DCL20-C. Always specify void even if a function takes no parameter

Functions that take no parameters should explicitly declare a void parameter in their parameter list. This holds true during both the declaration and definition sections (and they should match). Many compilers today still allow implicitly declared functions, even though C99 has eliminated them.

Defining a function with a void argument list differs from declaring it with no arguments because in the latter case, the compiler will not check whether the function is called with parameters at all [C void usage]. Consequently, function calling with arbitrary parameters will be accepted without a warning at compile time.

Failure to declare a void parameter will result in

• an ambiguous functional interface between the caller and callee
• sensitive information outflow

A similar rule deals with parameter type in a more general sense: DCL07-C. Include the appropriate type information in function declarators.

In C+, the usage of foo() and foo(void) has exactly the same meaning and effect, so this rule doesn't apply to C+. But it is still recommended to explicitly declare foo(void) instead of foo() to distinguish from foo(...), which will then take arbitrary parameters.

Noncompliant Code Example (Ambiguous Interface)

In this noncompliant code example, the caller gives a call to foo with a parameter specified as 3. Because foo() is declared without void, the compiler will not perform any caller check. Due to the accidental internal implementation, the function foo() outputs a value: 3 which matches with caller's temptation. In an inherited code base where foo and caller are developed at different times, this can lead to the caller's belief that foo() actually accepts one integer as parameter and foo() will output the corresponding message when the parameter is changed.

In fact, no parameter specified for a function has the same meaning as arbitrary parameters, which means that the caller may feed arbitrary number of parameters to the function.

void foo() {
  int i = 3;
  printf("i value: %d\n", i);
}

...

/* caller */
foo(3);

Compliant Solution (Ambiguous Interface)

In this compliant solution, void is explicitly specified as a parameter.

/* compile using gcc4.3.3 */
void foo (void) {
  int i = 3;
  printf("i value: %d\n", i);
}

Implementation Details (Ambiguous Interface)

When the above compliant solution is used and foo(3) is called, the GCC compiler will issue the following diagnostic, which alerts the programmer about the misuse of the function interface.

error: too many arguments to function ‘foo’

Noncompliant Code Example (Information Outflow)

Another possible vulnerability is the leak of privileged information. In this noncompliant code example, suppose a user with high privileges feeds some secret input into the caller that the caller then passes to foo(). Because of the way function foo() is defined, it is easy to assume that there is no way that foo() can retrieve info from caller. However, because the value of i is really passed into stack (before the return address of caller), a malicious programmer can change the internal implementation and copy the value manually and contain it in a less privileged file.

/* compile using gcc4.3.3 */
void foo() {
  /* use asm code to retrieve i
   * implicitly from caller
   * and transfer it to a less privilege file */
}

...

/* caller */
foo(i); /* i is fed from user input */

Compliant Solution (Information Outflow)

void foo(void) {
  int i = 3;
  printf("i value: %d\n", i);
}

Again, the simplest solution is to explicitly specify void as the only parameter.

Risk Assessment

Related Vulnerabilities

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

References

[[ISO/IEC 9899:1999]] Forward and Section 6.9.1, "Function definitions"
[C void usage]