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    <p>The backslash notation in a character literal allows you to specify the numeric value of a character instead of using the character itself. So <code>'\1'</code>[*] means "the character whose numeric value is 1", <code>'\2'</code> means "the character whose numeric value is 2", etc. Almost. Due to a quirk of C, character literals actually have type <code>int</code>, and indeed <code>int</code> is used to handle characters in other contexts too, such as the return value of <code>fgetc</code>. So <code>'\1'</code> means "the numeric value as an int, of the character whose numeric value is 1", and so on.</p> <p>Since characters <em>are</em> numeric values in C, "the character whose numeric value is 1" actually <em>is</em> <code>(char)1</code>, and the extra decoration in <code>'\1'</code> has no effect on the compiler - <code>'\1'</code> has the same type and value as <code>1</code> in C. So the backslash notation is more needed in string literals than it is in character literals, for inserting non-printable characters that don't have their own escape code.</p> <p>Personally, I prefer to write <code>0</code> when I mean 0, and let implicit conversions do their thing. Some people find that very difficult to understand. When working with those people, it's helpful to write <code>'\0'</code> when you mean a character with value 0, that is to say in cases where you expect your <code>0</code> is soon going to implicitly convert to <code>char</code> type. Similarly, it can help to write <code>NULL</code> when you mean a null pointer constant, <code>0.0</code> when you mean a double with value 0, and so on.</p> <p>Whether it makes any difference to the compiler, and whether it needs a cast, depends on context. Since <code>'\0'</code> has exactly the same type and value as <code>0</code>, it needs to be cast to <code>char</code> in exactly the same circumstances. So <code>'\0'</code> and <code>(char)0</code> differ in type, for exactly equivalent expressions you can either consider <code>(char)'\0'</code> vs <code>(char)0</code>, or <code>'\0'</code> vs <code>0</code>. <code>NULL</code> has implementation-defined type -- sometimes it needs to be cast to a pointer type, since it may have integer type. <code>0.0</code> has type <code>double</code>, so is certainly different from <code>0</code>. Still, <code>float f = 1.0;</code> is identical to <code>float f = 1;</code> and <code>float f = 1.0f</code>, whereas <code>1.0 / i</code>, where <code>i</code> is an int, usually has a different value from <code>1 / i</code>.</p> <p>So, any general rule whether to use <code>'\0'</code> or <code>0</code> is purely for the convenience of readers of your code - it's all the same to the compiler. Pick whichever you (and your colleagues) prefer the look of, or perhaps define a macro <code>ASCII_NUL</code>.</p> <p>[*] or <code>'\01'</code> - since the backslash introduces an <em>octal</em> number, not decimal, it's sometimes wise to make this a bit more obvious by ensuring it starts with a 0. Makes no difference for 0, 1, 2 of course. I say "sometimes", because backslash can only be followed by 3 octal digits, so you can't write <code>\0101</code> instead of <code>\101</code>, to remind the reader that it's an octal value. It's all quite awkward, and leads to even more decoration: <code>\x41</code> for a capital A, and you could therefore write <code>'\x0'</code> for 0 if you want.</p>
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