Two or more incompatible declarations of the same function or object must not appear in the same program because they result in undefined behavior. The C Standard, 6.2.7, 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:
UB | Description | Code |
|---|---|---|
Two declarations of the same object or function specify types that are not compatible (6.2.7). | All noncompliant code in this guideline | |
| 31 | Two identifiers differ only in nonsignificant characters (6.4.2.1). | Excessively Long Identifiers |
An object has its stored value accessed other than by an lvalue of an allowable type (6.5). | Incompatible Object Declarations | |
| A function is defined with a type that is not compatible with the type (of the expression) pointed to by the expression that denotes the called function (6.5.2.2). | Incompatible Function Declarations |
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In this noncompliant code example, the variable i is declared to have the type int in file a.c but defined to be of the 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 this noncompliant code example, the function buginf() is defined to take a variable number of arguments and expects them all to be signed integers , with a sentinel value of -1:
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Although this code appears to be well defined because of the prototype-less declaration of buginf(), it exhibits undefined behavior in accordance with the C Standard, 6.7.6.3, paragraph 15 [ISO/IEC 9899:2011]:,
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 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 promotions.
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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, introducing . 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 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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NOTE: The identifier bash_groupname_completion_function referenced here was taken from GNU Bash, version 3.2.
Compliant Solution (Excessively Long Identifiers)
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