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    <p>I believe the problem is the constraints specified in <em>C99</em> <strong>6.5.16.1(1)</strong>, which seem to prohibit mixing qualifications in assignments, except for pointers for which an inclusive-qualifier exception is defined. The problem is that with indirect pointers, you end up passing a pointer to one thing to a pointer to another. The assignment isn't valid because, if it was, you could fool it into modifying a const-qualified object with the following code:</p> <pre><code>const char **cpp; char *p; const char c = 'A'; cpp = &amp;p; // constraint violation *cpp = &amp;c; // valid *p = 0; // valid by itself, but would clobber c </code></pre> <p>It might seem reasonable that <code>cpp</code>, which promises not to modify any <code>char</code>s, might be assigned a pointer to an object pointing at non-qualified <code>char</code>s. After all, that's allowed for single-indirect pointers, which is why, e.g., you can pass a mutable object to the second parameter of <code>strcpy(3)</code>, the first parameter to <code>strchr(3)</code>, and many other parameters that are declared with <code>const</code>.</p> <p>But with the indirect pointer, at the next level, assignment from a qualified pointer is allowed, and now a perfectly unqualified pointer assignment will clobber a qualified object. </p> <p>I don't immediately see how a 2-D array could lead to this situation, but in any case it hits the same constraint in the standard. </p> <p>Since in your case, you aren't actually tricking it into clobbering a const, the right thing for <em>your</em> code would seem to be inserting the cast.</p> <hr> <p><strong>Update:</strong> OK guys, as it happens this issue is <strong><a href="http://c-faq.com/ansi/constmismatch.html" rel="noreferrer">in the C faq</a></strong>, and this entire discussion has also taken place several times <strong><a href="http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20230" rel="noreferrer">on the gcc bug list</a></strong> and on the gcc mailing list. </p> <ul> <li>Gcc bug list: <strong><a href="http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20230" rel="noreferrer">http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20230</a></strong>.</li> <li>C FAQ: it's question 11.10: <strong><a href="http://c-faq.com/ansi/constmismatch.html" rel="noreferrer">http://c-faq.com/ansi/constmismatch.html</a></strong></li> </ul> <p>The lesson: you can pass a <code>T *x</code> when <code>const T *x</code> is expected, by explicit exception, but <code>T *x</code> and <code>const T *x</code> are still distinct types, so you can't pass a pointer to either one to a pointer to the other.</p>
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    1. POStrange warning in a C function const multidimensional-array argument
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    1. POStrange warning in a C function const multidimensional-array argument
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    1. COSee my comment below. I don't see why you think that 6.5.16.1(1) is violated. You can add qualifiers (say, assign a non-const to a const), you just can't remove them.
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      1. USEugene Smith
    2. COThanks, but I see no constraint violation in your example. const char **cpp means that we are not allowed to change the memory pointed by (**cpp) right? If so, only violation here would be (**cpp) = 0. Correct me if I am wrong please.
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    3. CO+1 My conclusion is different than yours, but we both agree that this is a valid problem.
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      1. USChris Lutz
 

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