errno
The C library errno variable is defined in the implicit static data area of the library.
This area is identified
by __user_libspace(). The function that returns
the address of errno is:
(*(volatile int *) __aeabi_errno_addr())
You can define __aeabi_errno_addr() if
you want to place errno at a user-defined location instead
of the default location identified by __user_libspace().
Note
Legacy versions of errno.h might define errno in
terms of __rt_errno_addr() rather than __aeabi_errno_addr().
The function name __rt_errno_addr() is a legacy
from pre-ABI versions of the tools, and is still supported to ensure
that object files generated with those tools link successfully.
Returns
The return value is a pointer to a variable of type int,
containing the currently applicable instance of errno.