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ATOMIC_DEC_INT(9) Kernel Developer's Manual ATOMIC_DEC_INT(9)

atomic_dec_int, atomic_dec_int_nv, atomic_dec_long, atomic_dec_long_nv
atomic decrement operations

#include <sys/atomic.h>
atomic_dec_int(volatile unsigned int *p);
unsigned int
atomic_dec_int_nv(volatile unsigned int *p);
atomic_dec_long(volatile unsigned long *p );
unsigned long
atomic_dec_long_nv(volatile unsigned long *p);

The atomic_dec set of functions provide an interface for atomically performing decrement and decrement-and-fetch operations with respect to interrupts and multiple processors in the system.
The value referenced by the pointer p is decremented by 1.

atomic_dec_int(), atomic_dec_int_nv(), atomic_dec_long(), and atomic_dec_long_nv() can all be called during autoconf, from process context, or from interrupt context.

atomic_dec_int and atomic_dec_long perform the decrement without returning any knowledge of the value at p.
atomic_dec_int_nv and atomic_dec_long_nv return the value at p after the decrement was performed.

atomic_add_int(9), atomic_inc_int(9)

The atomic_add functions first appeared in NetBSD 5.0 and OpenBSD 5.5.
February 13, 2014 OpenBSD-current