Why is this an issue?
There are three distinct char types, (plain) char, signed char and unsigned char. signed
char and unsigned char should only be used for numeric data, and plain char should only be used for character data.
Since it is implementation-defined, the signedness of the plain char type should not be assumed.
Noncompliant code example
signed char a = 'a'; // Noncompliant, explicitly signed
unsigned char b = '\r'; // Noncompliant, explicitly unsigned
char c = 10; // Noncompliant
unsigned char d = c; // Noncompliant, d is explicitly signed while c is not
char e = a; // Noncompliant, a is explicitly signed while e is not
Compliant solution
char a = 'a';
char b = '\r';
unsigned char c = 10;
signed char c = 10;
Exceptions
- Since the integer value 0 is used as a sentinel for the end of a string, converting this value to char is ignored.
Resources
- MISRA C:2004, 6.1 - The plain char type shall be used only for the storage and use of character values
- MISRA C:2004, 6.2 - signed and unsigned char type shall be used only for the storage and use of number values
- MISRA C++:2008, 5-0-11 - The plain char type shall only be used for the storage and use of character values
- MISRA C++:2008, 5-0-12 - signed char and unsigned char type shall only be used for the storage and use of numeric values
- CERT, INT07-C. - Use only explicitly signed or unsigned char type for numeric values
- CERT, STR00-C. - Represent characters using an appropriate type
- CERT, STR04-C. - Use plain char for characters in the basic character set
