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Function declarators must be declared with the appropriate type information, including a return type , and 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 properly check function type information. When using standard library calls, the easiest (and preferred) way to obtain function declarators with appropriate type information is to include the appropriate header file.

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

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Noncompliant Code Example

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Non-

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Prototype-

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Format Declarators)

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

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

Section Subclause 6.11 of the C99 standard, "Future language directions", states that "The .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."

Compliant Solution

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Non-

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Prototype-

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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 the curly braces is the function body.:

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

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Noncompliant Code Example

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Function Prototypes)

Failure to specify function prototypes results in a function being implicitly defined. Without a function prototype, the compiler assumes the the 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 practice can result in unintended and undefined behavior.

In this non-compliant noncompliant code example, the definition of func() in file_a.c 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.:

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

However, because there is no prototype for func() in file_b.c, 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:

/* ...file_b.c source file */
func(1, 2);

Wiki MarkupC99 eliminated implicit function declarations from the C language \[[ISO/IEC 9899-1999|AA. C References#ISO/IEC 9899-1999]\]. However, many compilers still allow the compilation of programs containing implicitly defined declared functions, although they may issue a warning message. These warnings should be resolved \[[resolved. (See MSC00-AC. Compile cleanly at high warning levels]\].)

Compliant Solution

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Function Prototypes)

To correct this example, the appropriate This compliant solution correctly includes the function prototype for func() should be specified. in the compilation unit in which it is invoked, and the function invocation has been corrected to pass the right number of arguments:

int func(int, int, int);
/* ...file_b.c source file */
int func(int one, int two, int three){
  printf("%d %d %d", one, two, three);
  return 1;
}
/* ... */
func(1, 2, 3);

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Noncompliant Code Example

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Function Pointers)

If a function pointer refers to an incompatible function, invoking that function via the pointer may corrupt the process stack. As a result, unexpected data may be accessed by the called function.unmigrated-wiki-markup

In this non-compliant 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 an unexpected program behavior. This example also violates rule \[[DCL35-C. Do not convert a function pointer to an incompatible type]\].also violates EXP37-C. Call 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);  /* incorrectIncorrect */
   /* ... */
   return 0;
}

Compliant Solution

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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.

Automated Detection

Related Vulnerabilities

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

References

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Related Guidelines

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