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Inline functions are, in general, more suitable for this task . (See recommendation see 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 a do-while loop syntactically, so the macro can appear safely inside inside if clauses or other places that expect a single statement or a statement block. Note that this is only effective if none of the multiple statements are break or continue, as they would be captured by the do-while loop. (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 a do-while loop . See recommendation (see 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.:

Code Block

bgColor	#FFcccc
lang	c


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

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

Code Block

bgColor	#FFcccc
lang	c


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

This will expand toIt expands to the following, which is certainly not what the programmer intended:

Code Block

bgColor	#FFcccc
lang	c


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

This is certainly not what the programmer intendedFurthermore, this macro violates PRE02-C. Macro replacement lists should be parenthesized.

Noncompliant Code Example

This noncompliant code example parenthesizes its macro arguments, but inadequately bounds multiple statements.:

Code Block

bgColor	#FFcccc
lang	c


/*
 * Swaps two values. and requires
 * 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:

Code Block

bgColor	#FFcccc
lang	c


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:

Code Block

bgColor	#FFcccc
lang	c


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

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

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

Compliant Solution

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

Code Block

bgColor	#ccccFF
lang	c


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

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

This macro still violates the recommendation PRE12-C. Do not define unsafe macros, because both macro arguments are evaluated twice. It is expected that the arguments are simple lvalues.