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POEquivalence of p[0] and *p for incomplete array types
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copied!<p>Consider the following code (it came about as a result of <a href="https://stackoverflow.com/a/10183715/129570">this discussion</a>):</p> <pre><code>#include <stdio.h> void foo(int (*p)[]) { // Argument has incomplete array type printf("%d\n", (*p)[1]); printf("%d\n", p[0][1]); // Line 5 } int main(void) { int a[] = { 5, 6, 7 }; foo(&a); // Line 10 } </code></pre> <p>GCC 4.3.4 <a href="http://ideone.com/rsrRA" rel="nofollow noreferrer">complains</a> with the error message:</p> <pre><code>prog.c: In function ‘foo’: prog.c:5: error: invalid use of array with unspecified bounds </code></pre> <p>Same error message in GCC 4.1.2, and seems to be invariant of <code>-std=c99</code>, <code>-Wall</code>, <code>-Wextra</code>.</p> <p>So it's unhappy with the expression <code>p[0]</code>, but it's happy with <code>*p</code>, even though these should (in theory) be equivalent. If I comment out line 5, the code compiles and does what I would "expect" (displays <code>6</code>).</p> <p>Presumably one of the following is true:</p> <ol> <li>My understanding of the C standard(s) is incorrect, and these expressions <em>aren't</em> equivalent.</li> <li>GCC has a bug.</li> </ol> <p>I'd place my money on (1).</p> <p><strong>Question:</strong> Can anyone elaborate on this behaviour?</p> <p><strong>Clarification:</strong> I'm aware that this can be "solved" by specifying an array size in the function definition. That's not what I'm interested in.</p> <hr> <p><strong>For "bonus" points:</strong> Can anyone confirm that MSVC 2010 is in error when it rejects line 10 with the following message?</p> <pre><code>1><snip>\prog.c(10): warning C4048: different array subscripts : 'int (*)[]' and 'int (*)[3]' </code></pre>

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