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copied!<p>Well, scanf expects a char* pointer as the next argument when seeing a "%s". But what you give it is a pointer to a char[100]. You give it a <code>char(*)[100]</code>. It's not guaranteed to work at all, because the compiler may use a different representation for array pointers of course. If you turn on warnings for gcc, you will see also the proper warning displayed.</p> <p>When you provide an argument object that is an argument not having a listed parameter in the function (so, as in the case for scanf when has the vararg style "..." arguments after the format string), the array will degenerate to a pointer to its first element. That is, the compiler will create a <code>char*</code> and pass that to printf.</p> <p>So, <strong>never do it with <code>&a</code></strong> and pass it to scanf using "%s". Good compilers, as comeau, will warn you correctly:</p> <blockquote> <p>warning: argument is incompatible with corresponding format string conversion</p> </blockquote> <p>Of course, the <code>&a</code> and <code>(char*)a</code> <em>have</em> the same address stored. But that does not mean you can use <code>&a</code> and <code>(char*)a</code> interchangeably.</p> <hr> <p>Some Standard quotes to especially show how pointer arguments are <em>not</em> converted to <code>void*</code> auto-magically, and how the whole thing is undefined behavior. </p> <blockquote> <p>Except when it is the operand of the sizeof operator or the unary & operator, or is a string literal used to initialize an array, an expression that has type <em>‘‘array of type’’ is converted</em> to an expression with type <em>‘‘pointer to type’’</em> that points to the initial element of the array object. (<code>6.3.2.1/3</code>)</p> </blockquote> <p>So, that is done always - it isn't mentioned below explicitly anymore when listening valid cases when types may differ.</p> <blockquote> <p>The ellipsis notation in a function prototype declarator causes argument type conversion to stop after the last declared parameter. The default argument promotions are performed on trailing arguments. (<code>6.5.2.2/7</code>)</p> </blockquote> <p>About how <code>va_arg</code> behaves extracting the arguments passed to printf, which is a vararg function, emphasis added by me (<code>7.15.1.1/2</code>):</p> <blockquote> <p>Each invocation of the va_arg macro modifies ap so that the values of successive arguments are returned in turn. The parameter type shall be a type name specified such that the type of a pointer to an object that has the specified type can be obtained simply by postﬁxing a <code>*</code> to type. If there is no actual next argument, or if type is not compatible with the type of the actual next argument (as promoted according to the default argument promotions), the <em>behavior is undefined</em>, except for the following cases: </p> <ul> <li>one type is a signed integer type, the other type is the corresponding unsigned integer type, and the value is representable in both types;</li> <li>one type is <em>pointer to void</em> and the other is a pointer to a <em>character type</em>.</li> </ul> </blockquote> <p>Well, here is what that <em>default argument promotion</em> is:</p> <blockquote> <p>If the expression that denotes the called function has a type that does not include a prototype, the integer promotions are performed on each argument, and arguments that have type float are promoted to double. These are called the default argument promotions. (<code>6.5.2.2/6</code>)</p> </blockquote>

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