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atomic_clr_value()

Safely clear a variable, returning the previous value

Synopsis:

#include <atomic.h>

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

Arguments:

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

Library:

libc

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

Description:

The atomic_clr_value() 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.

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

Returns:

The previous value of loc's contents.

Examples:

To safely clear the 0x10101010 bits in a flag:

#include <atomic.h>
...

volatile unsigned flags;
unsigned previous;
...

previous = atomic_clr_value( &flags, 0x10101010 );

Classification:

QNX Neutrino

Safety:
Cancellation point	No
Interrupt handler	Yes
Signal handler	Yes
Thread	Yes