Home
Developer Resources

atomic_add

PDF Documents
Caution: This version of this document is no longer maintained. For the latest documentation, see http://www.qnx.com/developers/docs.

atomic_add()

Safely add to a variable

Synopsis:

#include <atomic.h>

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

Arguments:

loc
A pointer to the value that you want to add 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, even in a symmetric-multiprocessing system.

The atomic_*() functions are guaranteed to complete without being preempted by another thread.

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 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 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

See also:

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