atomic_set()

Safely set bits in a variable

Synopsis:

#include <atomic.h>

void atomic_set( 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 set.

Library:

libc

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

Description:

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.

Examples:

To safely set the 1 bit in a flag:

#include <atomic.h>
...

volatile unsigned flags;
...

atomic_set( &flags, 0x01 );

Classification:

QNX Neutrino

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