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POWhat if I omit the return type of main function in C?
 

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  1. POWhat if I omit the return type of main function in C?
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    <p>Before anyone may mark it duplicate of related questions. I emphasize I <em>DO</em> have read all those questions. But I still have some interrogations(yep, some little pedantic :) )</p> <p><strong>For C</strong></p> <p>Some conclusions:</p> <pre><code>1. In C89(C90), this is _undefined_ . 2. In C99(or C11), a type of int is madatory; control flow reached the closing } will return a value of 0. </code></pre> <p>Here comes my interrogations.</p> <ol> <li><p>In <a href="http://www.vmunix.com/~gabor/c/draft.html" rel="nofollow">c89</a>, I have found nothing about <strong>undefined</strong>, but <strong>unspecified</strong>?</p> <p><strong>Detail:</strong> The related parts in <strong><a href="http://www.vmunix.com/~gabor/c/draft.html" rel="nofollow">C89</a></strong> are <strong>5.1.2.2.1 Program startup</strong> and <strong>5.1.2.2.3 Program termination</strong> (<strong>NOTE</strong> : both are under the <strong>5.1.2.2 Hosted environment</strong> section, within which our later discussion is limitted)</p> <p><strong>Cite:</strong> -- 5.1.2.2.3 Program termination --</p> <p><code>A return from the initial call to the main function is equivalent to calling the exit function with the value returned by the main function as its argument.10 If the } that terminates the main function is reached, the termination status returned to the host environment is unspecified.</code></p> <p>Just note that part: <strong>If the } that terminates ...</strong> , it clearly says that if we omit the return type - thus the } will be reached at -<br> the termination status is <strong>unspecified</strong> </p> <p>According the definition of the standard of <strong>undefined</strong> and <strong>unspecified</strong>, Should I say that it gives <strong>unspecified value</strong> since whatever it return is a legal int value, but the <strong>consequese is undefined</strong>-we could not predict what value will lead to what catastrophic consequese?</p></li> <li><p>In c99, a type of int is madatory, but <code>gcc --std=c99</code> given a test without int type(no return type actually) gives only <strong>waring:return type of ‘main’ is not ‘int’</strong> ,but not <strong>error</strong> ?</p> <p><strong>Detail:</strong> the related parts are the same as that in c89.</p> <p><strong>Cite:</strong> -- 5.1.2.2.1 Program startup --</p> <p><code>It shall be defined with a return type of int and ...</code> </p> <p>and -- 4. Conformance --</p> <p><code>1. In this International Standard, ‘‘shall’’ is to be interpreted as a requirement on an implementation or on a program; conversely, ‘‘shall not’’ is to be interpreted as a prohibition.</code></p> <p>So <strong>shall</strong> should be interpreted as <strong>madatory</strong> in this standard, why gcc with swith <strong>--std=c99</strong> violated this? </p></li> </ol>
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    1. COReport a bug to gcc maintainers.
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    2. COuse `-pedantic-errors` to make ISO violations errors in GCC
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    3. CO`gcc -std=c99` is only a loose approximation, it still swallows most of the c89 stuff that was removed, and extensions. However, it gives a diagnostic, and more is not required. If you want a good approximation to the standard, also use `-pedantic-errors`.
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      1. USDaniel Fischer
 

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