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    <p>As others have pointed out, the problem is that you're passing the wrong type of argument to <code>userComFunction</code>. But the real problem is that your compiler isn't telling you that.</p> <p>As of C90 (that's two standards ago), calling a function with no visible declaration was legal, and the compiler would make assumptions (often incorrect) about what the function actually looks like. When the compiler sees the call to <code>userComFunction</code>, it hasn't seen a declaration or definition for <code>userComFunction</code>, so it can't diagnose your error.</p> <p>As of C99, calling a function with no visible declaration is a <em>constraint violation</em>, meaning that a compiler must at least warn you about it. C99 also dropped the "implicit <code>int</code>" rule, so you can no longer omit the return type in a function declaration; <code>main</code> should be declared with an <code>int</code> return type (<em>not</em> <code>void</code>!), and <code>userComFunction</code>, since it doesn't return anything, should be <code>void</code>.</p> <p>You can either move the full definition of <code>userComFunction</code> above the definition of <code>main</code>, or you can leave the definition where it is and add a "forward" declaration:</p> <pre><code>void userComFunction(struct userCom *return_type); int main(void) { /* ... */ } void userComFunction(struct userCom *return_type) { /* ... */ } </code></pre> <p>When you do that, the compiler should let you know that your call:</p> <pre><code>userComFunction(com); </code></pre> <p>is incorrect. (The fix is to change <code>com</code> to <code>&amp;com</code>.)</p> <p>You should also use gcc's command-line options to enable more warnings. For example:</p> <pre><code>gcc -std=c99 -pedantic -Wall -Wextra </code></pre> <p><code>-std=c99</code> says to enforce ISO C99 rules. <code>-pedantic</code> says to <em>really</em> enforce those rules. <code>-Wall</code> and <code>-Wextra</code> enable additional warnings.</p>
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    1. POStructs and functions in C
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    1. COI always wonder by reading your answer. You don't miss anything You should be teacher :)
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      1. USGrijesh Chauhan
    2. COShould also use `-pedantic-error` for more stick checks?
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    3. CO@GrijeshChauhan: (`-pedantic-errors` turns warnings enabled by `-pedantic` into fatal errors.) It depends. If you have a policy of treating all warnings as problems that need to be fixed, there's not much practical difference between `-pedantic` and `-pedantic-errors`. If you actually need to write non-portable code in some cases, and you really know what you're doing, `-pedantic` can warn you about potential problems, and you can decide for yourself whether to change the code accordingly. For beginning programmers trying to write portable code, `-pedantic-errors` is probably a good idea.
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