libcrt - Barrelfish C and C++ run-time support code

INTRODUCTION

This code supports the C and C++ run-times. It is responsible for
calling the main() function and (if available) all static, global
constructors, before doing so. On program exit, it is responsible for
calling the corresponding global destructors.

GLOBAL (DE-)CONSTRUCTION

This is supported for both C and C++. C global constructors are a GCC
extension. In libcrt, crtbegin.c and crtend.c are responsible for
global (de-)construction.

For global construction, the GCC C++ ABI mandates that a .ctors
section be contained in the ELF binary, which contains an array of
function pointers to global constructor functions, prefixed by a
processor word specifying its length. The length is equal to the
number of elements in the array, or -1, which specifies no length
information. For a length of -1, the array shall be NULL terminated
instead. libcrt uses a NULL terminated array and a length of -1, but
supports loading the other format.

In order for this to work, crtbegin.o has to be linked before any
other object file, except crt0.o, and crtend.o after any other object
file, respectively. crtbegin.o starts the .ctors section by giving the
length word of -1. crtend.o gives the NULL terminator. GCC inserts its
function pointers into the other object files, where appropriate.

Global deconstruction is unimplemented.

UPDATE: In recent versions of binutils (e.g. 2.21.51.20110523), the
linker converts .ctors into .init_array and creates two new symbols
__init_array_start, and __init_array_end, which point to the start and
after the end of the .init_array section, respectively. .init_array
seems to be a forward-walked array of function pointers, without a
length header.


RUN-TIME INITIALIZATION

crt0.S defines _start, the entry point for all Barrelfish ELF
binaries. It is responsible for setting up an initial stack and will
call barrelfish_init_disabled() for libbarrelfish initialization.

A special entry point is _start_init, which passes a third parameter
to barrelfish_init_disabled() to mark an "init domain". Refer to
libbarrelfish documentation for more information.


CALLING MAIN

crtbegin.c contains _main(), which is expected to be called by
libbarrelfish after initialization. This will call all global
constructors and then call main().