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copied!<p><code>floats</code> are automatically promoted to <code>doubles</code> when passed as <code>...</code> parameters (similarly to how <code>chars</code> and <code>short ints</code> are promoted to <code>ints</code>). When <code>printf()</code> looks at a format specifier (<code>%d</code> or <code>%f</code> or whatever), it grabs and interprets the raw data it has received according to the format specifier (as <code>int</code> or <code>double</code> or whatever) and then prints it.</p> <p><code>printf("%d\n",x)</code> is wrong because you are passing a <code>double</code> to <code>printf()</code> but lie to it that it's going to be an <code>int</code>. <code>printf()</code> makes you pay for the lie. It takes 4 bytes (most likely, but not necessarily 4 bytes) from its parameters, which is the size of an <code>int</code>, it grabs those bytes from where you have previously put 8 bytes (again, most likely, but not necessarily 8 bytes) of the <code>double</code> 4.0, of which those 4 bytes happen to be all zeroes and then it interprets them as an integer and prints it. Indeed, powers of 2 in the IEEE-754 double precision format normally have 52 zero bits, or more than 6 8-bit bytes that are zero.</p> <p>Those "most likely, but not necessarily" words mean that the C standard does not mandate a fixed size and range for types and they may vary from compiler to compiler, from OS to OS. These days 4-byte <code>ints</code> and 8-byte <code>doubles</code> are the most common (if we consider, e.g. the x86 platform).</p>

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