Safely subtract from a variable, returning the previous value
#include <atomic.h> unsigned atomic_sub_value( volatile unsigned * loc, unsigned decr );
- A pointer to the value that you want to subtract from.
- The number that you want to subtract.
Use the -l c option to qcc to link against this library. This library is usually included automatically.
The atomic_sub_value() 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.
The previous value of loc's contents.
Safely subtract 1 from a counter:
#include <atomic.h> … volatile unsigned count; unsigned previous; … previous = atomic_sub_value( &count, 1 );