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Macros are often frequently used to generalize several segments of code which might be used multiple times in the code. This guideline focuses on macros consisting of single statements, which do not usually have to be enclosed in a do()....while(). (See also 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 PRE10-C. Wrap multi-statement multistatement macros in a do-while loop.) When writing macros, a C programmer should not include a semicolon at the end of a macro definition. The use of a If required, the semicolon should be determined when included following the macro is used. A expansion. Inadvertently inserting a semicolon at the end of a the macro definition can unexpectedly change the control flow of the program, depending upon its usage in the program code.

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. A for loop should require braces, even if it contains only a single body statement. (See 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 user programmer has provided inserted a semicolon at the end of the macro definition by mistake.

#define FOR_LOOP(n)  for(i=0; i<(n); i++);
main()
{
 	int i;
    	FOR_LOOP(3)
    	{
             printfputs("Inside for loop\n");
    	}
}

The user 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.

Compliant Solution

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++)
main()
{
 	int i;
    	FOR_LOOP(3)
    	{
        	printfputs("Inside for loop\n");
    	}
}

Noncompliant Code Example

In the this noncompliant code belowexample, the programmer defines a macro which that increments the value in of the first argument, x, by 1 by one and then wraps modulates it along with the max value provided by the user.of the second argument, max:

#define INCREMENTINCREMOD(x, max)    ((x) = ((x) + 1) % (max));
main()
{
     int index = 0;
     int value;
     value = INCREMENTINCREMOD(index, 10) + 2;
/*     ...........
     ...........

}
 */

In this caseIn the above code, the programmer intends to increment index and then use that as a value by adding 2 to it. Unfortunately, the value will always be is equal to the incremented value of index because of the semicolon present at the end of the macro. The '+ 2;' will be is treated as another 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 whether or not to have a semicolon or not up to the person who is using the macro.:

#define INCREMENTINCREMOD(x, max)    ((x) = ((x) + 1) % (max))
main()
{
     int index = 0;
     int value;
     value = INCREMENT(index, 10) + 2;
     .........
     ..........

}

Mitigation Strategies

The programmer should ensure that there is no semicolon at the end of a macro definition with single statement. The responsibility for having a semicolon where needed during the use of the macro should be given to the person using the macro.

Compliant Solution

This compliant solution uses an inline function as recommended by PRE00-C. Prefer inline or static functions to function-like macros.

inline int incremod(int *x, int 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.

Automated Detection

Related Vulnerabilities

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

Related Guidelines

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Image Added Image Added Image AddedPRE10-C. Wrap multi-statement macros in a do-while loop      01. Preprocessor (PRE)      PRE12-C. Define numeric constants in a portable way