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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:

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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 1514. Furthermore, accessing the object using an lvalue of an incompatible type, as shown in function f(), is undefined behavior 37 with 36 with possible observable results ranging from unintended information exposure to memory overwrite to a hardware trap.

/* In a.c */
extern int i;   /* UB 1514 */

int f(void) {
  return ++i;   /* UB 3736 */
}

/* In b.c */
short i;   /* UB 1514 */

Compliant Solution (Incompatible Object Declarations)

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In this noncompliant code example, the variable a is declared to have a pointer type in file a.c but defined to have an array type in file b.c. The two declarations are incompatible, resulting in undefined behavior 1514. As before, accessing the object in function f() is undefined behavior 3736 with the typical effect of triggering a hardware trap.

/* In a.c */
extern int *a;   /* UB 1514 */

int f(unsigned int i, int x) {
  int tmp = a[i];   /* UB 3736: read access */
  a[i] = x;         /* UB 3736: write access */
  return tmp;
}

/* In b.c */
int a[] = { 1, 2, 3, 4 };   /* UB 1514 */

Compliant Solution (Incompatible Array Declarations)

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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 1514. Furthermore, invoking the function is undefined behavior 4137 and typically has catastrophic consequences.

/* In a.c */
extern int f(int a);   /* UB 1514 */

int g(int a) {
  return f(a);   /* UB 4137 */
}

/* In b.c */
long f(long a) {   /* UB 1514 */
  return a * 2;
}

Compliant Solution (Incompatible Function Declarations)

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Although this code appears to be well defined because of the prototype-less declaration of of buginf(), it exhibits undefined behavior in accordance with the C Standard, 6.7.67.34, paragraph 15 14 [ISO/IEC 9899:20112024],

For two function types to be compatible, both shall specify compatible return types. Moreover, the parameter type lists , if both are present, shall agree in the number of parameters and in use of the final ellipsis terminator; corresponding parameters shall have compatible types. If one type has a parameter type list and the other type is specified by a function declarator that is not part of a function definition and that contains an empty identifier list, the parameter list shall not have an ellipsis terminator and the type of each parameter shall be compatible with the type that results from the application of the default argument promotionsIn the determination of type compatibility and of a composite type, each parameter declared with function or array type is taken as having the adjusted type and each parameter declared with qualified type is taken as having the unqualified version of its declared type.

Compliant Solution (Incompatible Variadic Function Declarations)

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In this noncompliant code example, the length of the identifier declaring the function pointer bash_groupname_completion_function() in the file bashline.h exceeds by 3 the minimum implementation limit of 31 significant initial characters in an external identifier. This introduces 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 3130. It results in two incompatible declarations of the same function. (See undefined behavior 1514.) In addition, invoking the function leads to undefined behavior 4137 with typically catastrophic effects.

/* In bashline.h */
/* UB 1514, UB 3130 */
extern char * bash_groupname_completion_function(const char *, int);

/* In a.c */
#include "bashline.h"

void f(const char *s, int i) {
  bash_groupname_completion_function(s, i);  /* UB 4137 */
}

/* In b.c */
int bash_groupname_completion_funct;  /* UB 1514, UB 3130 */

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

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