What is built-in function in c?

Answer 1

There are no built-in functions in C++. The definition of a built-in function is a function that does not need to be declared before it is used, but every function in C++ is user-defined and must be declared before it can be used. This includes functions provided by the C++ standard library which we declare by including the appropriate headers.

Some articles mistakenly describe keywords such as while, switch and if as being built-in functions, however these are statements, not functions.

C++ does provide several built-in operators, some of which look and behave very much like functions. For example, the built-in sizeof() operator can be used without any declaration and looks very much like a function, but its argument is not a value, it is a type name, and we cannot (easily) define a function that accepts a type name as an argument.

The built-in typeid() operator also takes a type name argument, but it is often mistakenly regarded as being a user-defined function because we must include the C++ standard library header in order to use it. But that header is only required because the return value is a std::typeinfo object which is not a built-in data type.

Unlike sizeof() and typeid(), the default global new and delete operators can be overridden with user-defined function operators, thus these are also mistakenly regarded as being built-in functions. However, only the overrides are functions because that's the only way to define an operator overload; the default global operators are built-in operators, not built-in functions.

The semantic difference between a built-in operator and a built-in function may seem insignificant, however a real built-in function would be no different to a user-defined function other than the fact that it need not be declared before using it. But a user-defined function also has identity (a memory address) and we can pass that identity to other functions using a function pointer argument. But we cannot pass a built-in operator to a user-defined function because it has no identity, thus it cannot be regarded as being a built-in function.

Answer 2

1. Compiler-dependent, tipically: memcpy, memcmp, strcpy, strlen, strcmp.

2. sizeof -- but that isn't really a function

Answer 3

There are no built-in functions in C. All functions are user-defined, including those defined by the C standard library, which is a user-defined library just like any other.