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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.

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Incompatible Object Declarations Incompatible Object Declarations

Noncompliant Code Example (Incompatible Object Declarations)

In this noncompliant code example, the 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 15. Furthermore, accessing the object using an lvalue of an incompatible type, as shown in function f(), is undefined behavior 37 with possible observable results ranging from unintended information exposure to memory overwrite to a hardware trap.

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

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

/* In b.c */
int i;   

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Incompatible Array Declarations Incompatible Array Declarations

Noncompliant Code Example (Incompatible Array Declarations)

In this noncompliant code example, 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() is undefined behavior 37 with the typical effect of triggering a hardware trap.

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

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

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

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Incompatible Function Declarations Incompatible Function Declarations

Noncompliant Code Example (Incompatible Function Declarations)

In this noncompliant code example, the 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 is undefined behavior 41 and typically has catastrophic consequences.

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/* In a.c */
void buginf(const char *fmt, ...) {
   /* ... */
}

/* In b.c */
void buginf(const char *fmt, ...);

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Excessively Long Identifiers Excessively Long Identifiers

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 undefined behavior 31. It 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.

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