# Initializers/Constructors and Finalizers/Destructors in Picolibc

The `__libc_init_array` and `__libc_fini_array` functions in Picolibc
are called by Picocrt during application startup and shutdown. If you
have custom start up code that replaces Picocrt, you probably want to
call these as well.

## `__libc_init_array`

This function performs application initialization by calling:

 1) Functions listed in the preinit array
 2) _init function
 3) Functions listed in the init array

### _init function

Your program may declare an optional initialization function called
`_init`. If such a function is included in the binary it will be
called before the init array contents are evaluated.

## `__libc_fini_array`

Most embedded applications never return from main, but if yours does,
Picocrt will call:

 1) Functions listed in the fini array
 2) _fini function

### _fini function

Your program may declare an optional finalization function called
`_fini`. If such a function is included in the binary it will be
called after the fini array contents are evaluated.

## Automatically generated arrays of function pointers

The `.preinit_array`, `.init_array` and `.fini_array` segments are
built by the linker using data generated by the compiler. When you
decorate a function with `__attribute__((constructor))`, the compiler
places the address of the function into the `.init_array`
segment. Decorate a function with `__attribute__((destructor))` and
the compiler will place its address in the `.fini_array` segment. The
`.preinit_array` segment is generated internally by the compiler for
things like C++ vtable validation

The linker script should collect all of the data from these segments
into memory (probably flash as it doesn't change) so that the image
ends up with arrays of function pointers. For data from the
`.init_array` segment, the array should start with the symbol
`__init_array_start` and ending with `__init_array_end`. For data from
the `.fini_array` segment, the array should start with the symbol
`__fini_array_start` and end with `__fini_array_end`. Finally, for
data from the `.preinit_array` segment, the array starts with
`__preinit_array_start` and ends with `__preinit_array_end`.

Each of these arrays are complicated by the optional priority assigned
to destructors and constructors, and the presense of the deprecated
`.ctors` and `.dtors` segments.