atomic operations in multithreaded c code?
Post: #1
Are there any operations in C that are sure to be atomic? I used to think the addition/assignment operations were atomic, but I've been lead to believe this isn't the case.
Post: #2
There are no inherently atomic operations in the C language, though on some implementations intrinsic atomic ops are provided by the compiler, which are used as functions.
Post: #3JeroMiya Wrote:Are there any operations in C that are sure to be atomic? I used to think the addition/assignment operations were atomic, but I've been lead to believe this isn't the case.Yep, they definitely aren't atomic.
I remember once having to do a project in C where we had two threads running at the same time on the same variable - one thread would increment a global int, and one thread would decrement it. It also kept track of how many times each one incremented and decremented the variable. Every once in a while, the value of the global int wouldn't match the number of increments and decrements we did on the value, which meant that it wasn't atomic. Something like that.
Post: #4
You can assume that word-sized loads and stores are atomic; just be certain that your compiler is doing what you think it is -- volatile may be necessary.
More complex operations are provided by the OS in C. For Mac OS X, look at <libkern/OSAtomic.h>; for Windows look at InterlockedCompareExchangePointer and friends; for Linux there is an equivalent compare_and_swap or something in glibc.
More complex operations are provided by the OS in C. For Mac OS X, look at <libkern/OSAtomic.h>; for Windows look at InterlockedCompareExchangePointer and friends; for Linux there is an equivalent compare_and_swap or something in glibc.
Post: #5
If you happen to be using pthreads and don't need specifically atomic operations, look into
Code:
int pthread_mutex_destroy(pthread_mutex_t *mutex)
Destroy a mutex.
int pthread_mutex_init(pthread_mutex_t *mutex, const pthread_mutexattr_t
*attr)
Initialize a mutex with specified attributes.
int pthread_mutex_lock(pthread_mutex_t *mutex)
Lock a mutex and block until it becomes available.
int pthread_mutex_trylock(pthread_mutex_t *mutex)
Try to lock a mutex, but don't block if the mutex is
locked by another thread, including the current
thread.
int pthread_mutex_unlock(pthread_mutex_t *mutex)
Unlock a mutex.Did you ever wonder why we had to run for shelter when the promise of a brave new world unfurled beneath the clear blue sky?
