Threads in the new ISO C Standard from 2011

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Syntax for Declaring Atomic Objects A new type qualifier: _Atomic. The other type qualifiers are: const, volatile, restrict. Examples: _Atomic int a; // atomic int int* _Atomic b; // atomic pointer _Atomic int* c; // pointer to atomic int _Atomic int* _Atomic c; // atomic pointer to atomic int _Atomic struct { int d; } e; // atomic struct In C++ it’s written as atomic a. For easier porting C allows: _Atomic (type-name) Jonas Skeppstedt (js@cs.lth.se) Lecture 6 2013 4 / 70
Using Atomic Objects Members of an atomic struct/union may not be accessed individually. The whole struct must first be copied to a non-atomic variable of compatible type. The ++, --, and compound assignment operators (e.g. +=) are atomic read-modify-write operations. The size of atomic and non-atomic compatible types is typically different as well as the alignment requirements. The memory ordering when using these operators is sequential consistency, which means costly memory fences are used. We will see functions for performing the same atomic operations which use relaxed memory ordering and may be preferable. Recall that alignment requirement refers to that for example a four byte int must be given an address that is a multiple of four etc. Jonas Skeppstedt (js@cs.lth.se) Lecture 6 2013 5 / 70
Page 1 and 2: Threads in the new ISO C Standard f
Page 3: Atomic Objects Atomic objects are n
Page 7 and 8: Lock Free Property To know whether
Page 9 and 10: Memory Order for Atomic Operations
Page 11 and 12: Answer: Yes The code may execute in
Page 13 and 14: Atomic Compare and Exchange There a
Page 15 and 16: Weak Compare and Exchange Functions
Page 17 and 18: Atomic Fetch and Modify Functions T
Page 19 and 20: Atomic Flag The atomic flag type an
Page 21 and 22: The C11 Memory Consistency Model fo
Page 23 and 24: Release and Acquire, and Consume Re
Page 25 and 26: Happens Before and Data Races We wi
Page 27 and 28: Sequenced Before The most common se
Page 29 and 30: An Example _Atomic int m; int u; //
Page 31 and 32: Dependences Dependences are intra-t
Page 33 and 34: Definition of Dependency in C11 A c
Page 35 and 36: Inter-Thread Happens Before An eval
Page 37 and 38: Read-Copy Update RCU is an alternat
Page 39 and 40: A Simplified Example Consider the f
Page 41 and 42: More Details About Dependences Reca
Page 43 and 44: Constant Propagation Example 2(3) T
Page 45 and 46: One More Data Dependency a = atomic
Page 47 and 48: Load and Reserve/Store Conditional
Page 49 and 50: Power Memory Barrier Instructions T
Page 51 and 52: lwsync Memory Barrier for Release O
Page 53 and 54: c;isync Memory Barrier for Load Acq
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Cumulative Ordering If the memory a
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Implementation on Power The followi
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Store Store relaxed st Store releas
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Acquire and Release a Spin Lock # a
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Initialization Function void call_o
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Mutex Options for mtx_init Function
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Condition Variable Functions int cn
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Thread Functions int thrd_create(th
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Threads in the new ISO C Standard from 2011

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