# atomic_fetch_or, atomic_fetch_or_explicit

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< c‎ | atomic

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.

### Parameters

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`.

### References

- C17 standard (ISO/IEC 9899:2018):

  - 7.17.7.5 The atomic_fetch and modify generic functions (p: 208)

- C11 standard (ISO/IEC 9899:2011):

  - 7.17.7.5 The atomic_fetch and modify generic functions (p: 284-285)

### See also

atomic_fetch_and atomic_fetch_and_explicit (C11) | atomic bitwise AND (function)
atomic_fetch_xor atomic_fetch_xor_explicit (C11) | atomic bitwise exclusive OR (function)

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