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    <p>The problem with most answers to this question is portability. I've provided a portable answer <a href="https://stackoverflow.com/questions/15529994/most-efficient-way-to-store-an-unsigned-16-bit-integer-to-a-file/15532021">here</a>, but this recieved relatively little positive feedback. Note that C defines undefined behavior as: behavior, upon use of a nonportable or erroneous program construct or of <em>erroneous data</em>, for which this International Standard imposes no requirements.</p> <p>The answer I'll give here won't assume that <code>int</code> is 16 bits in width; It'll give you an idea of how to represent "larger int" values. It's the same concept, but uses a dynamic loop rather than two <code>fputc</code>s.</p> <p>Declare an array of <code>sizeof int</code> unsigned chars: <code>unsigned char big_endian[sizeof int];</code></p> <p>Separate the sign and the absolute value.</p> <pre><code>int sign = value &lt; 0; value = sign ? -value : value; </code></pre> <p>Loop from <code>sizeof int</code> to 0, writing the least significant bytes:</p> <pre><code>size_t foo = sizeof int; do { big_endian[--foo] = value % (UCHAR_MAX + 1); value /= (UCHAR_MAX + 1); } while (foo &gt; 0); </code></pre> <p>Now insert the sign: <code>foo[0] |= sign &lt;&lt; (CHAR_BIT - 1);</code></p> <p>Simple, yeh? Little endian is equally simple. Just reverse the order of the loop to go from 0 to <code>sizeof int</code>, instead of from <code>sizeof int</code> to 0:</p> <pre><code>size_t foo = 0; do { big_endian[foo++] = value % (UCHAR_MAX + 1); value /= (UCHAR_MAX + 1); } while (foo &lt; sizeof int); </code></pre> <p>The portable methods make more sense, because they're well defined.</p>
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    1. POConverting little endian to big endian using Bitshift Operators
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    1. CO"The problem with most answers to this question is portability." -- You actually have it backwards. The results of your code depends on the value of UCHAR_MAX and CHAR_BIT, but *file formats* are defined independently of the host machine, and almost invariably in terms of 8-bit bytes. The OP's teacher did not define exactly what file format is wanted, but it almost certainly isn't the big-endian host-specific sign-and-magnitude format your code produces.
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      1. USJim Balter
    2. COJustify your comment: "Your program is nonsensical, as it reads two numbers in ASCII, then reverses the nibbles (not bytes, as would be the case for changing endianness) of one of them and writes it out in binary."
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