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POStrange warning in a C function const multidimensional-array argument
 

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    <p>I'm getting some strange warnings about this code:</p> <pre><code>typedef double mat4[4][4]; void mprod4(mat4 r, const mat4 a, const mat4 b) { /* yes, function is empty */ } int main() { mat4 mr, ma, mb; mprod4(mr, ma, mb); } </code></pre> <p><code>gcc</code> output as follows:</p> <pre><code>$ gcc -o test test.c test.c: In function 'main': test.c:13: warning: passing argument 2 of 'mprod4' from incompatible pointer type test.c:4: note: expected 'const double (*)[4]' but argument is of type 'double (*)[4]' test.c:13: warning: passing argument 3 of 'mprod4' from incompatible pointer type test.c:4: note: expected 'const double (*)[4]' but argument is of type 'double (*)[4]' </code></pre> <p>If I define the function as:</p> <pre><code>void mprod4(mat4 r, mat4 a, mat4 b) { } </code></pre> <p>Or defining matrices in main as:</p> <pre><code>mat4 mr; const mat4 ma; const mat4 mb; </code></pre> <p>Or call the function in main as:</p> <pre><code>mprod4(mr, (const double(*)[4])ma, (const double(*)[4])mb); </code></pre> <p>Or even defining <code>mat4</code> as:</p> <pre><code>typedef double mat4[16]; </code></pre> <p>Makes the warning go away. What is happening here? Am I doing something invalid?</p> <p>The gcc version is 4.4.3, if relevant.</p> <p>I also posted on gcc bugzilla: <a href="http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47143" rel="nofollow noreferrer">http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47143</a></p> <p>My current workaround is making ugly macros that cast stuff for me:</p> <pre><code>#ifndef _NO_UGLY_MATRIX_MACROS #define mprod4(r, a, b) mprod4(r, (const double(*)[4])a, (const double(*)[4])b) #endif </code></pre> <hr> <p>Answer from Joseph S. Myers on gcc bugzilla:</p> <blockquote> <p>Not a bug. The function parameters are of type "pointer to array[4] of const double" because const on an array type applies to the element type, recursively, and then the outermost array type, only, of a parameter of array type decays to a pointer, and the arguments passed are of type "pointer to array[4] of double" after array-to-pointer decay, and the only case where qualifiers are permitted to be added in assignment, argument passing etc. is qualifiers on the immediate pointer target, not those nested more deeply.</p> </blockquote> <p>Sounds pretty confusing to me, like the function expects:</p> <pre><code>pointer to array[4] of const doubles </code></pre> <p>and we are passing</p> <pre><code>pointer to const array[4] of doubles </code></pre> <p>intead.</p> <p>Or would it be the inverse? The warnings suggest that the function expects a:</p> <pre><code>const double (*)[4] </code></pre> <p>which seems to me more like a</p> <pre><code>pointer to const array[4] of doubles </code></pre> <p>I'm really confused with this answer. Could somebody who understands what he said clarify and exemplify?</p>
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