atomic_sub_value()
QNX SDP8.0C Library ReferenceAPIDeveloper
Safely subtract from a variable, returning the previous value (QNX OS)
Synopsis:
#include <atomic.h>
unsigned atomic_sub_value( volatile unsigned * loc,
unsigned decr );
Arguments:
- loc
- A pointer to the location that you want to subtract a value 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_value() function is a thread-safe way of doing a
(*loc) -= decr operation.
Note:
The atomic_sub_value() function may be slower than
atomic_sub().
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.
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.
Returns:
The previous value at loc.
Examples:
Safely subtract 1 from a counter:
#include <atomic.h>
…
volatile unsigned count;
unsigned previous;
…
previous = atomic_sub_value( &count, 1 );
Classification:
| Safety: | |
|---|---|
| Cancellation point | No |
| Signal handler | Yes |
| Thread | Yes |
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