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According to the C Standard, subclause 6.7.6.3, paragraph 14 [ISO/IEC 9899:2011],

An identifier list declares only the identifiers of the parameters of the function. An empty list in a function declarator that is part of a definition of that function specifies that the function has no parameters. The empty list in a function declarator that is not part of a definition of that function specifies that no information about the number or types of the parameters is supplied.

Subclause 6.11.6 states that

The use of function declarators with empty parentheses (not prototype-format parameter type declarators) is an obsolescent feature.

Consequently, functions that accept no arguments Functions that takes no parameter should explicitly declare a void parameter as in their parameter list. This holds true during in both the declaration and definition section sections (and they which should match), especially given that many compilers today still allow implicitly declared functions though C99 has eliminated it.

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 [TIGCC, void usage]. Consequently, function calling with arbitrary parameters will be accepted without a warning at compile time.

Failure to declare a void parameter will results result in

• An ambiguous functional interface between the caller and callee.
• Sensitive information outflow.

A similar recommendation deals with parameter type in a more general sense: DCL07-C. Include the appropriate type information in function declarators, and even sensitive information outflow.

Noncompliant Code Example (

...

Ambiguous Interface)

In this noncompliant code example, the caller calls foo() with an argument of 3. The caller expects foo() to accept a single int argument and to output the argument as part of a longer message. Because foo() is declared without the void parameter, the compiler will not perform any caller check. It is therefore possible that the caller may not detect the error. In this example, for instance, foo() might output the value 3 as expected.

Because no function parameter has the same meaning as an arbitrary parameter, the caller can provide an arbitrary number of arguments to the function.

/* In foo.h */
void foo();

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

...

/* In caller.c */
    #include "foo.h"

foo(3);

In this particular example, caller gives a call to foo with parameter specified as 3. When compiled with gcc-4.3.3 in Linux, no warning will be issued. Due to the accidental internal implementation, the function foo() outputs i value: 3 which matches with caller's temptation. In an inherited code base where foo and caller are developed at different time, this leads to caller's belief that foo actually accept one integer as parameter and foo will output the corresponding message when parameter is changed.

In fact, no parameter specified for a function has the same meaning as arbitrary paramemters, which means the caller can feed arbitrary number of parameters to the function. Arbitrary parameter list is useful for system logging purpose, e.g., but not the callee's intention in other situation.

Compliant Solution (Ambiguous Interface)

In this compliant solution, void is specified explicitly as a parameter in the declaration of foo's prototype:

...

/* In foo.h */
void foo(void);

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

In this example, void is explicitly specified as a parameter. If the caller calls foo(3) again, a warning message will be issued as follows:

/* In caller.c */
#include "foo.h"

foo(3);

Implementation Details (Ambiguous Interface)

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

error: too many arguments to function ‘foo’

Because it is an error message, the programmer can not ignore it and thus previous belief regarding the interface can be rectified.

"€˜foo"

Noncompliant Code Example (Information Outflow)

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

...

/* compileCompile using gcc4.3.3 */
void foo () {
  /* 
 /  * useUse asmassembly code to retrieve i
     * implicitly from caller
     * and transfer it to a less privilegeprivileged file.
   */
}

...

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

Another possible vulnerability is the leak of privileged information. In the above example, suppose a user with high privilege feed some secret input into the caller, then caller will pass the info to foo. When doing static code analysis of foo interface, because no parameter is specified, it is easy to assume there's 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.

...

Compliant Solution (Information Outflow)

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

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

Risk Assessment

Related Vulnerabilities

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

Other Languages

Automated Detection

Related Vulnerabilities

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

Related Guidelines

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

...

accepts an arbitrary number and type of arguments.

Bibliography

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