EXP19-C. Use braces for 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.

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 (valid_login())
  if (is_normal())
    privileges = NORMAL;
  else
    privileges = ADMINISTRATOR;

It works as expected by setting the privileges variable accordingly, depending on whether the logged in user is an administrator or not.

However, when the programmer adds another statement to the body of the first if statement, the code functions differently.

int privileges;

if (valid_login())
  printf("Login Successful\n");  /* debugging line added here */
  if (is_normal())               /* this if-else clause is always evaluated even if the user is not logged in! */
    privileges = NORMAL;
  else
    privileges = ADMINISTRATOR;

Because of the additional statement in the body of the first if statement, the user can easily gain administrator privileges, without having to even provide valid login credentials.

Compliant Solution

Adding braces ensures that privileges are correctly assigned.

int privileges;

if (valid_login()) {
  printf("Login Successful\n");  /* debugging line added here */
  if (is_normal()) {
    privileges = NORMAL;
  } else {
    privileges = ADMINISTRATOR;
  }
}

Noncompliant Code Example

Macros can be used to execute a sequence of multiple statements as group.

Note that a macro with multiple statements should be wrapped in a do-while loop (see PRE10-C. Wrap multi-statement macros in a do-while loop), but for the purposes of this example it is not. However, the situation can still be salvaged if braces are used in the if statement.

#define DEC(x,y) \
  printf("Initial value was %d\n", x); \
  x -= y; \
  printf("Current value is %d\n", x)

This macro will expand correctly in a normal sequence of statements, but not as the then-clause in an if statement:

int x, y, z;

if (z == 0)
  DEC(x, y);

This will expand to:

int x, y, z;

if (z == 0)
  printf("Initial value was %d\n", x);
x -= y;
printf("Current value is %d\n", x);

Compliant Solution

Given an if statement bounded with opening and closing braces, the macro would expand as intended.

int x, y, z;

if (z == 0) {
  printf("Initial value was %d\n", x);
  x -= y;
  printf("Current value is %d\n", x)
}

Risk Assessment

References

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