Macros are dangerous because their use resembles that of real functions, but they have different semantics. C99 adds inline functions to the C programming language. Inline functions should be preferred over macros when they can be used interchangeably. Making a function an inline function suggests that calls to the function be as fast as possible by using, for example, an alternative to the usual function call mechanism, such as inline substitution. (See also guidelines rules PRE31-C. Avoid side-effects in arguments to unsafe macros, PRE01-C. Use parentheses within macros around parameter names, and recommendation PRE02-C. Macro replacement lists should be parenthesized.)
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int a = 81 / ((++i) * (++i) * (++i)); |
which is undefined. (See guideline rule EXP30-C. Do not depend on order of evaluation between sequence points.)
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In this noncompliant code example, the programmer has written a macro called {{EXEC_BUMP()}} to call a specified function and increment a global counter \[[Dewhurst 2002|AA. Bibliography#Dewhurst 02]\]. When the expansion of a macro is used within the body of a function, as in this example, identifiers refer to the declarations in scope where the body occurs. As a result, when the macro is called in the {{aFunc()}} function, it inadvertently increments a local counter with the same name as the global variable. Note that this example also violates guidelinerecommendation [DCL01-C. Do not reuse variable names in subscopes]. |
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This noncompliant code example also violates guideline rule EXP30-C. Do not depend on order of evaluation between sequence points.
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calculates only one of the two expressions depending on the selector's value. See guideline recommendation PRE05-C. Understand macro replacement when concatenating tokens or performing stringification for more information.
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An example of the use of function-like macros to create type-generic functions is shown in guideline recommendation MEM02-C. Immediately cast the result of a memory allocation function call into a pointer to the allocated type.
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Search for vulnerabilities resulting from the violation of this rule on the CERT website.
Related Guidelines
CERT C++ Secure Coding Standard: PRE00-CPP. Avoid defining macros
ISO/IEC 9899:1999 Section 6.7.4, "Function specifiers"
ISO/IEC TR 24772 "NMP Pre-processor Directives"
MISRA Rule 19.7
Bibliography
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\[[FSF 2005|AA. Bibliography#FSF 05]\] Section 5.34, "[An Inline Function Is as Fast as a Macro|http://gcc.gnu.org/onlinedocs/gcc/Inline.html]" \[[Dewhurst 2002|AA. Bibliography#Dewhurst 02]\] Gotcha #26, "#define Pseudofunctions" \[[ISO/IEC 9899:1999|AA. Bibliography#ISO/IEC 9899-1999]\] Section 6.7.4, "Function specifiers" \[[ISO/IEC PDTR 24772|AA. Bibliography#ISO/IEC PDTR 24772]\] "NMP Pre-processor Directives" \[[Kettlewell 2003|AA. Bibliography#Kettlewell 03]\] \[[MISRA 2004|AA. Bibliography#MISRA 04]\] Rule 19.7 \[[Summit 2005|AA. Bibliography#Summit 05]\] Question 10.4 |
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