[avr-libc-dev] Malloc hooks

[Taavo Raykoff]

The avr-libc malloc and friends such as realloc and free are implemented in a 
way that is not re-entrant.

When using FreeRTOS, any application code can be pre-empted and needs to be 
thread-safe.  FreeRTOS does have its own allocators independent of avr-libc, 
and one allocator which wraps avr-libc in what is effectively a critical 
section.

However this does not solve the problem of legacy library code which use 
avr-libc directly, such as 
Arduino String library
avr-lib’s strdup() and possibly others
Other libraries

When pre-empted, the allocator structures (freelist, etc) can be corrupted 
resulting in hard-to-find bugs.

I think a good solution to this would be to provide entry/exit hooks on the 
malloc and friends implementations, that can optionally be implemented to 
ensure that the avr-libc implementation is in a section that will prevent 
pre-emption.  This can easily be done using weak linker symbols, that can be 
ignored by existing code, but provide a hook for the entry/exit handlers.

This is one approach:  
https://www.gnu.org/software/libc/manual/html_node/Hooks-for-Malloc.html 
<https://www.gnu.org/software/libc/manual/html_node/Hooks-for-Malloc.html>

I’d like to submit a patch for this and get included upstream…

Taavo.