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The C Standard, subclause 3.14 .paragraph 3 [ISO/IEC 9899:2011], states:
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Thread 1: register 0 = flags Thread 1: register 0 &= ~mask(flag1) Thread 2: register 0 = flags Thread 2: register 0 &= ~mask(flag2) Thread 1: register 0 |= 1 << shift(flag1) Thread 1: flags = register 0 Thread 2: register 0 |= 2 << shift(flag2) Thread 2: flags = register 0 |
Compliant Solution (Bit-field,
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C11, Mutex)
This compliant solution protects all accesses of the flags with a mutex, thereby preventing any data races. Finally, the flags are embedded in a union alongside a long, and a static assertion guarantees that the flags do not occupy more space than the long. This technique prevents any data not checked by the mutex from being accessed or modified with the bit-fields on platforms that do not comply with C11 memory semantics.
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#include <threads.h>
#include <assert.h>
struct multi_threaded_flags {
unsigned int flag1 : 2;
unsigned int flag2 : 2;
};
static_assert(
sizeof(long) >= sizeof(struct multi_threaded_flags),
"A long type will not hold the flags on this architecture."
);
union mtf_protect {
struct multi_threaded_flags s;
long padding;
};
struct mtf_mutex {
union mtf_protect u;
mtx_t mutex;
};
struct mtf_mutex flags;
int thread1(void *arg) {
if (thrd_success != mtx_lock(&flags.mutex)) {
/* Handle error */
}
flags.u.s.flag1 = 1;
if (thrd_success != mtx_unlock(&flags.mutex)) {
/* Handle error */
}
return 0;
}
int thread2(void *arg) {
if (thrd_success != mtx_lock(&flags.mutex)) {
/* Handle error */
}
flags.u.s.flag2 = 2;
if (thrd_success != mtx_unlock(&flags.mutex)) {
/* Handle error */
}
return 0;
}
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