Variables and functions should be declared in the minimum scope from which all references to the identifier are still possible.

By using a larger scope than is necessary, code becomes less readable, harder to maintain, and more likely to reference unintended variables. (See recommendation DCL01-C. Do not reuse variable names in subscopes.)

Noncompliant Code Example

In this noncompliant code example, the function counter() increments the global variable count and then returns immediately if this variable exceeds a maximum value.

unsigned int count = 0;

void counter() {
  if (count++ > MAX_COUNT) return;
  /* ... */

}

Assuming that the variable count is only accessed from this function, this example is noncompliant because it does not define count within the minimum possible scope.

Compliant Solution

In this compliant solution, the variable count is declared within the scope of the counter() function as a static variable. The static modifier, when applied to a local variable (one inside of a function), modifies the lifetime (duration) of the variable so that it persists for as long as the program does, and does not disappear between invocations of the function.

void counter() {
  static unsigned int count = 0;
  if (count++ > MAX_COUNT) return;
  /* ... */

}

The keyword static also prevents re-initialization of the variable.

Noncompliant Code Example

The counter variable i is declared outside of the for loop. This goes against this recommendation because it is not declared in the block in which it is used. If this snippet were reused with another index variable j, but there was a previously declared variable i, the loop could iterate over the wrong variable.

public void doStuff(...){
  size_t i = 0;

  for (i=0; i < 10; i++){
    /* Perform operations */
  }
}

Compliant Solution

Complying with this recommendation requires that you declare variables where they are used, thus improving readability and reusability. In this example, this would be done by declaring the loop's index variable i within the initialization of the for loop. This was recently relaxed in the C99 standard.

public void doStuff(...){
  for (size_t i=0; i < 10; i++) {
    /* Perform operations */
  }
}

Risk Assessment

Failure to minimize scope could result in less reliable, readable, and reusable code.

Automated Detection

TODO

Related Guidelines

CERT C++ Secure Coding Standard: DCL07-CPP. Minimize the scope of variables and methods

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

Related Guidelines

ISO/IEC 9899:1999 Appendix D.1.15, "Declaration in for-Loop Statement"

Bibliography

      02. Declarations and Initialization (DCL)      DCL20-C. Always specify void even if a function accepts no arguments