bugprone-signed-char-misuse

Finds signed char -> integer conversions which might indicate a programming error. The basic problem with the signed char, that it might store the non-ASCII characters as negative values. The human programmer probably expects that after an integer conversion the converted value matches with the character code (a value from [0..255]), however, the actual value is in [-128..127] interval. This also applies to the plain char type on those implementations which represent char similar to signed char.

To avoid this kind of misinterpretation, the desired way of converting from a signed char to an integer value is converting to unsigned char first, which stores all the characters in the positive [0..255] interval which matches with the known character codes.

It depends on the actual platform whether char is handled as signed char by default and so it is caught by this check or not. To change the default behavior you can use -funsigned-char and -fsigned-char compilation options.

Currently, this check is limited to assignments and variable declarations, where a signed char is assigned to an integer variable. There are other use cases where the same misinterpretation might lead to similar bogus behavior.

See also: STR34-C. Cast characters to unsigned char before converting to larger integer sizes

A good example from the CERT description when a char variable is used to read from a file that might contain non-ASCII characters. The problem comes up when the code uses the -1 integer value as EOF, while the 255 character code is also stored as -1 in two's complement form of char type. See a simple example of this bellow. This code stops not only when it reaches the end of the file, but also when it gets a character with the 255 code.

#define EOF (-1)

int read(void) {
  char CChar;
  int IChar = EOF;

  if (readChar(CChar)) {
    IChar = CChar;
  }
  return IChar;
}

A proper way to fix the code above is converting the char variable to an unsigned char value first.

#define EOF (-1)

int read(void) {
  char CChar;
  int IChar = EOF;

  if (readChar(CChar)) {
    IChar = static_cast<unsigned char>(CChar);
  }
  return IChar;
}