Page History

...

Attempting to compile a program with a function declarator that does not include the appropriate type information typically generates a warning but does not prevent program compilation. These warnings should be resolved. (See MSC00-C. Compile cleanly at high warning levels.)

Noncompliant Code Example (Non-Prototype-Format Declarators)

...

Section 6.11.7 of the C Standard [ISO/IEC 9899:2011] states that "the use of function definitions with separate parameter identifier and declaration lists (not prototype-format parameter type and identifier declarators) is an obsolescent feature."

...

Declaring a function without any prototype forces the compiler to assume that the correct number and type of parameters have been supplied to a function. This can result in unintended and undefined behavior.

In this noncompliant code example, the definition of func() in file_a.c expects three parameters but is supplied only two.

...

C99 eliminated implicit function declarations from the C language. However, many compilers still allow the compilation of programs containing implicitly declared functions, although they may issue a warning message. These warnings should be resolved. (See MSC00-C. Compile cleanly at high warning levels.)

Compliant Solution (Function Prototypes)

...

In this noncompliant code example, the function pointer fn_ptr refers to the function add(), which accepts three integer arguments. However, fn_ptr is specified to accept two integer arguments. Setting fn_ptr to refer to add() results in unexpected program behavior. This example also violates DCL35-C. Do not invoke a function using a type that does not match the function definitionCall functions with the correct number and type of arguments.

int add(int x, int y, int z) {
   return x + y + z;
}

int main(int argc, char *argv[]) {
   int (*fn_ptr) (int, int);
   int res;
   fn_ptr = add;
   res = fn_ptr(2, 3);  /* incorrect */
   /* ... */
   return 0;
}

...

...

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

Related Guidelines

Bibliography

...