Submitter: Jens Gustedt
Submission Date:
2012-10-08
Source:
Reference Document: N/A
Version: 1.0
Date: 2012-10-08
Subject: initialization of atomic_flag
Summary
C11 expresses the intention to have atomic_flag as a
primitive that should allow to emulate all other atomic types and
operations, 7.17.8 p3 in a note says:
The remaining types can be emulated with atomic_flag,
though with less than ideal properties.
With the current semantic for the initialization
of atomic_flag this goal cannot be achieved.
Details
This is a very good concept as far as I can see, but I have one
problem to achieve this, initialization. The phrasing for atomic
types in general and for atomic_flag are different.
For atomic_flag we have:
An atomic_flag that is not explicitly initialized with
ATOMIC_FLAG_INIT is initially in an indeterminate
state.
The problem is how to emulate an atomic type
with atomic_flag during initialization. Say we emulate
with something like
struct atomic_int_struct {
atomic_flag cat;
int val;
};
Then the ATOMIC_VAR_INIT macro could simply look
like:
#define ATOMIC_VAR_INIT(V) { .cat = ATOMIC_FLAG_INIT, .val = (V), }
But if I’d have a variable of atomic_int_struct
with static storage duration
struct atomic_int_struct v;
is supposed to do the right thing, namely to guarantee
that v has a valid state at startup, so it should
contain a 0 for .val,
and .cat must be in a determinate state. Since all
atomic operations should work without problems on v,
the state of .cat can’t be anything else than
“clear”.
Now looking into the possible implementations
of atomic_flag in assembler I didn’t yet meet a
processor where the “clear” state would not be naturally
represented by an all 0 value. So I guess in any
reasonable implementation we would just have
#define ATOMIC_FLAG_INIT { 0 }
(or some equivalent formulation.)
If this is so, why ATOMIC_FLAG_INIT at all? Why not
phrasing the same as for the other atomic types
Suggested Technical Corrigendum
Eliminate the mention of ATOMIC_FLAG_INIT in 7.17.1p3,
B.16 and the index.
Proposed change for the initialization of atomic_flag,
7.17.8p4:
The default initializer { 0 } may be used to initialize
an atomic_flag to the clear
state. An atomic_flag object with automatic storage
duration that is not explicitly initialized using { 0 }
is initially in an indeterminate state; however, the default
(zero) initialization for objects with static or thread-local
storage duration initializes an atomic_flag to the
clear state.
EXAMPLE
atomic_flag guard = { 0 };
If the committee would want to keep the
macro ATOMIC_FLAG_INIT arround, a partial alternative
to the above text would be to modify the text in 7.17.1
ATOMIC_FLAG_INIT
which expands to a default initializer ({ 0 } or
equivalent) for an object of type atomic_flag.