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<title>Using the GNU Compiler Collection (GCC)</title>
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<h3 class="section">Double-Word Integers</h3>
<p>ISO C99 supports data types for integers that are at least 64 bits wide,
and as an extension GCC supports them in C89 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 <code>LL</code>
to the integer. To make an integer constant of type <code>unsigned long
long int</code>, add the suffix <code>ULL</code> 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 fullword-to-doubleword a 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, unless you declare 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 will result because the caller and the
subroutine will 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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