STR04-C. Use plain char for characters in the basic character set

The three types char, signed char, and unsigned char are collectively called the character types. Compilers have the latitude to define char to have the same range, representation, and behavior as either signed char or unsigned char. Irrespective of the choice made, char is a separate type from the other two and is not compatible with either.

For characters in the basic character set, it doesn't matter which data type is used, except for type compatibility. Consequently, it is best to use plain char for character data for compatibility with standard string handling functions.

In most cases, the only portable operators on plain char types are assignment and equality operators (=, ==, != ). An exception is the translation to and from digits.  For example, if the char c is a digit, c - '0' is a value between 0 and 9.

Noncompliant Code Example

The following noncompliant code example simply shows calling the standard string handling function strlen() with a plain character string, a signed character string, and an unsigned character string. The strlen() functions takes a single argument of type const char *.

size_t len;
char cstr[] = "char string";
signed char scstr[] = "signed char string";
unsigned char ucstr[] = "unsigned char string";

len = strlen(cstr);
len = strlen(scstr);  /* warns when char is unsigned */
len = strlen(ucstr);  /* warns when char is signed */

Compiling at high warning levels in compliance with MSC00-C. Compile cleanly at high warning levels causes warnings to be issued when converting from unsigned char[] to const char * when char is signed and from signed char[] to const char * when char is defined to be unsigned. Casts are required to eliminate these warnings, but excessive casts can make code difficult to read and hide legitimate warning messages.

If this C code were compiled using a C++ compiler, conversions from unsigned char[] to const char * and from signed char[] to const char * would be flagged as errors requiring casts.

Compliant Solution

The compliant solution uses plain char for character data.

size_t len;
char cstr[] = "char string";

len = strlen(cstr);

Conversions are not required and the code compiles cleanly at high warning levels without casts.

Risk Assessment

Failing to use plain char for characters in the basic character set can lead to excessive casts and less effective compiler diagnostics.

Automated Detection

Fortify SCA Version 5.0 with CERT C Rule Pack can detect violations of this recommendation, except cases involving signed char.

The EDG Front End to Compass Rose can detect violation of this recommendation.

Compass/ROSE can detect violations of this recommendation.

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

Other Languages

This rule appears in the C++ Secure Coding Standard as STR04-CPP. Use plain char for characters in the basic character set.

References

[[ISO/IEC 9899:1999]] Section 6.2.5, "Types"
[[MISRA 04]] Rule 6.1, "The plain char type shall be used only for the storage and use of character values"

STR03-C. Do not inadvertently truncate a null-terminated byte string      07. Characters and Strings (STR)