EXP05-J. Use braces for the body of an if, for, or while statement

Opening and closing braces for if, for, or while statements should always be used, even if the statement's body contains only a single statement.

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

Braces improve the uniformity and readability of code.

More importantly, when inserting an additional statement into a body containing only a single statement, it is easy to forget to add braces because the indentation gives strong (but misleading) guidance to the structure.

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

Noncompliant Code Example

This noncompliant code example uses an if statement without braces to authenticate a user.

int login;

if (invalid_login())
  login = 0;
else
  login = 1;

A developer might add a debugging statement to determine when the login is valid, but forget to add opening and closing braces.

int login;

if (invalid_login())
  login = 0;
else
  printf("Login is valid\n");  /* debugging line added here */
  login = 1;                   /* this line always gets executed, regardless of a valid login! */

Due to the indentation of the code, it is difficult to tell that the code will not function as intended by the programmer, potentially leading to a security breach.

Compliant Solution

In the compliant solution, opening and closing braces are used even when the body is a single statement.

int login;

if (invalid_login()) {
  login = 0;
} else {
  login = 1;
}

Noncompliant Code Example

This noncompliant code example has an if statement nested in another if statement without braces around if and else bodies.

int privileges;

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

The indentation could lead the programmer to believe that a user is given administrator privileges only when the user's login is valid. However, the else statement actually attaches to the inner if statement:

int privileges;

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

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

Compliant Solution

In the compliant solution, adding braces removes the ambiguity and ensures that privileges are correctly assigned.

int privileges;

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

Risk Assessment

Related Guidelines

ISO/IEC 9899:1999 Section 6.8.4, "Selection statements"

MISRA Rule 14.8

Bibliography

[[GNU 2010]] Coding Standards, Section 5.3, "Clean Use of C Constructs"

EXP18-C. Do not perform assignments in selection statements      03. Expressions (EXP)      EXP20-C. Perform explicit tests to determine success, true and false, and equality