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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-A. Compile cleanly at high warning levels).

Non-Compliant Code Example (

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Non-

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Prototype-

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Format Declarators)

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

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Section 6.11 of the C99 standard, "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 (

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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;
}

Non-Compliant 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 that the correct number and type of parameters have been supplied to a function. This can result in unintended and undefined behavior.

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C99 eliminated implicit function declarations from the C language \[[ISO/IEC 9899-1999|AA. C References#ISO/IEC 9899-1999]\]. However, many compilers still allow compilation of programs containing implicitly defined functions, although they may issue a warning message. These warnings should be resolved (see [MSC00-A. Compile cleanly at high warning levels]).

Compliant Solution (

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Function Prototypes)

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

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

Non-Compliant 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.

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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 (

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Function Pointers)

To correct this example, the declaration of fn_ptr is changed to accept three arguments.

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