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(libc.info)Atomic Types

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Atomic Types
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   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.

 - Data Type: sig_atomic_t
     This is an integer data type.  Objects of this type are always
     accessed atomically.

   In practice, you can assume that `int' and other integer types no
longer than `int' are 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.