In C, the isdigit() function is a standard library function declared in the <ctype.h> header file. It is used to determine whether a given character is a digit or not. Here are the rules for isdigit():

1. The isdigit() function takes a single argument of type int, which represents the character to be tested.
2. The function returns an integer value of non-zero if the character is a digit (0-9), and returns 0 if the character is not a digit.
3. The isdigit() function does not distinguish between different character sets or encodings. It operates based on the character’s ASCII value.
4. The function only works with individual characters, not strings or sequences of characters.

Here’s an example usage of isdigit():

<span class="hljs-meta">#<span class="hljs-keyword">include</span> <span class="hljs-string"><stdio.h></span></span>

<span class="hljs-meta">#<span class="hljs-keyword">include</span> <span class="hljs-string"><ctype.h></span></span>

<span class="hljs-type">int</span> <span class="hljs-title function_">main</span><span class="hljs-params">()</span> {
<span class="hljs-type">char</span> ch = <span class="hljs-string">'5'</span>;
<span class="hljs-keyword">if</span> (<span class="hljs-built_in">isdigit</span>(ch)) {
<span class="hljs-built_in">printf</span>(<span class="hljs-string">"%c is a digit.\n"</span>, ch);
} <span class="hljs-keyword">else</span> {
<span class="hljs-built_in">printf</span>(<span class="hljs-string">"%c is not a digit.\n"</span>, ch);
}

<span class="hljs-keyword">return</span> <span class="hljs-number">0</span>;
}


Output:

<span class="hljs-number">5</span> <span class="hljs-keyword">is</span> a digit.


In the example above, the isdigit() function is used to check whether the character ch is a digit. Since it is the digit ‘5’, the condition evaluates to true, and the message “5 is a digit.” is printed.