atomic_sub()

Safely subtract from a variable

Synopsis:

#include <atomic.h>

void atomic_sub( volatile unsigned * loc,
                 unsigned decr );

Library:

libc

Description:

The atomic_sub() function is a thread-safe way of doing a (*loc) -= decr 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 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 subtract 1 from a counter:

#include <atomic.h>
...

volatile unsigned count;
...

atomic_sub( &count, 1 );

Classification:

QNX 6

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