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Assertions are a valuable diagnostic tool for finding and eliminating software defects that may result in vulnerabilities.  (See see MSC11-C. Incorporate diagnostic tests using assertions). ) The runtime assert() macro has some limitations, however, in that it incurs a runtime overhead and because it calls abort(). Consequently, the runtime assert() macro is useful only for identifying incorrect assumptions and not for runtime error checking. As a result, runtime assertions are generally unsuitable for server programs or embedded systems.

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static_assert(constant-expression, string-literal);

Section Subclause 6.7.10 of the C Standard [ISO/IEC 9899:2011] states:

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#include <assert.h>
 
struct timer {
  unsigned char MODE;
  unsigned int DATA;
  unsigned int COUNT;
};
 
int func(void) {
  assert(sizeof(struct timer) == sizeof(unsigned char) + sizeof(unsigned int) + sizeof(unsigned int));
}

Although the use of the runtime assertion is better than nothing, it needs to be placed in a function and executed. This means that it is usually far away from the definition of the actual structure to which it refers. The diagnostic occurs only at runtime and only if the code path containing the assertion is executed.

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For assertions involving only constant expressions, a preprocessor conditional statement may be used, as in this examplecompliant solution:

struct timer {
  unsigned char MODE;
  unsigned int DATA;
  unsigned int COUNT;
};

#if (sizeof(struct timer) != (sizeof(unsigned char) + sizeof(unsigned int) + sizeof(unsigned int)))
  #error "Structure must not have any padding"
#endif

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#include <assert.h>
 
struct timer {
  unsigned char MODE;
  unsigned int DATA;
  unsigned int COUNT;
};

static_assert(sizeof(struct timer) == sizeof(unsigned char) + sizeof(unsigned int) + sizeof(unsigned int),
              "Structure must not have any padding");

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Other uses of static assertion are shown in STR07-C. Use the bounds-checking interfaces for remediation of existing string manipulation code and FIO35FIO34-C. Use feof() and ferror() to detect end-of-file and file errors when sizeof(int) == sizeof(char)Distinguish between characters read from a file and EOF or WEOF.

Risk Assessment

Static assertion is a valuable diagnostic tool for finding and eliminating software defects that may result in vulnerabilities at compile time. The absence of static assertions, however, does not mean that code is incorrect.

Automated Detection

Related Vulnerabilities

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

Related Guidelines

 Bibliography

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