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

• Wiki Markup
  converting from {{unsigned char\[\]}} to {{const char *}} when {{char}} is signed

• Wiki Markupconverting 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.

Wiki MarkupIf 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);

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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"

Bibliography

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