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

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

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

Compliant Solution

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 (Array Declarations)

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/* 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

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 (Function Declarations)

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/* 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

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_func integer variable defined in file b.c, which is exactly 30 characters long. On an implementation that exactly meets this limit, the behavior of the program is undefined (see undefined behavior 15. In addition, invoking the function leads to undefined behavior 41 with typically catastrophic effects.

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

Compliant Solution

In this compliant solution the length of the identifier declaring the function pointer bash_groupname_completion() in bashline.h is less than 31 characters. 

/* 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_func; 

Exceptions

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