Use and Implementation of Interfaces
Each of the following statements applies unless explicitly stated otherwise in the detailed descriptions that follow. If an argument to a function has an invalid value (such as a value outside the domain of the function, or a pointer outside the address space of the program, or a null pointer), the behaviour is undefined. Any function declared in a header may also be implemented as a macro defined in the header, so a library function should not be declared explicitly if its header is included. Any macro definition of a function can be suppressed locally by enclosing the name of the function in parentheses, because the name is then not followed by the left parenthesis that indicates expansion of a macro function name. For the same syntactic reason, it is permitted to take the address of a library function even if it is also defined as a macro. The use of the C-language #undef construct to remove any such macro definition will also ensure that an actual function is referred to. Any invocation of a library function that is implemented as a macro will expand to code that evaluates each of its arguments exactly once, fully protected by parentheses where necessary, so it is generally safe to use arbitrary expressions as arguments. Likewise, those function-like macros described in the following sections may be invoked in an expression anywhere a function with a compatible return type could be called.
Provided that a library function can be declared without reference to any type defined in a header, it is also permissible to declare the function, either explicitly or implicitly, and use it without including its associated header. If a function that accepts a variable number of arguments is not declared (explicitly or by including its associated header), the behaviour is undefined.
As a result of changes in this issue of this specification, application writers are only required to include the minimum number of headers. Implementations of XSI-conformant systems will make all necessary symbols visible as described in the Headers section of this specification.
Use of File System InterfacesThe Interfaces in this volume that operate on files can behave differently if the file that is being operated on has been made available by a network file system. If the network file system is an XSI-conformant system conforming to the XNFS specification, the differences that can occur are detailed in Appendices A and B of that document.