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POHow bit endianness affects bitwise shifts and file IO in C?
 

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    <p>Let <strong>L</strong> and <strong>B</strong> be two machines. <strong>L</strong> order its <em>bits</em> from LSB (Least Significant Bit) to MSB (Most Significant Bit) while <strong>B</strong> order from MSB to LSB. Or, in other words, <strong>L</strong> uses Little Endian while <strong>B</strong> uses Big Endian <em>bit</em> - not to be confused with byte - ordering.</p> <p>Problem 1 <strong>SOLVED</strong>:</p> <p>We are writing the following code which we want to be portable:</p> <pre><code>#include &lt;stdio.h&gt; int main() { unsigned char a = 1; a &lt;&lt;= 1; printf("a = %d\n", (int) a); return 0; } </code></pre> <p>on <strong>L</strong>, it will print 2, but what happens on <strong>B</strong>? Will it shift the 1 out and print 0?.</p> <p><strong>SOLUTION:</strong> The C99 definition at 6.5.7 says it that, at least on unsigned integer types, <code>&lt;&lt;</code> and <code>&gt;&gt;</code> will multiply and divide by 2 respectively.</p> <p>Problem 2:</p> <p>We are writing the following code which we want to be portable:</p> <p>READ program:</p> <pre><code>/* program READ */ #include &lt;stdio.h&gt; int main() { FILE* fp; unsigned char a; fp = fopen("data.dat", "rb"); fread(&amp;a, 1, 1, fp); fclose(fp); return 0; } </code></pre> <p>and WRITE program:</p> <pre><code>/* program WRITE */ #include &lt;stdio.h&gt; int main() { FILE* fp; unsigned char a = 1; fp = fopen("data.dat", "wb"); fwrite(&amp;a, 1, 1, fp); fclose(fp); return 0; } </code></pre> <p>what happens if we run WRITE on <strong>L</strong>, move the data file to <strong>B</strong> and run READ there? And if we run WRITE on <strong>B</strong> and then READ on <strong>L</strong>?</p> <p>Sorry if this is a FAQ. I googled for hours without luck.</p>
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