PRE31-C. Never invoke an unsafe macro with arguments containing assignment, increment, decrement, volatile access, or function call

An unsafe macro function is one that evaluates a parameter more than once in the code expansion, or potentially never evaluates the parameter at all. Never invoke an unsafe macro with arguments containing an assignment, increment, decrement, volatile access, or other side effects including function calls which may cause side effects. Any input or output is also a side effect, even though it might be accomplished through function calls or volatile access. Thus input and output must similarly be avoided in arguments to unsafe macros.

The documentation for unsafe macros must warn about putting side effects on the invocation, and the responsibility is on the programmer using the macro. Because of the risks associated with their use, it is recommended that you avoid the creation of unsafe macro functions. See also VOID PRE00-A. Prefer inline or static functions to function-like macros.

The assert() macro is an excellent example of an unsafe macro. Its argument may be evaluated once or not at all, depending on the NDEBUG macro. For more information, see EXP39-C. Avoid side effects in assertions.

Non-Compliant Coding Example

One problem with unsafe macros is side effects on macro arguments. The following non-compliant code, which increments n twice, is a typical example:

#define ABS(x) (((x) < 0) ? -(x) : (x))
/* ... */
m = ABS(++n); /* undefined behavior */

Compliant Solution

One compliant solution is simply not to invoke an unsafe macro with arguments containing an assignment, increment, decrement, or function call, as in the following example:

#define ABS(x) (((x) < 0) ? -(x) : (x)) /* UNSAFE */
/* ... */
++n;
m = ABS(n);

Note the comment declaring the macro unsafe as a warning for programmers.

A second, preferable, compliant solution is to declare ABS() as an inline function (see VOID PRE00-A. Prefer inline or static functions to function-like macros).

inline int abs(int x) {
  return (((x) < 0) ? -(x) : (x));
}
/* ... */
m = abs(++n);

This eliminates the problem of recalling which macros are safe, and which are not.

Exceptions

PRE31-EX1: An exception can be made for calling functions that have no side effects. However, it is easy to forget about obscure side effects that a function might have, especially library functions for which source code is not available; even changing errno is a side effect. Unless the function is user-written and does nothing but perform a computation and return its result without calling any other functions, it is likely that many developers will forget about some side effect. Consequently, while this exception is allowed, it is not recommended.

Risk Assessment

Invoking an unsafe macro with an argument that has side effects may cause those side effects to occur more than once. This can lead to unexpected program behavior.

Automated Detection

The LDRA tool suite V 7.6.0 is able to detect violations of this rule.

Related Vulnerabilities

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

References

[[ISO/IEC 9899-1999]] Section 5.1.2.3, "Program execution"
[[MISRA 04]] Rule 19.6
[[Plum 85]] Rule 1-11

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