These are the only permitted uses of macros. Storage class specifiers and type qualifiers include keywords such as extern, static and const. Any other use of #define could lead to unexpected behaviour when substitution is made, or to very hard-to-read code.

In particular macros shall not be used to define statements or parts of statements except the use of the do-while construct. Nor shall macros redefine the syntax of the language. All brackets of whatever type ( ) { } [ ] in the macro replacement list shall be balanced.

The do-while-zero construct (see example below) is the only permitted mechanism for having complete statements in a macro body. The do-while-zero construct is used to wrap a series of one or more statements and ensure correct behaviour. Note: the semicolon must be omitted from the end of the macro body.

Example

/* The following are compliant */
#define PI 3.14159F                   /* Constant                      */
#define XSTAL 10000000                /* Constant                      */
#define CLOCK (XSTAL/16)              /* Constant expression           */
#define PLUS2(X) ((X) + 2)            /* Macro expanding to expression */
#define STOR extern                   /* storage class specifier       */
#define INIT(value){ (value), 0, 0}   /* braced initialiser            */
#define CAT (PI)                      /* parenthesised expression      */
#define FILE_A "filename.h"           /* string literal                */
#define READ_TIME_32() \
   do { \
      DISABLE_INTERRUPTS (); \
      time_now = (uint32_t)TIMER_HI << 16; \
      time_now = time_now | (uint32_t)TIMER_LO; \
      ENABLE_INTERRUPTS (); \
   } while (0) /* example of do-while-zero */
/* the following are NOT compliant */
#define int32_t long /* use typedef instead                     */
#define STARTIF if(  /* unbalanced () and language redefinition */
#define CAT PI       /* non-parenthesised expression            */

External guidance

PRE02-C: Macro replacement lists should be parenthesized