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Functions should always be declared with the appropriate function prototype. A function prototype is a declaration of a function that declares the types of its parameters. If a function prototype is not available, the compiler cannot perform checks on the number and type of arguments being passed to functions. Argument type checking in C is only performed during compilation, and does not occur during linking, or dynamic loading.

Non-Compliant Code Example 1

This non-compliant program makes use of function declarators with empty parentheses. Consequently, the program compiles cleanly at high warning levels but contains serious errors.

...

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.h>
#include <string.h>

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

Non-Compliant Code Example 2

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

...

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 2

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 3

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 3

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.

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"