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Safely clear a variable, returning the previous value


#include <atomic.h>

unsigned atomic_clr_value( volatile unsigned * loc,
                           unsigned bits );


A pointer to the value that you want to clear bits in.
The bits that you want to clear.



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


The atomic_clr_value() function is a thread-safe way of doing an (*loc) &= ~bits operation.

Note: The atomic_clr_value() function may be slower than atomic_clr().

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.


The previous value of loc's contents.


To safely clear the 0x10101010 bits in a flag:

#include <atomic.h>

volatile unsigned flags;
unsigned previous;

previous = atomic_clr_value( &flags, 0x10101010 );


QNX Neutrino

Cancellation point No
Interrupt handler Yes
Signal handler Yes
Thread Yes

See also:

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