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    <p>There is a function-like macro that can determine the ****value bits**** of an integer type, but only if you already know that type's maximum value. Whether or not you'll get a compile-time constant depends on your compiler but I would guess in most cases the answer is yes.</p> <p>Credit to Hallvard B. Furuseth for his IMAX_BITS() function-like macro that he posted in reply to a <a href="http://groups.google.com/group/comp.lang.c/tree/browse_frm/thread/35f79d1050452749" rel="nofollow noreferrer">question on comp.lang.c</a> </p> <pre><code>/* Number of bits in inttype_MAX, or in any (1&lt;&lt;b)-1 where 0 &lt;= b &lt; 3E+10 */ #define IMAX_BITS(m) ((m) /((m)%0x3fffffffL+1) /0x3fffffffL %0x3fffffffL *30 \ + (m)%0x3fffffffL /((m)%31+1)/31%31*5 + 4-12/((m)%31+3)) </code></pre> <blockquote> <p>IMAX_BITS(INT_MAX) computes the number of bits in an int, and IMAX_BITS((unsigned_type)-1) computes the number of bits in an unsigned_type. Until someone implements 4-gigabyte integers, anyway:-)</p> </blockquote> <p><br> And <strike>credit to Eric Sosman</strike> for this <a href="http://groups.google.com/group/comp.lang.c/msg/e998153ef07ff04b?dmode=source" rel="nofollow noreferrer">alternate version</a> that should work with less than 2040 bits:<br> <strong>(EDIT 1/3/2011 11:30PM EST: It turns out this version was also written by Hallvard B. Furuseth)</strong> </p> <pre><code>/* Number of bits in inttype_MAX, or in any (1&lt;&lt;k)-1 where 0 &lt;= k &lt; 2040 */ #define IMAX_BITS(m) ((m)/((m)%255+1) / 255%255*8 + 7-86/((m)%255+12)) </code></pre> <p><br> <strong>Remember that although the width of an unsigned integer type is equal to the amount of value bits, the width of a signed integer type is one greater (§6.2.6.2/6).</strong> This is of special importance as in my original comment to your question I had incorrectly stated that the IMAX_BITS() macro calculates the width, when it actually calculates the amount of value bits. Sorry about that! </p> <p>So for example <code>IMAX_BITS(INT64_MAX)</code> will create a compile-time constant of 63. However in this example we are dealing with a signed type so you must add 1 to account for the sign bit if you want the actual width of an int64_t, which is of course 64. </p> <p>In a separate comp.lang.c discussion a user named blargg gives a breakdown of how the macro works:<br> <a href="http://coding.derkeiler.com/Archive/C_CPP/comp.lang.c/2009-01/msg02242.html" rel="nofollow noreferrer">Re: using pre-processor to count bits in integer types...</a></p> <p>Note that the macro only works with 2^n-1 values (ie all 1s in binary), as would be expected with any MAX value. Also note that while it is easy to get a compile-time constant for the maximum value of an unsigned integer type (<code>IMAX_BITS((unsigned type)-1)</code>), at the time of this writing I don't know any way to do the same thing for a signed integer type without invoking implementation defined behavior. If I ever find out I'll answer my own related SO question, here:<br> <a href="https://stackoverflow.com/questions/4514572/c-question-off-t-and-other-signed-integer-types-minimum-and-maximum-values">C question: off_t (and other signed integer types) minimum and maximum values - Stack Overflow</a></p>
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    1. POIs there any way to compute the width of an integer type at compile-time?
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    1. POIs there any way to compute the width of an integer type at compile-time?
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    1. COWhat's the implementation-defined-behavior-dependent way to do it for signed types? I assume it depends on casting an unsigned value to signed, but it would be nice to see what you've got and what assumptions it depends on.
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    2. COBy the way, the result of this macro is defined by the C language to be a constant expression, as long as the argument `m` is a constant expression.
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    3. CO@R re signed types: the way I came up with was if we were to start with the value of 1 in a signed integer type and continuously multiply by 2 until the value became negative we could determine the amount of value bits. What is implementation defined is whether or not the value will actually become negative. Since I had asked my question I've googled and found some interesting macros that check bits in a signed type sequentially using bitwise operations, but that wouldn't be portable (not that my own example is, anyway). I'll search my history on another computer for a link or two.
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