atomic_fetch_or, atomic_fetch_or_explicit

Defined in header <stdatomic.h>
C atomic_fetch_or( volatile A* obj, M arg );
(1) (since C11)
C atomic_fetch_or_explicit( volatile A* obj, M arg, memory_order order );
(2) (since C11)

Atomically replaces the value pointed by obj with the result of bitwise OR between the old value of obj and arg, and returns the value obj held previously. The operation is read-modify-write operation. The first version orders memory accesses according to memory_order_seq_cst, the second version orders memory accesses according to order.

This is a generic function defined for all atomic object types A. The argument is pointer to a volatile atomic type to accept addresses of both non-volatile and volatile (e.g. memory-mapped I/O) atomic objects, and volatile semantic is preserved when applying this operation to volatile atomic objects. M is either the non-atomic type corresponding to A if A is atomic integer type, or ptrdiff_t if A is atomic pointer type.

It is unspecified whether the name of a generic function is a macro or an identifier declared with external linkage. If a macro definition is suppressed in order to access an actual function (e.g. parenthesized like (atomic_fetch_or)(...)), or a program defines an external identifier with the name of a generic function, the behavior is undefined.


obj - pointer to the atomic object to modify
arg - the value to bitwise OR to the value stored in the atomic object
order - the memory synchronization ordering for this operation: all values are permitted

Return value

The value held previously be the atomic object pointed to by obj.


  • C17 standard (ISO/IEC 9899:2018):
    • The atomic_fetch and modify generic functions (p: 208)
  • C11 standard (ISO/IEC 9899:2011):
    • The atomic_fetch and modify generic functions (p: 284-285)

See also

atomic bitwise AND
atomic bitwise exclusive OR
C++ documentation for atomic_fetch_or, atomic_fetch_or_explicit

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