To avoid uncertainty about interrupting access to a variable, you can
use a particular data type for which access is always atomic:
sig_atomic_t. Reading and writing this data type is guaranteed
to happen in a single instruction, so there's no way for a handler to
run “in the middle” of an access.
The type sig_atomic_t is always an integer data type, but which
one it is, and how many bits it contains, may vary from machine to
machine.
This is an integer data type. Objects of this type are always accessed atomically.
In practice, you can assume that int is atomic.
You can also assume that pointer
types are atomic; that is very convenient. Both of these assumptions
are true on all of the machines that the GNU C library supports and on
all POSIX systems we know of.