atomic_add()

Updated: May 06, 2022

Safely add to a variable (QNX Neutrino)

Synopsis:

#include <atomic.h>

void atomic_add( volatile unsigned * loc,
                 unsigned incr );

Arguments:

loc
A pointer to the location that you want to add a value to.
incr
The number that you want to add.

Library:

libc

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

Description:

The atomic_add() function is a thread-safe way of doing an (*loc) += incr operation.

When modifying a variable shared between a thread and an interrupt handler, you must either disable interrupts or use atomic operations.

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, atomic operations will cause a fault if the object is allocated in uncached memory.

The implementation of atomic functions may depend on the architecture. For more information, see LL/SC vs LSE atomic operations in the description in Building Embedded Systems of the cpuinfo area of the system page.

Examples:

To safely increment a counter shared between multiple threads:

#include <atomic.h>
…

volatile unsigned count;
…

atomic_add( &count, 1 );

Classification:

QNX Neutrino

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