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In this noncompliant code example, two threads simultaneously modify two distinct members of a struct.:
| Code Block | ||||
|---|---|---|---|---|
| ||||
struct multi_threaded_flags {
unsigned char flag1;
unsigned char flag2;
};
struct multi_threaded_flags flags;
void thread1(void) {
flags.flag1 = 1;
}
void thread2(void) {
flags.flag2 = 2;
}
|
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Adjacent bit-fields may be stored in a single memory location. Consequently, modifying adjacent bit-fields in different threads is undefined behavior, even in C11.:
| Code Block | ||||
|---|---|---|---|---|
| ||||
struct multi_threaded_flags {
unsigned int flag1 : 2;
unsigned int flag2 : 2;
};
struct multi_threaded_flags flags;
void thread1(void) {
flags.flag1 = 1;
}
void thread2(void) {
flags.flag2 = 2;
}
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The C Standard, section Section 3.14.3 [ISO/IEC 9899:2011], states:
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| [ISO/IEC 9899:2011] | Section 3.14, "Memory Location" |
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