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

Functions that takes no parameter should explicitly declare a void parameter in their parameter list. This holds true during both the declaration and definition section (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 the function 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]. Thus, function calling with arbitrary parameters will be accepted without a warning at compile time.

Failure to declare a void parameter will result in ambiguous functional interface between caller and callee, and even sensitive information outflow.

There is a similar rule that 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 particular 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 paramemters, which means that the caller can 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 example, 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)

In gcc-4.3.3, when the above compliant solution is used and foo(3) is called, gcc will issue the following diagnostic, which alerts the programmer of 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 the following example, suppose a user with high privileges feeds some secret input into the caller, then caller will pass the info 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 implmentation 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]