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Macros are frequently used to make source code more readable. Macro definitions, regardless of whether they expand to a single or multiple statements should not conclude with a semicolon. (See recommendation PRE10-C. Wrap multi-statement macros in a do-while loop.) If required, the semicolon should be included following the macro expansion. Inadvertently inserting a semicolon at the end of the macro definition can unexpectedly change the control flow of the program.

Another way to avoid this problem is to prefer inline or static functions over function-like macros. (See also recommendation PRE00-C. Prefer inline or static functions to function-like macros.)

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This noncompliant code example creates a macro definition for a for loop in the program. for loops should require braces, even if it contains only a single body statement. (See recommendation EXP19-C. Use braces for the body of an if, for, or while statement.) This macro takes an integer argument, which is the number of times the loop should run. The programmer has provided a semicolon at the end of the macro definition by mistake.

#define FOR_LOOP(n)  for(i=0; i<(n); i++);

int i;
FOR_LOOP(3)
{
  puts("Inside for loop\n");
}

The programmer expects to get the following output from the code:

Inside for loop
Inside for loop
Inside for loop

But because of the semicolon at the end of the macro definition, the for loop in the program has a null statement, so the statement "Inside for loop" gets printed just once. Essentially, the semicolon at the end of the macro definition changes the program control flow.

Though the above Although this example might not actually be used in code, it shows the effect a semicolon in a macro definition can have.

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The compliant solution is to write the macro definition without the semicolon at the end, leaving the decision to whether or not to have a semicolon or not up to the person who is using the macro.

#define FOR_LOOP(n)  for(i=0; i<(n); i++)

int i;
FOR_LOOP(3)
{
  puts("Inside for loop\n");
}

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In this noncompliant code example, the programmer defines a macro which that increments the value of the first argument, x, by one and modulates it with the value of the 2nd second argument, max.

#define INCREMENT(x, max) ((x) = ((x) + 1) % (max));

int index = 0;
int value;
value = INCREMENT(index, 10) + 2;
/* ...*/

In this case, the programmer intends to increment index and then use that as a value by adding 2 to it. Unfortunately, the value is equal to the incremented value of index because of the semicolon present at the end of the macro. The '+ 2;' is treated as a separate statement by the compiler. The user will not get any compilation errors. If the user has not enabled warnings while compiling, the effect of the semicolon in the macro cannot be detected at an early stage.

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The compliant solution is to write the macro definition without the semicolon at the end, leaving the decision whether or not to have a semicolon or not up to the person who is using the macro.

#define INCREMENT(x, max) ((x) = ((x) + 1) % (max))

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