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<h3 class="section">6.9 Double-Word Integers</h3>
<p><a name="index-g_t_0040code_007blong-long_007d-data-types-2905"></a><a name="index-double_002dword-arithmetic-2906"></a><a name="index-multiprecision-arithmetic-2907"></a><a name="index-g_t_0040code_007bLL_007d-integer-suffix-2908"></a><a name="index-g_t_0040code_007bULL_007d-integer-suffix-2909"></a>
ISO C99 supports data types for integers that are at least 64 bits wide,
and as an extension GCC supports them in C90 mode and in C++.
Simply write <code>long long int</code> for a signed integer, or
<code>unsigned long long int</code> for an unsigned integer. To make an
integer constant of type <code>long long int</code>, add the suffix ‘<samp><span class="samp">LL</span></samp>’
to the integer. To make an integer constant of type <code>unsigned long
long int</code>, add the suffix ‘<samp><span class="samp">ULL</span></samp>’ to the integer.
<p>You can use these types in arithmetic like any other integer types.
Addition, subtraction, and bitwise boolean operations on these types
are open-coded on all types of machines. Multiplication is open-coded
if the machine supports a fullword-to-doubleword widening multiply
instruction. Division and shifts are open-coded only on machines that
provide special support. The operations that are not open-coded use
special library routines that come with GCC.
<p>There may be pitfalls when you use <code>long long</code> types for function
arguments without function prototypes. If a function
expects type <code>int</code> for its argument, and you pass a value of type
<code>long long int</code>, confusion results because the caller and the
subroutine disagree about the number of bytes for the argument.
Likewise, if the function expects <code>long long int</code> and you pass
<code>int</code>. The best way to avoid such problems is to use prototypes.
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