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

Inline functions are, in general, more suitable for this task. (See recommendation PRE00-C. Prefer inline or static functions to function-like macros.) Occasionally, however, they are not feasible (when macros are expected to operate on variables of different types, for example).

When multiple statements are used in a macro, they should be bound together in a do-while loop syntactically, so the macro can appear safely inside if clauses or other places that expect a single statement or a statement block. (Alternatively, when an if, for or while statement uses braces even for a single body statement, then multiple statements in a macro will expand correctly even without a do-while loop. See recommendation EXP19-C. Use braces for the body of an if, for, or while statement.

Noncompliant Code Example

This noncompliant code example contains multiple, unbound statements.

/*
 * 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;

This is certainly not what the programmer intended.

Noncompliant Code Example

This noncompliant code example inadequately bounds multiple statements.

/*
 * Swaps two values.
 * Requires tmp variable to be defined.
 */
#define SWAP(x,y) { tmp=x; x=y; y=tmp; }

This macro fails to expand correctly in some case such as the following example which is meant to be an if-statement with two branches:

if (x > y)
  SWAP(x,y);          /* Branch 1 */
else  
  do_something();     /* Branch 2 */

Following macro expansion, however, this code is interpreted as an if-statement with only one branch:

if (x > y) { /* Single-branch if-statement!!! */

  tmp = x;   /* The one and only branch consists */
  x = y;     /* of the block. */
  y = tmp;
}
;            /* empty statement */
else         /* ERROR!!! "parse error before else" */
  do_something();

The problem is the semi-colon ';' following the block.

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 wrapped statement macros can result in unexpected and difficult to diagnose behavior.

Related Vulnerabilities

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

Related Guidelines

CERT C++ Secure Coding Standard: PRE10-CPP. Wrap multi-statement macros in a do-while loop

ISO/IEC TR 24772 "NMP Pre-processor Directions"

Bibliography

Linux Kernel Newbies FAQ FAQ/DoWhile0

      01. Preprocessor (PRE)