PRE11-C. Do not conclude macro definitions with a semicolon

Macros are frequently used to make source code more readable. Macro definitions, regardless of whether they expand to a single or multiple statements (see PRE10-C. Wrap multi-statement macros in a do-while loop) should not conclude with a semicolon. 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 PRE00-C. Prefer inline or static functions to function-like macros.)

In general, the programmer should ensure that there is no semicolon at the end of a macro definition. The responsibility for having a semicolon where needed during the use of such a macro should be delegated to the person invoking the macro.

Noncompliant Code Example

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 EXP19-C. Use braces for 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 example might not actually be used in code, it shows the effect a semicolon in a macro definition can have.

Compliant Solution

The compliant solution is to write the macro definition without the semicolon at the end, leaving the decision 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");
}

Noncompliant Code Example

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

Compliant Solution

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

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

Risk Assessment

Using a semicolon at the end of a macro definition can result in the change of program control flow and thus unintended program behavior.

Related Vulnerabilities

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

Other Languages

This rule appears in the C++ Secure Coding Standard as PRE11-CPP. Do not conclude macro definitions with a semicolon.

PRE10-C. Wrap multi-statement macros in a do-while loop      01. Preprocessor (PRE)