Editing C standard library (section)

=== Compiler built-in functions ===
Some compilers (for example, [[GNU Compiler Collection|GCC]]<ref>{{cite book |author=FSF |author-link=Free Software Foundation |date=2024 |chapter=6.64 Other Built-in Functions Provided by GCC |chapter-url=https://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html |title=A GNU Manual |url=https://gcc.gnu.org/onlinedocs/gcc/index.html |publisher=self-published}}</ref>) provide built-in versions of many of the functions in the C standard library; that is, the implementations of the functions are written into the compiled [[object file]], and the program calls the built-in versions instead of the functions in the C library [[shared object]] file. This reduces function-call overhead, especially if function calls are replaced with [[inline function|inline]] variants, and allows other forms of [[compiler optimization|optimization]] (as the compiler knows the [[control flow|control-flow]] characteristics of the built-in variants), but may cause confusion when debugging (for example, the built-in versions cannot be replaced with [[instrumentation (computer programming)|instrumented]] variants).

However, the built-in functions must behave like ordinary functions in accordance with ISO C. The main implication is that the program must be able to create a pointer to these functions by taking their address, and invoke the function by means of that pointer. If two pointers to the same function are derived in two different translation units in the program, these two pointers must compare equal; that is, the address comes by resolving the name of the function, which has external (program-wide) linkage.