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

In the event that either of these statements are If an if, while, or for statement is used in a macro, this implies that the macro definition should not be concluded conclude with a semicolon. (see See PRE11-C. Do not conclude macro definitions with a semicolon.).

Braces help improve the uniformity , and therefore readability of code. More importantlyimportant, when inserting an additional statement in into a body containing only a single statement, it is easy to forget to add braces when because the indentation tends to give a gives strong (but probably misleading) guide 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 PRE10-C. Wrap multistatement 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 developer might add a debugging statement to determine when the login is valid , but forgets forget to add opening and closing braces.:

int login;

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

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

Compliant Solution

Opening 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

When you have This noncompliant code example has an if-else statement nested in another if statement , always put without braces around the if-else. This noncompliant code example does not use braces. and else bodies:

int privileges;

if (validinvalid_login())
  if (isallow_normalguests())
    privileges = NORMALGUEST;
  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.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 (validinvalid_login())
  printf("Login Successful\n");  /* debugging line added here */
  if (isallow_normalguests())               /* this if-else clause is always evaluated even if the user is not logged in! */
    privileges = NORMALGUEST;
  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 credentialsThis is a security loophole: users with invalid logins can still obtain administrator privileges.

Compliant Solution

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

int privileges;

if (validinvalid_login()) {
  printf("Login Successful\n");  /* debugging line added here */
  if (isallow_normalguests()) {
    privileges = NORMALGUEST;
  } 
} else {
    privileges = ADMINISTRATOR;
  }
}

Noncompliant Code Example

...

(empty block)

This noncompliant code example has a while statement with no block:

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

while (invalid_login());

Note that if invalid_login() has no side effects (such as warning the user if their login failed), this code also violates MSC12-C. Detect and remove code that has no effect or is never executed.

Compliant Solution (empty block)

This compliant solution features an explicit empty block, which clarifies the developer's intent:Given an if statement bounded with opening and closing braces, the macro would expand as intended.

while (invalid_login()) {
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

Automated Detection

Related Vulnerabilities

CVE-2014-1266 was due, in large part, to failing to follow this recommendation. There is a spurious "goto fail" statement on line 631 of sslKeyExchange.c. This "goto" gets executed unconditionally, even though it is indented as if it were part of the preceding "if" statement.  As a result, the call to sslRawVerify (which performs the actual signature verification) is rendered dead code.  [ImperialViolet 2014]. If the body of the "if" statement had been enclosed in braces, then this defect likely would not have happened.

Related Guidelines

 Bibliography

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Image Added Image Added Image Added Wiki Markup\[[ISO/IEC 9899-1999|AA. References#ISO/IEC 9899-1999]\] Section 6.8.4, "Selection statements" \[GNU Coding Standards|http://www.gnu.org/prep/standards/standards.html#Syntactic-Conventions\] Section 5.3, "Clean Use of C Constructs"