Macros are often used to execute a sequence of multiple statements as a group.

While inline functions are, in general, more suitable for this task (see PRE00-A. Prefer inline or static functions to function-like macros), occasionally they are not feasible. For instance, the macros are expected to operate on variables of different types; this is well-handled by macros, but not by inline functions.

When multiple statements are used in a macro, they should be bound together in a loop syntactically, so the macro can appear safely inside if-clauses, or other places that expect a single statement or a statement block.

Non-Compliant Code Example

/* Swaps two values, requires tmp variable to be defined */
#define SWAP(x,y) \
  tmp = x; \
  x = y; \
  y = tmp

This macro will expand correctly in a normal sequence of statements, but not as the then-clause in an if statement:

int x, y, z, tmp;
if (z == 0)
  SWAP( x, y);

This will expand to:

int x, y, z, tmp;
if (z == 0)
  tmp = x;
x = y;
y = tmp;

which is certainly not what the author intended.

Compliant Solution

Wrapping the macro inside a do-while loop mitigates the problem.

/* Swaps two values, requires tmp variable to be defined */
#define SWAP(x,y) \
  do { \
    tmp = x; \
    x = y; \
    y = tmp; \
  while (0)

The do-while loop will always be executed exactly once.

Risk Assessment

Improperly sealed statement macros will cause behavior that is unexpected and difficult to diagnose.

Related Vulnerabilities

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

References

PRE09-A. Do not replace secure functions with less secure functions      01. Preprocessor (PRE)