrt’ [enabled by default] for(i=2; i<=sqrt(N); i++) ^ user.c:10:11: warning: ignoring return value of ‘fscanf’, declared with attribute warn_unused_result [-Wunused-result] fscanf(f,"%d",&N); ^