printf bug ??!

[lanael]

void main() {
ulong u =0b100000000000000000000000000000000000000000000000; // 1 and 
47 zeros
ulong u1=0b1000000000000000000000000000000000000000000000000; // 1 and 
48 zeros
printf("u  = \t %llx \t %llb\n",u,u);
printf("u1 = \t %llx \t %llb\n",u1,u1);
}

output :
u  =     800000000000    
100000000000000000000000000000000000000000000000
u1 =     1000000000000    
000000000000000000000000000000000000000000000000

it looks like a buffer overflow...