atomic_exchange, atomic_exchange_explicit

Defined in header <stdatomic.h>
C atomic_exchange( volatile A* obj, C desired );
(1) (since C11)
C atomic_exchange_explicit( volatile A* obj, C desired, memory_order order );
(2) (since C11)

Atomically replaces the value pointed by obj with desired 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. C is the non-atomic type corresponding to A.

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_exchange)(...)), 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
desired - the value to replace the atomic object with
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_exchange generic functions (p: 207)
  • C11 standard (ISO/IEC 9899:2011):
    • The atomic_exchange generic functions (p: 283)

See also

swaps a value with an atomic object if the old value is what is expected, otherwise reads the old value
C++ documentation for atomic_exchange, atomic_exchange_explicit

© cppreference.com
Licensed under the Creative Commons Attribution-ShareAlike Unported License v3.0.