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    <p>I like both solutions. Here is another way that comes to my mind (probably not better).</p> <p><code> ((~((unsigned int)0) &lt;&lt; k) >> (k + n)) &lt;&lt; n </code></p> <p>EDIT: There was a bug in my previous version (it was without the unsigned int cast). The problem was that <code>~0 &gt;&gt; n</code> adds 1s at the front and not 0s.</p> <p>And yes this approach has one big downside; it assumes that you know the number of bits of the default integer type or in other words it assumes that you really know k, whereas the other solutions are independent of k. This makes my version less portable, or at least harder to port. (It also uses 3 shifts, and addition and a bitwise negation operator, which is two extra operations.)</p> <p><strong>So you would do better to use one of the other examples.</strong></p> <p>Here is a little test app, done by Jonathan Leffler, to compare and verify the output of the different solutions:</p> <pre><code>#include &lt;stdio.h&gt; #include &lt;limits.h&gt; enum { ULONG_BITS = (sizeof(unsigned long) * CHAR_BIT) }; static unsigned long set_mask_1(int k, int m, int n) { return ~(~0 &lt;&lt; m) &lt;&lt; n; } static unsigned long set_mask_2(int k, int m, int n) { return ((1 &lt;&lt; m) - 1) &lt;&lt; n; } static unsigned long set_mask_3(int k, int m, int n) { return ((~((unsigned long)0) &lt;&lt; k) &gt;&gt; (k + n)) &lt;&lt; n; } static int test_cases[][2] = { { 1, 0 }, { 1, 1 }, { 1, 2 }, { 1, 3 }, { 2, 1 }, { 2, 2 }, { 2, 3 }, { 3, 4 }, { 3, 5 }, }; int main(void) { size_t i; for (i = 0; i &lt; 9; i++) { int m = test_cases[i][0]; int n = test_cases[i][1]; int k = ULONG_BITS - (m + n); printf("%d/%d/%d = 0x%08lX = 0x%08lX = 0x%08lX\n", k, m, n, set_mask_1(k, m, n), set_mask_2(k, m, n), set_mask_3(k, m, n)); } return 0; } </code></pre>
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    1. COOn the assumption that this answer can be made to work, the obvious downside in comparison with the other two is the presence of the third shift operation, which makes it more time consuming.
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    2. COThe other problem is that it uses the parameter k which the other two solutions are able to ignore (it doesn't use m, though, so it still only uses two of the three parameters).
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    3. CORight there was a bug in it, I fixed it now and add a comment that the other solutions are preferable. I haven't removed it completely, maybe someone can learn from my mistakes and it'd be sad to loose your nice test code :).
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