lib: libc: minimal: add check on returned value

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• Fix [Coverity CID :219508] Unchecked return value in lib/libc/minimal/source/stdlib/malloc.c #32938 [Coverity CID :219508] Unchecked return value in lib/libc/minimal/source/stdlib/malloc.c
• Fix [Coverity CID :219508] Unchecked return value in lib/libc/minimal/source/stdlib/malloc.c #32937 [Coverity CID :219508] Unchecked return value in lib/libc/minimal/source/stdlib/malloc.c

The Coverity complains about sys_mutex_lock() which returns 0 if
locked. I added also the same check on returned value for
sys_mutex_unlock() which returns 0 if unlocked.

Signed-off-by: Guðni Már Gilbert gudni.m.g@gmail.com

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I'm wondering how we could track the returned value from sys_mutex_lock() and sys_mutex_unlock(). Could we use the errno variable?

Something like this: https://github.com/zephyrproject-rtos/zephyr/blob/ee79143ec80b1f9e83ab25d74a654517e214f303/lib/libc/minimal/source/stdlib/malloc.c#L51

Also, is there any advantage in using a macro CHECKIF() instead of if() directly?

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Added a (void) cast to sys_mutex_unlock() in free() to make it clear we are not interested in the returned value. Hopefully that will be enough to supress a false alarm Coverity report.

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@nashif friendly reminder :) do you know of anyone else to review this so we can proceed with the fix?