PRE31-C. Avoid side effects in arguments to unsafe macros

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:

#define ABS(x) (((x) < 0) ? -(x) : (x))
 
void func(int n) {
  /* Validate that n is within the desired range */
  int m = ABS(++n);

  /* ... */
}

The invocation of the ABS() macro in this example expands to

m = (((++n) < 0) ? -(++n) : (++n));

The resulting code is well defined but causes n to be incremented twice rather than once.

Compliant Solution

In this compliant solution, the increment operation ++n is performed before the call to the unsafe macro.

#define ABS(x) (((x) < 0) ? -(x) : (x)) /* UNSAFE */
 
void func(int n) {
  /* Validate that n is within the desired range */
  ++n;
  int m = ABS(n);

  /* ... */
}

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.)

Compliant Solution

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.

#include <complex.h>
#include <math.h>
 
static inline int iabs(int x) {
  return (((x) < 0) ? -(x) : (x));
}
 
void func(int n) {
  /* Validate that n is within the desired range */

int m = iabs(++n);

  /* ... */
}

Compliant Solution

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, 6.5.2.1, paragraph 3 [ISO/IEC 9899:2024]:

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 default generic 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.

#include <complex.h>
#include <math.h>
 
static inline long long llabs(long long v) {
  return v < 0 ? -v : v;
}
static inline long labs(long v) {
  return v < 0 ? -v : v;
}
static inline int iabs(int v) {
  return v < 0 ? -v : v;
}
static inline int sabs(short v) {
  return v < 0 ? -v : v;
}
static inline int scabs(signed char v) {
  return v < 0 ? -v : v;
}
 
#define ABS(v)  _Generic(v, signed char : scabs, \
                            short : sabs, \
                            int : iabs, \
                            long : labs, \
                            long long : llabs, \
                            float : fabsf, \
                            double : fabs, \
                            long double : fabsl, \
                            double complex : cabs, \
                            float complex : cabsf, \
                            long double complex : cabsl)(v)
 
void func(int n) {
  /* Validate that n is within the desired range */
  int m = ABS(++n);
  /* ... */
}

Generic selections were introduced in C11 and are not available in C99 and earlier editions of the C Standard.

Compliant Solution (GCC)

GCC's __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:

#define ABS(x) __extension__ ({ __typeof (x) tmp = x; \
                    tmp < 0 ? -tmp : tmp; })

Note that relying on such extensions makes code nonportable and violates MSC14-C. Do not introduce unnecessary platform dependencies.

Noncompliant Code Example (assert())

The 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:

#include <assert.h>
#include <stddef.h>
  
void process(size_t index) {
  assert(index++ > 0); /* Side effect */
  /* ... */
}

Compliant Solution (assert())

This compliant solution avoids the possibility of side effects in assertions by moving the expression containing the side effect outside of the assert() macro.

#include <assert.h>
#include <stddef.h>
  
void process(size_t index) {
  assert(index > 0); /* No side effect */
  ++index;
  /* ... */
}

Exceptions

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.

Risk Assessment

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.

Automated Detection

Related Vulnerabilities

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

Related Guidelines

Key here (explains table format and definitions)

Key here (explains table format and definitions)

Bibliography