Caution: This version of this document is no longer maintained. For the latest documentation, see


Safely subtract from a variable


#include <atomic.h>

void atomic_sub( volatile unsigned * loc,
                 unsigned decr );


A pointer to the value that you want to subtract from.
The number that you want to subtract.



Use the -l c option to qcc to link against this library. This library is usually included automatically.


The atomic_sub() function is a thread-safe way of doing a (*loc) -= decr operation, even in a symmetric-multiprocessing system.

The atomic_*() functions are guaranteed to complete without being preempted by another thread. When modifying a variable shared between a thread and an interrupt, you must either disable interrupts or use the atomic_*() functions.

The atomic_*() functions are also useful for modifying variables that are referenced by more than one thread (that aren't necessarily in the same process) without having to use a mutex.

Caution: Perform atomic operations only on objects that were allocated in normal memory mappings. On certain processors (e.g. some PPC ones), atomic operations will cause a fault if the object is allocated in uncached memory.


Safely subtract 1 from a counter:

#include <atomic.h>

volatile unsigned count;

atomic_sub( &count, 1 );


QNX Neutrino

Cancellation point No
Interrupt handler Yes
Signal handler Yes
Thread Yes

See also:

atomic_add(), atomic_add_value(), atomic_clr(), atomic_clr_value(), atomic_set(), atomic_set_value(), atomic_sub_value(), atomic_toggle(), atomic_toggle_value()