atomic_sub()

Safely subtract from a variable

Synopsis:

#include <atomic.h>

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

Arguments:

loc: A pointer to the value that you want to subtract from.
decr: The number that you want to subtract.

Library:

libc

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

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.

CAUTION:

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.

Examples:

Safely subtract 1 from a counter:

#include <atomic.h>
…

volatile unsigned count;
…

atomic_sub( &count, 1 );

Classification:

QNX Neutrino

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