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    <p><em>All</em> C functions should have prototypes. They're not actually mandatory, but there's no good reason not to use them (unless you're stuck with a pre-ANSI compiler that doesn't support them). (But see the bottom of this answer.)</p> <p>If you want a function that takes a variable number of arguments, that prototype should end with <code>, ...</code>, and the function itself should use the <code>&lt;stdarg.h&gt;</code> mechanism to process its arguments. (This requires at least one argument with a defined type; that argument is used as an anchor for the following arguments.) It's documented <a href="http://man7.org/linux/man-pages/man3/stdarg.3.html" rel="nofollow">here</a> and elsewhere.</p> <p>As I was typing this, you updated your question with "<strong>NOTE: No libraries (such as (...) )should be used</strong>". <code>&lt;stdarg.h&gt;</code> is one of the handful headers that's required for all conforming C implementations, including freestanding (embedded) ones -- because it doesn't define any functions, just types and macros. Your C implementation should support it. If it doesn't, then it's not a conforming C implementation, and you'll need to tell us exactly what compiler you're using and/or read its documentation to find out how it handles variadic functions, or an equivalent.</p> <p>If you really can't use <code>, ...</code> and <code>&lt;stdarg.h&gt;</code>, (or perhaps the older <code>&lt;varargs.h&gt;</code>), then you can define your function with a fixed number of arguments, enough for all uses, then have callers pass extra null pointers.</p> <p><strong>EDIT:</strong></p> <p>This is an update based on new information in comments and chat.</p> <p>The OP has a homework assignment to implement <code>printf</code> for some TI microcontroller, for some reason <em>not</em> using either the <code>, ...</code> notation or <code>&lt;stdarg.h&gt;</code>. The compiler in question apparently implements C89/C90, so it does support both features; this is an arbitrary restriction.</p> <p>This information should have been in the question, which is why I'm downvoting it until the OP updates it.</p> <p>There is no portable way to achieve this -- which is exactly why <code>, ...</code> is part of the standard language, and <code>&lt;stdarg.h&gt;</code> is part of the standard library.</p> <p>Probably the best approach would be to write a program that uses <code>, ...</code> and <code>&lt;stdarg.h&gt;</code>, then invoke the compiler so it shows just the output of the preprocessor (resolving the various <code>va_*</code> macros and the <code>va_list</code> type), and then imitate that. <em>And</em> you'd have to assume, or verify using the compiler documentation, that the calling convention for variadic and non-variadic functions is compatible. In other words, find out what this particular implementation does, and reinvent a similar wheel.</p> <p>(I hope that the point of the homework assignment is to demonstrate how much better the standard techniques are.)</p> <p><strong>UPDATE 2:</strong></p> <p>I wrote above that all C functions should have prototypes. This may actually be a rare exception to this rule. At least one of these calls:</p> <pre><code>printf("Hello\n"); printf("x = %d\n", 42); </code></pre> <p>must produce a diagnostic from a conforming compiler unless either <code>printf</code> is declared with <code>, ...</code> (which is forbidden by the homework assignment), or there is no visible prototype for <code>printf</code>. If there's no prototype, then at least one of the calls will have undefined behavior (behavior that's not defined by the C standard, though it may be defined by a particular compiler).</p> <p>In effect, to meet the homework requirements, you'll have to pretend that you're using a pre-ANSI C compiler.</p>
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    1. CO@RobertoNovelo: Prototypes aren't required by the language, but if you have a that conforms to the 1989 standard or later, it almost *never* makes sense not to use them. Non-prototyped function declarations and definitions were retained in the language only for compatibility with pre-ANSI code.
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    2. CO@RobertoNovelo: And compilers do not "guess" the function type for given call; they follow specific rules given in the language standard. If a call doesn't match a function definition, and there's no visible prototype, compilers typically *don't* produce error messages; you just get misbehaving code. Always use prototypes, unless you have a *very* good and specific reason not to.
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