(implementation_standard)=

# Convention for implementing entrypoints

The implementations of LLVM-libc entrypoints live in the `src/` directory,
organized by the public header they belong to. Some entrypoints are platform-
specific, and so their implementations are in a subdirectory with the name of
the platform (e.g., `stdio/linux/remove.cpp`).

For a complete overview of what an entrypoint is and how it is managed in the
build system, see the {ref}`entrypoints` documentation.

Implementation of entrypoints can span multiple `.cpp` and `.h` files, but
there will be at least one header file with name of the form
`<entrypoint name>.h` for every entrypoint. This header file is called the
implementation header file. For the `isalpha` function, the path to the
implementation header file is `src/ctype/isalpha.h`.

## Implementation Header File Structure

We will use the `isalpha` function from the public `ctype.h` header file as an
example. The `isalpha` function will be declared in an internal header file
`src/ctype/isalpha.h` as follows:

```
// --- isalpha.h --- //
#ifndef LLVM_LIBC_SRC_CTYPE_ISALPHA_H
#define LLVM_LIBC_SRC_CTYPE_ISALPHA_H

namespace LIBC_NAMESPACE_DECL {

int isalpha(int c);

} // namespace LIBC_NAMESPACE_DECL

#endif LLVM_LIBC_SRC_CTYPE_ISALPHA_H
```

All LLVM-libc implementation constructs must be enclosed in the
`LIBC_NAMESPACE_DECL` namespace. See {ref}`code_style` for the full technical
rationale and macro definitions.

## `.cpp` File Structure

The main `.cpp` file is named `<entrypoint name>.cpp` and is usually in the
same folder as the header. It contains the signature of the entrypoint function,
which must be defined with the `LLVM_LIBC_FUNCTION` macro. For example, the
`isalpha` function from `ctype.h` is defined as follows, in the file
`src/ctype/isalpha.cpp`:

```
// --- isalpha.cpp --- //

namespace LIBC_NAMESPACE_DECL {

LLVM_LIBC_FUNCTION(int, isalpha, (int c)) {
  // ... implementation goes here.
}

} // namespace LIBC_NAMESPACE_DECL
```

Notice the use of the macro `LLVM_LIBC_FUNCTION`. This macro helps us define
a C alias symbol for the C++ implementation. For example, for a library build,
the macro is defined as follows:

```
#define LLVM_LIBC_FUNCTION(type, name, arglist)
    LLVM_LIBC_FUNCTION_IMPL(type, name, arglist)
#define LLVM_LIBC_FUNCTION_IMPL(type, name, arglist)
    LLVM_LIBC_FUNCTION_ATTR decltype(LIBC_NAMESPACE::name)
        __##name##_impl__ __asm__(#name);
    decltype(LIBC_NAMESPACE::name) name [[gnu::alias(#name)]];
    type __##name##_impl__ arglist
```

The LLVM_LIBC_FUNCTION_ATTR macro is normally defined to nothing, but can be
defined by vendors who want to set their own attributes.

When defining a variable, use the `LLVM_LIBC_VARIABLE` macro:

```
LLVM_LIBC_VARIABLE(char **, environ) = nullptr;
```
