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| <h4 class="subsection">A.5.1 Computing the Width of an Integer Data Type</h4> |
| |
| <p><a name="index-integer-type-width-3741"></a><a name="index-width-of-integer-type-3742"></a><a name="index-type-measurements_002c-integer-3743"></a> |
| The most common reason that a program needs to know how many bits are in |
| an integer type is for using an array of <code>long int</code> as a bit vector. |
| You can access the bit at index <var>n</var> with |
| |
| <pre class="smallexample"> vector[<var>n</var> / LONGBITS] & (1 << (<var>n</var> % LONGBITS)) |
| </pre> |
| <p class="noindent">provided you define <code>LONGBITS</code> as the number of bits in a |
| <code>long int</code>. |
| |
| <p><a name="index-limits_002eh-3744"></a>There is no operator in the C language that can give you the number of |
| bits in an integer data type. But you can compute it from the macro |
| <code>CHAR_BIT</code>, defined in the header file <samp><span class="file">limits.h</span></samp>. |
| |
| <dl> |
| <!-- limits.h --> |
| <!-- ISO --> |
| <dt><code>CHAR_BIT</code><dd>This is the number of bits in a <code>char</code>—eight, on most systems. |
| The value has type <code>int</code>. |
| |
| <p>You can compute the number of bits in any data type <var>type</var> like |
| this: |
| |
| <pre class="smallexample"> sizeof (<var>type</var>) * CHAR_BIT |
| </pre> |
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