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For characters in the basic character set, it doesn't matter does not 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 This noncompliant code example simply shows the standard string-handling function strlen() being called with a plain character string, a signed character string, and an unsigned character string. The strlen() functions function 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);  /* warnsWarns when char is unsigned */
len = strlen(ucstr);  /* warnsWarns when char is signed */

Compiling at high warning levels in compliance with recommendation MSC00-C. Compile cleanly at high warning levels causes warnings to be issued when

• converting Converting from unsigned char[] to const char * when char is signed
• converting Converting from signed char[] to const char * when char is defined to be unsigned

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The compliant solution uses plain char for character data.:

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

len = strlen(cstr);

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

Automated Detection

Related Vulnerabilities

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

Related Guidelines

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ISO/IEC 9899:1999 Section 6.2.5, "Types"

MISRA Rule 6.1, "The plain char type shall be used only for the storage and use of character values"

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