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Safely set bits in a variable


#include <atomic.h>

void atomic_set( volatile unsigned * loc,
                 unsigned bits );


A pointer to the location whose bits you want to toggle.
The bits that you want to set.



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


The atomic_set() function is a thread-safe way of doing an (*loc) |= bits operation.

The atomic_*() functions are guaranteed to complete without being preempted by another thread, even in a symmetric-multiprocessing system. 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.


To safely set the 1 bit in a flag:

#include <atomic.h>

volatile unsigned flags;

atomic_set( &flags, 0x01 );


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_sub(), atomic_sub_value(), atomic_toggle(), atomic_toggle_value()