multithreading - pthreads: If I increment a global from two different threads, can there be sync issues?

Question

Suppose I have two threads A and B that are both incrementing a ~global~ variable "count". Each thread runs a for loop like this one:

for(int i=0; i<1000; i++)
    count++; //alternatively, count = count + 1;

i.e. each thread increments count 1000 times, and let's say count starts at 0. Can there be sync issues in this case? Or will count correctly equal 2000 when the execution is finished? I guess since the statement "count = count + 1" may break down into TWO assembly instructions, there is potential for the other thread to be swapped in between these two instructions? Not sure. What do you think?

Answer 1

Yes there can be sync issues in this case. You need to either protect the count variable with a mutex, or use a (usually platform specific) atomic operation.

Example using pthread mutexes

static pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;

for(int i=0; i<1000; i++) {
    pthread_mutex_lock(&mutex);
    count++;
    pthread_mutex_unlock(&mutex);
}

Using atomic ops

There is a prior discussion of platform specific atomic ops here: UNIX Portable Atomic Operations

If you only need to support GCC, this approach is straightforward. If you're supporting other compilers, you'll probably have to make some per-platform decisions.