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We use weak linking to ensure we clean up at exit time correctly. If
libc is linked, then we call our cleanup function by registering an
atexit callback with the library. If libc is not linked, we have a weak
symbol that results in a noop. Similarly, if we call rt·exit while
linked with libc, this immediately calls libc's exit (which will call
our cleanup as we registered it). If we are not linked to libc, exit()
is given as a weak link to a noop function.
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This is a large change. In order to remove myself from libc's arcane
interface, I implemented an independent runtime layer. It is based on
musl's wonderful implementation mostly. Critically, if libc is linked to
the program, then we cooperate. Namely, we call start main and let libc
do all initialization. If not, then we have a noop defined in rt3.a.
The general structure of the file is:
1. sys/$os/$arch contains all architecture dependent code
2. sys/$os/port contains all code that depends on the os, but is
portable
3. rt/$arch contains all the runtime architecture dependent code
4. rt/* contains the portable runtime code.
Obviously testing is needed. Specifically, while code is checked in for
the most popular architectures, it only has been tested on one computer!
Overall this is exciting and as been educational.
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