def l2c(l):
    "unsigned long to char[4]"
    c = []
    c.append(int(l & U32(0xFF)))
    c.append(int((l >> 8) & U32(0xFF)))
    c.append(int((l >> 16) & U32(0xFF)))
    c.append(int((l >> 24) & U32(0xFF)))
    return c
unsigned long to char[4]