atomic_fetch_add(), atomic_fetch_add_explicit()

Updated: May 06, 2022

Atomically add to the value of an atomic object (C11)

Synopsis:

#include <stdatomic.h>

C atomic_fetch_add( volatile A *obj,
                    M amount );

C atomic_fetch_add_explicit( volatile A *obj,
                             M amount,
                             memory_order order );

Arguments:

obj
A pointer to the atomic object (see the atomic_* types) whose value you want to add to.
amount
The value you want to add to the object. “M” is either the non-atomic type corresponding to A if A is an atomic integer type, or ptrdiff_t if A is an atomic pointer type.
order
(atomic_fetch_add_explicit() only) The memory_order to use for the operation.

Library:

Description:

The atomic_fetch_add() and atomic_fetch_add_explicit() functions are generic functions that atomically add the given value to the given atomic object. The atomic_fetch_add() function orders memory access according to memory_order_seq_cst; atomic_fetch_add_explicit() orders them as specified by order.

The implementation of atomic functions may depend on the architecture. For more information, see LL/SC vs LSE atomic operations in the description in Building Embedded Systems of the cpuinfo area of the system page.

Returns:

The previous value of the atomic object. The “C” represents the non-atomic data type that corresponds to the atomic object.

Classification:

C11

Safety:  
Cancellation point No
Interrupt handler Read the Caveats
Signal handler Read the Caveats
Thread Yes

Caveats:

If this function is lock-free (see atomic_is_lock_free()), it's safe to call it from an ISR or signal handler.