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    <p>I think the following works, but I will state my assumptions first:</p> <ul> <li>floating-point numbers are stored in IEEE-754 format on your implementation,</li> <li>No overflow,</li> <li>You have <code>nextafterf()</code> available (it's specified in C99).</li> </ul> <p>Also, most likely, this method is not very efficient.</p> <pre><code>#include &lt;stdio.h&gt; #include &lt;stdlib.h&gt; #include &lt;math.h&gt; int main(int argc, char *argv[]) { /* Change to non-zero for superior, otherwise inferior */ int superior = 0; /* double value to convert */ double d = 0.1; float f; double tmp = d; if (argc &gt; 1) d = strtod(argv[1], NULL); /* First, get an approximation of the double value */ f = d; /* Now, convert that back to double */ tmp = f; /* Print the numbers. %a is C99 */ printf("Double: %.20f (%a)\n", d, d); printf("Float: %.20f (%a)\n", f, f); printf("tmp: %.20f (%a)\n", tmp, tmp); if (superior) { /* If we wanted superior, and got a smaller value, get the next value */ if (tmp &lt; d) f = nextafterf(f, INFINITY); } else { if (tmp &gt; d) f = nextafterf(f, -INFINITY); } printf("converted: %.20f (%a)\n", f, f); return 0; } </code></pre> <p>On my machine, it prints:</p> <pre><code>Double: 0.10000000000000000555 (0x1.999999999999ap-4) Float: 0.10000000149011611938 (0x1.99999ap-4) tmp: 0.10000000149011611938 (0x1.99999ap-4) converted: 0.09999999403953552246 (0x1.999998p-4) </code></pre> <p>The idea is that I am converting the <code>double</code> value to a <code>float</code> value&mdash;this could be less than or greater than the double value depending upon the rounding mode. When converted back to <code>double</code>, we can check if it is smaller or greater than the original value. Then, if the value of the <code>float</code> is not in the right direction, we look at the next <code>float</code> number from the converted number in the original number's direction.</p>
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    1. POConverting double to float without relying on the FPU rounding mode
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    1. USAlok Singhal
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    1. POConverting double to float without relying on the FPU rounding mode
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    1. COThank you very much for this code. I was slowly becoming convinced that this was the least error-prone solution. Thanks for pointing out `nextafterf` too, that's much better than in/decrementing the bits of the `float` as if it was an `int`. To alleviate the risk of `f+1` being equal to `f`, may I write `nextafterf(f, INFINITY)` instead?
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    2. COI just read the man pages, the C standard draft, and tried it out, and looks like `INFINITY` should work.
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    3. COOK, I have edited my post. Thanks for the comment.
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