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Noncompliant Code Example

Wiki MarkupIn 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 recommendation [DCL01-C. Do not reuse variable names in subscopes].

size_t count = 0;

#define EXEC_BUMP(func) (func(), ++count)

void g(void) {
  printf("Called g, count = %zu.\n", count);
}

void aFunc(void) {
  size_t count = 0;
  while (count++ < 10) {
    EXEC_BUMP(g);
  }
}

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Platform-Specific Details

Wiki MarkupGNU C (and some other compilers) supported inline functions before they were added to C99 and, as a result, have significantly different semantics. Richard Kettlewell provides a good explanation of differences between the C99 and GNU C rules \[ [Kettlewell 2003|AA. Bibliography#Kettlewell 03]\].

Exceptions

PRE00-EX1: Macros can be used to implement local functions (repetitive blocks of code that have access to automatic variables from the enclosing scope) that cannot be achieved with inline functions.

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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" \[[Kettlewell 2003|AA. Bibliography#Kettlewell 03]\] \[[Summit 2005|AA. Bibliography#Summit 05]\] Question "
[Dewhurst 2002] Gotcha #26, "#define Pseudofunctions"
[Kettlewell 2003]
[Summit 2005] Question 10.4

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01. Preprocessor (PRE)      01. Preprocessor (PRE)