DCL40-C. Do not create incompatible declarations of the same function or object

Two or more incompatible declarations of the same function or object must not appear in the same program  because they result in undefined behavior. Subclause 6.2.7 of the C standard mentions that two types may be distinct yet compatible, and addresses precisely when two distinct types are compatible.

The C Standard identifies four situations in which undefined behavior (UB) may arise as a result of incompatible declarations of the same function or object:

Although the effect of two incompatible declarations simply appearing in the same program may be benign on most implementations, the effects of invoking a function through an expression whose type is incompatible with the function definition are typically catastrophic. Similarly, the effects of accessing an object using an lvalue of a type that is incompatible with the object definition may range from unintended information exposure to memory overwrite to a hardware trap.

Noncompliant Code Example (Incompatible Object Declarations)

In this noncompliant code example, variable i is declared to have type int in file a.c but defined to be of type short in file b.c. The declarations are incompatible, resulting in undefined behavior undefined behavior 15. Furthermore, accessing the object using an lvalue of an incompatible type as done in function f() results in undefined behavior 37 with possible observable results ranging from unintended information exposure to memory overwrite to a hardware trap.

/* In a.c */
extern int i;   /* UB 15 */

int f(void) {
  return ++i;   /* UB 37 */
}

/* In b.c */
short i;   /* UB 15 */

Compliant Solution (Incompatible Object Declarations)

This compliant solution has compatible declarations of the variable i:

/* In a.c */
extern int i;   

int f(void) {
  return ++i;   
}

/* In b.c */
int i;   

Noncompliant Code Example (Incompatible Array Declarations)

In this noncompliant code, the variable a is declared to have pointer type in file a.c but defined to have array type in file b.c. The two declarations are incompatible, resulting in undefined behavior 15. As before, accessing the object in function f() results in undefined behavior 37 with the typical effect of triggering a hardware trap.

/* In a.c */
extern int *a;   /* UB 15 */

int f(unsigned i, int x) {
  int tmp = a[i];   /* UB 37: read access */
  a[i] = x;         /* UB 37: write access */
  return tmp;
}

/* In b.c */
int a[] = { 1, 2, 3, 4 };   /* UB 15 */

Compliant Solution (Incompatible Array Declarations)

This compliant solution declares a as an array in a.c and b.c:

/* In a.c */
extern int a[];   

int f(unsigned i, int x) {
  int tmp = a[i];   
  a[i] = x;         
  return tmp;
}

/* In b.c */
int a[] = { 1, 2, 3, 4 };  

Noncompliant Code Example (Incompatible Function Declarations)

In this noncompliant code example, function f() is declared in file a.c with one prototype but defined in file b.c with another. The two prototypes are incompatible, resulting in undefined behavior 15. Furthermore, invoking the function results in undefined behavior 41 with typically catastrophic effects.

/* In a.c */
extern int f(int a);   /* UB 15 */

int g(int a) {
  return f(a);   /* UB 41 */
}

/* In b.c */
long f(long a) {   /* UB 15 */
  return a * 2;
}

Compliant Solution (Incompatible Function Declarations)

This compliant solution has compatible declarations of the function f():

/* In a.c */
extern int f(int a);   

int g(int a) {
  return f(a);   
}

/* In b.c */
int f(int a) {   
  return a * 2;
}

Noncompliant Code Example (Excessively Long Identifiers)

In this noncompliant code example, the length of the identifier declaring the function pointer bash_groupname_completion_function() in file bashline.h exceeds by 3 the minimum implementation limit of 31 significant initial characters in an external identifier, introducing the possibility of colliding with the bash_groupname_completion_funct integer variable defined in file b.c, which is exactly 31 characters long.  On an implementation that exactly meets this limit, this is a violation of undefined behavior 31.  This results in two incompatible declarations of the same function (see undefined behavior 15). In addition, invoking the function leads to undefined behavior 41 with typically catastrophic effects.

/* In bash/bashline.h */
extern char* bash_groupname_completion_function(const char*, int); /* UB 15, UB 31 */

/* In a.c */
#include <bashline.h>

void f(const char *s, int i) {
  bash_groupname_completion_function(s, i);   /* UB 41 */
}

/* In b.c */
int bash_groupname_completion_funct;   /* UB 15, UB 31 */

Note: The identifier bash_groupname_completion_function referenced here was taken from GNU Bash version 3.2.

Compliant Solution (Excessively Long Identifiers)

In this compliant solution, the length of the identifier declaring the function pointer bash_groupname_completion() in bashline.h is less than 32 characters. Consequently it cannot clash with bash_groupname_completion_funct on any compliant platform.

/* In bash/bashline.h */
extern char* bash_groupname_completion(const char*, int);   

/* In a.c */
#include <bashline.h>

void f(const char *s, int i) {
  bash_groupname_completion(s, i);  
}

/* In b.c */
int bash_groupname_completion_funct; 

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