An unsafe function-like macro is one whose expansion results in evaluating one of its parameters more than once or not at all. Never invoke an unsafe macro with arguments containing an assignment, increment, decrement, volatile access, input/output, or other expressions with side effects (including function calls, which may cause side effects).
The documentation for unsafe macros should warn against invoking them with arguments with side effects, but the responsibility is on the programmer using the macro. Because of the risks associated with their use, it is recommended that the creation of unsafe function-like macros be avoided. (See PRE00-C. Prefer inline or static functions to function-like macros.)
This rule is similar to EXP44-C. Do not rely on side effects in operands to sizeof, _Alignof, or _Generic.
Noncompliant Code Example
One problem with unsafe macros is side effects on macro arguments, as shown by this noncompliant code example:
The invocation of the
ABS() macro in this example expands to
The resulting code is well defined but causes
n to be incremented twice rather than once.
In this compliant solution, the increment operation
++n is performed before the call to the unsafe macro.
Note the comment warning programmers that the macro is unsafe. The macro can also be renamed
ABS_UNSAFE() to make it clear that the macro is unsafe. This compliant solution, like all the compliant solutions for this rule, has undefined behavior if the argument to
ABS() is equal to the minimum (most negative) value for the signed integer type. (See INT32-C. Ensure that operations on signed integers do not result in overflow for more information.)
This compliant solution follows the guidance of PRE00-C. Prefer inline or static functions to function-like macros by defining an inline function
iabs() to replace the
ABS() macro. Unlike the
ABS() macro, which operates on operands of any type, the
iabs() function will truncate arguments of types wider than
int whose value is not in range of the latter type.
A more flexible compliant solution is to declare the
ABS() macro using a
_Generic selection. To support all arithmetic data types, this solution also makes use of inline functions to compute integer absolute values. (See PRE00-C. Prefer inline or static functions to function-like macros and PRE12-C. Do not define unsafe macros.)
According to the C Standard, 188.8.131.52, paragraph 3 [ISO/IEC 9899:2011]:
The controlling expression of a generic selection is not evaluated. If a generic selection has a generic association with a type name that is compatible with the type of the controlling expression, then the result expression of the generic selection is the expression in that generic association. Otherwise, the result expression of the generic selection is the expression in the
defaultgeneric association. None of the expressions from any other generic association of the generic selection is evaluated.
Because the expression is not evaluated as part of the generic selection, the use of a macro in this solution is guaranteed to evaluate the macro parameter
v only once.
Generic selections were introduced in C11 and are not available in C99 and earlier editions of the C Standard.
Compliant Solution (GCC)
__typeof extension makes it possible to declare and assign the value of the macro operand to a temporary of the same type and perform the computation on the temporary, consequently guaranteeing that the operand will be evaluated exactly once. Another GCC extension, known as statement expression, makes it possible for the block statement to appear where an expression is expected:
Note that relying on such extensions makes code nonportable and violates MSC14-C. Do not introduce unnecessary platform dependencies.
Noncompliant Code Example (
assert() macro is a convenient mechanism for incorporating diagnostic tests in code. (See MSC11-C. Incorporate diagnostic tests using assertions.) Expressions used as arguments to the standard
assert() macro should not have side effects. The behavior of the
assert() macro depends on the definition of the object-like macro
NDEBUG. If the macro
NDEBUG is undefined, the
assert() macro is defined to evaluate its expression argument and, if the result of the expression compares equal to 0, call the
abort() function. If
NDEBUG is defined,
assert is defined to expand to
((void)0). Consequently, the expression in the assertion is not evaluated, and no side effects it may have had otherwise take place in non-debugging executions of the code.
This noncompliant code example includes an
assert() macro containing an expression (
index++) that has a side effect:
Compliant Solution (
This compliant solution avoids the possibility of side effects in assertions by moving the expression containing the side effect outside of the
PRE31-C-EX1: An exception can be made for invoking an unsafe macro with a function call argument provided that the function has 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, this exception must be used with great care.
Invoking an unsafe macro with an argument that has side effects may cause those side effects to occur more than once. This practice can lead to unexpected program behavior.
|Axivion Bauhaus Suite|
Can detect the specific instance where assertion contains an operation/function call that may have a side effect
|LDRA tool suite|
9 S, 562 S, 572 S, 35 D, 1 Q
A full expression containing an increment (++) or decrement (--) operator should have no other potential side effects
|Polyspace Bug Finder|
|CERT C: Rule PRE31-C||Checks for side effect in arguments to unsafe macro (rule partially covered)|
3462, 3463, 3464, 3465, 3466, 3467
|3225, 3226, 3227, 3228, 3229|
Key here (explains table format and definitions)
Key here (explains table format and definitions)
|[Dewhurst 2002]||Gotcha #28, "Side Effects in Assertions"|
|[ISO/IEC 9899:2011]||Subclause 184.108.40.206, "Generic Selection"|
|[Plum 1985]||Rule 1-11|