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Function declarators must be declared with the appropriate type information, including a return type, parameter list, and function prototype (if the declarator is part of a function definition). If type information is not properly specified in a function declarator, the compiler cannot perform checks on the number and type of arguments being passed to the function.

Attempting to compile a program with a function declarator that does not include the appropriate type information typically generates a warning. These warnings should be resolved \[[MSC00-A. Compile cleanly at high warning levels]\], but do not prevent program compilation.

Non-Compliant Code Example (malloc())

The following example is based on rule \[[MEM02-A. Do not cast the return value from malloc()]\]. The header file {{stdlib.h}} contains the function prototype for {{malloc()}}. Failing to include {{stdlib.h}} causes {{malloc()}} to be improperly defined.

char *p = malloc(10);

Compliant Solution: (malloc())

Including stdlib.h ensures the function prototype for malloc() is declared.

#include <stdlib.h>
/* ... */
char *p = malloc(10);

Non-Compliant Code Example: (non-prototype-format declarators)

The non-compliant code example uses the identifier-list form for the parameter declarations.

extern int max(a, b)
int a, b;
{
  return a > b ? a : b;
}

Section 6.11 of the C99 standards, "Future language directions", 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."

Compliant Solution: (non-prototype-format declarators)

In this compliant solution, extern is the storage-class specifier and int is the type specifier; max(int a, int b) is the function declarator; and the block within {} is the function body.

extern int max(int a, int b)
{
  return a > b ? a : b;
}

Non-Compliant Code Example: (function prototypes)

Failure to specify function prototypes results in a function being implicitly defined. Without a function prototype, the compiler assumes the the correct number and type of parameters have been supplied to a function. This can result in unintended and undefined behavior.

In this non-compliant code example, the definition of func() expects three parameters but is supplied only two. However, because there is no prototype for func(), the compiler assumes that the correct number of arguments has been supplied, and uses the next value on the program stack as the missing third argument.

func(1, 2);
/* ... */
int func(int one, int two, int three){
  printf("%d %d %d", one, two, three);
  return 1;
}

C99 eliminated implicit function declarations from the C language \[[ISO/IEC 9899-1999:TC2|AA. C References#ISO/IEC 9899-1999TC2]\]. However, many compilers allow compilation of programs containing implicitly defined functions, although they may issue a warning message. These warnings should be resolved \[[MSC00-A. Compile cleanly at high warning levels]\], but do not prevent program compilation.

Compliant Solution: (function prototypes)

To correct this example, the appropriate function prototype for func() should be specified.

int func(int, int, int);
/* ... */

func(1, 2);
/* ... */
int func(int one, int two, int three){
  printf("%d %d %d", one, two, three);
  return 1;
}

Non-Compliant Code Example: (function pointers)

If a function pointer is set to refer to an incompatible function , invoking that function via the pointer may cause unexpected data to be taken from the process stack. As a result, unexpected data may be accessed by the called function \[[DCL35-C. Do not convert a function pointer to a function of an incompatible type]\].

In this example, the function pointer fn_ptr is set to refer 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 an unexpected program behavior.

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;
}

Compliant Solution: (function pointers)

To correct this example, the declaration of fn_ptr is changed to accept three 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) ;
   int res;
   fn_ptr = add;
   res = fn_ptr(2, 3, 4);
   /* ... */
   return 0;
}

Risk Assessment

Failing to include type information for function declarators can result in unexpected or unintended program behavior.

Related Vulnerabilities

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

References

\[[ISO/IEC 9899-1999:TC2|AA. C References#ISO/IEC 9899-1999TC2]\] Forward, Section 6.9.1, "Function definitions"
\[[Spinellis 06|AA. C References#Spinellis 06]\] Section 2.6.1, "Incorrect Routine or Arguments"