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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. These warnings should be resolved \[[MSC00-A. Compile cleanly at high warning levels]\], but do not prevent program compilation.

Non-Compliant Code Example

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In this example a function declarators is specified with empty parentheses. Consequently, the program compiles cleanly at high warning levels but contains serious errors.

(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 <stdio.h>
extern char *strchr();

int main(void) {
  char *c = strchr("world", 'w');
  printf("Hello %c!\n", *c);
  return 0;
}

C99 Section 6.11, "Future language directions", states that "The use of function declarators with empty parentheses (not prototype-format parameter type declarators) is an obsolescent feature." The use of these declarations prevents the compiler from performing type checking.

Compliant Solution 1

The following compliant solution includes the header file containing the appropriate library function prototype.

#include <stdio<stdlib.h>
#include/* <string.h>

int main(void) {
  ... */
char *cp = strchr("world", 'w');
  printf("Hello %c!\n", *c);
  return 0;
}
malloc(10);

Non-Compliant Code Example

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: (non-prototype-format declarators)

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

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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

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: (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

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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 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 \[[ISO/IEC 9899-1999:TC2|AA. C References#ISO/IEC 9899-1999TC2]\]. 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

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: (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 4

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 4

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

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

Risk Assessment

Failing to specify function prototypes can result in unexpected or unintended program behavior.

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