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/* Passes */
static_assert(
  sizeof(int) &lt;<= sizeof(void*), 
  &quot;"sizeof(int) &lt;<= sizeof(void*)&quot;"
); 

/* Fails */
static_assert(
  sizeof(double) &lt;<= sizeof(int), 
  &quot;"sizeof(double) &lt;<= sizeof(int)&quot;"
);

Static assertion is not available in C99, but the facility is being considered for inclusion in C1X by the ISO/IEC WG14 international standardization working group.

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struct timer {
  uint8_t MODE;
  uint32_t DATA;
  uint32_t COUNT;
};

#if (offsetof(timer, DATA) != 4)
  #error ""DATA must be at offset 4""
#endif

Using #error directives allows for clear diagnostic messages. Because this approach evaluates assertions at compile time, there is no runtime penalty.

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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 DCL03-CPP. Use a static assertion to test the value of a constant expression.

References

\[[Becker 08|AA. C References#Becker 08]\] 
\[[Eckel 07|AA. C References#Eckel 07]\]
\[[ISO/IEC 9899:1999|AA. C References#ISO/IEC 9899-1999]\] Section 6.10.1, ""Conditional inclusion,"" and Section 6.10.3.3, ""The ## operator,"" and Section 7.2.1, ""Program diagnostics""
\[[Klarer 04|AA. C References#Klarer 04]\]
\[[Saks 05|AA. C References#Saks 05]\]
\[[Saks 08|AA. C References#Saks 08]\]

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              02. Declarations and Initialization (DCL)