atomic_toggle_value()

Safely toggle a variable, returning the previous value

Synopsis:

#include <atomic.h>

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

Arguments:

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

Library:

libc

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

Description:

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

The atomic_toggle_value() function may be slower than atomic_toggle() .

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.

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.

Returns:

The previous value of loc's contents.

Examples:

To safely toggle the 0xdeadbeef bits in a flag: 

#include <atomic.h>
…

volatile unsigned flags;
unsigned previous;
…

previous = atomic_toggle_value( &flags, 0xdeadbeef );

Classification:

QNX Neutrino

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

Related links