It is necessary to understand how macro replacement works in C, particularly in the context of concatenating tokens using the ## operator and converting macro parameters to strings using the # operator.

Concatenating Tokens

The ## preprocessing operator is used to merge two tokens into one while expanding macros, which is called token pasting or token concatenation. When a macro is expanded, the two tokens on either side of each ## operator are combined into a single token that replaces the ## and the two original tokens in the macro expansion [FSF 2005].

Token pasting is most useful when one or both of the tokens come from a macro argument. If either of the tokens next to a ## is a parameter name, it is replaced by its actual argument before ## executes. The actual argument is not macro expanded first.

Stringification

Parameters are not replaced inside string constants, but the # preprocessing operator can be used instead. When a macro parameter is used with a leading #, the preprocessor replaces it with the literal text of the actual argument converted to a string constant [FSF 2005].

Noncompliant Code Example

The following definition for static_assert() from DCL03-C. Use a static assertion to test the value of a constant expression uses the JOIN() macro to concatenate the token assertion_failed_at_line_ with the value of __LINE__:

#define static_assert(e) \
  typedef char JOIN(assertion_failed_at_line_, __LINE__) \
    [(e) ? 1 : -1]

__LINE__ is a predefined macro name that expands to an integer constant representing the presumed line number of the current source line within the current source file. If the intention is to expand the __LINE__ macro, which is likely the case here, the following definition for JOIN() is noncompliant because the __LINE__ is not expanded, and the character array is subsequently named assertion_failed_at_line___LINE__:

#define JOIN(x, y) x ## y

 Compliant Solution

To get the macro to expand, a second level of indirection is required, as shown by this compliant solution:

#define JOIN(x, y) JOIN_AGAIN(x, y)
#define JOIN_AGAIN(x, y) x ## y

JOIN(x, y) calls JOIN_AGAIN(x, y) so that if x or y is a macro, it is expanded before the ## operator pastes them together.

Note also that macro parameters cannot be individually parenthesized when concatenating tokens using the ## operator, converting macro parameters to strings using the # operator, or concatenating adjacent string literals. This is an exception, PRE01-C-EX2, to PRE01-C. Use parentheses within macros around parameter names.

Noncompliant Code Example

This example is noncompliant if the programmer's intent is to expand the macro before stringification:

#define str(s) #s
#define foo 4

str(foo)

The macro invocation str(foo) expands to foo.

Compliant Solution

To stringify the result of expansion of a macro argument, two levels of macros must be used:

#define xstr(s) str(s)
#define str(s) #s
#define foo 4

The macro invocation xstr(foo) expands to 4 because s is stringified when it is used in str(), so it is not macro expanded first. However, s is an ordinary argument to xstr(), so it is completely macro expanded before xstr() is expanded. Consequently, by the time str() gets to its argument, it has already been macro expanded.

Risk Assessment

Use parenthesis around any macro definition that contains operators.

Non-Compliant Coding Example

In this non-compliant coding example, EOF is defined as -1. This macro definition consists of a unary negation operator '-' followed by an integer literal '1'.

#define EOF -1
/* ... */
if (c EOF) {
   /* ... */
}

In this example, the programmer has mistakenly omitted the comparison operator (see \[[MSC02-A. Avoid errors of omission]\]) from the conditional statement which should be {{c != EOF}}. After macro expansion, the conditional expression is incorrectly evaluated as a binary operation: {{c-1}}. This is syntactically correct, even though it is certainly not what the programmer intended. 

Parenthesizing the -1 in the declaration of EOF ensures that the macro expansion is evaluated correctly.

#define EOF (-1)

Once this modification is made, the non-compliant code example no longer compiles as the macro expansion results in the conditional expression c (-1), which is no longer syntactically valid.

Compliant Solution

The following compliant solution uses parenthesis around the macro definition and adds the (previously omitted) comparison operator.

#define EOF (-1)
/* ... */
if (c != EOF) {
   /* ... */
}

Risk Assessment

Failure to use parenthesis around macro definitions that contain operators can result in unintended program behavior.

Automated Detection

Related Vulnerabilities

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

References

Related Guidelines

Bibliography

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Image Added Image Added Image Added Wiki Markup\[[Plum 85|AA. C References#Plum 85]\] Rule 1-1 \[[ISO/IEC 9899-1999|AA. C References#ISO/IEC 9899-1999]\] Section 6.10, "Preprocessing directives," and Section 5.1.1, "Translation environment"