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Date: Sun, 27 Apr 2014 13:00:46 -0500
From: Richard Pennington <>
CC: Rich Felker <>
Subject: Re: Bare metal progress and an oddity.

On 04/27/2014 01:00 PM, Rich Felker wrote:
> On Sun, Apr 27, 2014 at 12:37:13PM -0500, Richard Pennington wrote:
>> As part of my quixotic quest to run musl on ARM bare metal I've
>> started to handle interrupts and an ARM timer to implement the
>> clock_* functions to prepare for pre-emptive scheduling of
>> pthread_create'd threads. The current sources are here:
>> An oddity: If I use printf() to print out values in an interrupt
>> handler it sort of works. The format string prints perfectly, but it
>> seems that the va_arg values are getting messed up. The stack is
>> large enough, fp and sp are sane. Am I missing something in
>> pop_arg() that is causing this? Come to think of it, do I have to
>> insure a certain stack alignment, for example? Most of the messed up
>> values are "long long".
>> Any thoughts appreciated.
> Yes, ARM EABI requires the stack to be aligned to at least an 8-byte
> boundary (maybe 16 but I think 8 is all that's required). If your
> stack is only aligned to 4 bytes, this will mess things up badly.
> Rich


You're absolutely right. As I was writing my initial email I thought of 
the stack alignment. It turns out that my idle thread pushed 12 bytes on 
the stack making it unaligned. The idle thread is a leaf function (of 
course) so I guess clang didn't feel the need to keep the stack aligned.
After fixing the alignment, interrupt printf() works fine.


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