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Date: Sat, 16 Nov 2019 02:08:36 +0100
From: cinap_lenrek@felloff.net
To: 9front@9front.org
Subject: Re: [9front] unofficial sdcard image for raspberry pi 4 testing
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X-Glyph: ➈
X-Bullshit: abstract base pipelining event 

thats the way the kernel does exception handling.

basically, when error() is called, it jumps to the last invocation of
waserror(), and returns 1.

waserror() sets up the jumplabel. on first invocation, it returns 0, so
the print does not happen the first time.

but if some error condition happens, and someone calls error() then it
jumps right back to waserror(), and waserror returns with 1.

poperror() removes the jump label.

example:

if(waserror()){
	print("an error happend\n");
	return;
}

...

if(somevar)
	error("something went wrong");

...

print("everything went fine\n");

poperror();

will print
"an error happend"

once error() is called and somevar is non-zero.

but it somevar is zero, it prints

"everything went fine"

this is often used to cleanup in the error case. you'll find
code like:

x = allocatesomething()
if(waserror()){
	freesomething(x);
	nexterror();
}

...

dosomethingwith(x);

...

poperror();

when dosomethingwithx() calls error(), this will clean up. but in
normal case, it will complete fine and once finished, remove the
error label pushed by waserror() so the error handling can be
nested.

hope this explains it.

--
cinap