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If an if, while, or for statement is used in a macro, then the macro definition should not be concluded with a semicolon. (see See guideline PRE11-C. Do not conclude macro definitions with a semicolon.).

Braces help improve the uniformity and readability of code.

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Braces also help ensure that macros with multiple statements are properly expanded. Such a macro should be wrapped in a do-while loop. (see See guideline PRE10-C. Wrap multi-statement macros in a do-while loop.) ; howeverHowever, when the do-while loop is not present, braces can still ensure that the macro expands as intended.

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However, in reality, the else statement actually attaches to the inner if statement, like so:

int privileges;

if (invalid_login())
  if (allow_guests())
    privileges = GUEST;
  else
    privileges = ADMINISTRATOR;

This is a security loophole - users loophole—users with invalid logins can still obtain administrator privileges.

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\[[ISO/IEC 9899-1999|AA. Bibliography#ISO/IEC 9899-1999]\] Section 6.8.4, "Selection statements"
\[[MISRA 042004|AA. Bibliography#MISRA 04]\] Rule 14.8
\[[GNU Coding Standards|http://www.gnu.org/prep/standards/standards.html#Syntactic-Conventions]\] Section 5.3, "Clean Use of C Constructs"

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