Defined in header <stdatomic.h>
_Bool atomic_is_lock_free( const volatile A* obj );
(since C11)

Determines if the atomic operations on all objects of the type A (the type of the object pointed to by obj) are lock-free. In any given program execution, the result of calling atomic_is_lock_free is the same for all pointers of the same type.

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.

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_is_lock_free)(...)), 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 inspect

Return value

true if the operations on all objects of the type A are lock-free, false otherwise.


#include <stdio.h>
#include <stdatomic.h>
_Atomic struct A { int a[100]; } a;
_Atomic struct B { int x, y; } b;
int main(void)
    printf("_Atomic struct A is lock free? %s\n", 
            atomic_is_lock_free(&a) ? "true" : "false");
    printf("_Atomic struct B is lock free? %s\n", 
            atomic_is_lock_free(&b) ? "true" : "false");

Possible output:

_Atomic struct A is lock free? false
_Atomic struct B is lock free? true

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C standards.

DR Applied to Behavior as published Correct behavior
DR 465 C11 this function was per-object this functions is per-type


  • C17 standard (ISO/IEC 9899:2018):
    • The atomic_is_lock_free generic function (p: 205)
  • C11 standard (ISO/IEC 9899:2011):
    • The atomic_is_lock_free generic function (p: 280)

See also

indicates that the given atomic type is lock-free
(macro constant)
C++ documentation for atomic_is_lock_free

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