# V2662\. MISRA\. Any value passed to a function from <ctype\.h\> should be representable as an unsigned character or be the value EOF\.

This diagnostic rule is based on the [MISRA](https://misra.org.uk/) \(Motor Industry Software Reliability Association\) software development guidelines\.

This diagnostic rule is relevant only for C\.

The functions from the [`<ctype.h>`](https://en.cppreference.com/w/c/header/ctype) standard library header accept `int` arguments, but the passed value should be within the `unsigned char` range or equal to the `EOF` constant\. If the value is passed outside this range, it results in undefined behavior \([C11, Section 7\.4](https://open-std.org/JTC1/SC22/WG14/www/docs/n1570.pdf#page=218)\)\.

Each function from `<ctype.h>` uses the passed argument as an index to access the set of characters\. If the passed value lies outside the `unsigned char` range and is not equal to `EOF`, an out\-of\-bounds access occurs\. This leads to reading outside the array, which, in turn, leads to undefined behavior\.

The example:

```cpp
int generate_random_value();

char *generate_random_name(size_t len)
{
  char *res = (char *) malloc(len * sizeof(char));
  if (!res) return NULL;

  for (size_t i = 0; i < len; ++i)
  {
    int c;
    do
    {
      c = generate_random_value();
    }
    while (c < 0 || c > 256 || !isalnum(c));
    
    res[i] = c;
  }

  return res;
}
```

The function generates a string of the specified length that contains randomly generated content with letters and numbers\. However, due to an incorrect condition in the `do` loop, the `256` value may be passed to the `isalnum` function, which is outside the `unsigned char` range \(`[0 .. 255]`\)\.

The fixed code:

```cpp
....
do
{
  c = generate_random_value();
}
while (c < 0 || c > 255 || !isalnum(c));
....
```