EXP19-C. 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 said statement's body contains only a single statement.

In the event that either of these statements are used in a macro, this implies that the macro definition should not be concluded with a semicolon (see PRE11-C. Do not conclude macro definitions with a semicolon).

Braces help improve the uniformity, and therefore readability of code.

More importantly, when inserting an additional statement in a body containing only a single statement, it is easy to forget to add braces when the indentation tends to give a strong (but probably misleading) guide 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 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-else statement without braces to authenticate a user.

int login;

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

The programmer adds a debugging statement to determine when the login is valid, but forgets 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 is not functioning as intended by the programmer, leading to a possible security breach.

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

When you have an if-else statement nested in another if statement, always put braces around the if-else.

This noncompliant code example does not use braces.

int privileges;

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

According to the indentation, the programmer may be led to believe that a user is given administrator privileges only when his login is valid.

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 with invalid logins can still obtain administrator privileges.

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

References

[[ISO/IEC 9899-1999]] Section 6.8.4, "Selection statements"
[[MISRA 04]] Rule 14.8
[GNU Coding Standards] Section 5.3, "Clean Use of C Constructs"