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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 \[[MSC00-A|MSC00-A. Compile cleanly at high warning levels]\], but do not prevent program compilation.

Non-Compliant Code Example

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.

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Section 6.11 of the C99 standards, "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

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);
}

Non-Compliant Code Example

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

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

In this 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);
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int func(int one, int two, int three){
  printf("%d %d %d", one, two, three);
  return 1;
}

Compliant Solution

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

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

Non-Compliant Code Example

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

char *p = malloc(10);

Compliant Solution

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

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

Risk Assessment

Failure to specify function prototypes can result in undefined, and perhaps unintended program behavior.

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Examples of vulnerabilities resulting from the violation of this rule can be found on the CERT website.

References

\[[ISO/IEC 9899-1999:TC2|AA. C References#ISO/IEC 9899-1999TC2]\] Forward, Section 6.9.1, "Function definitions"

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