Reading Critical Section Data using pthreads

Question

I have a multi-threaded application, I'm using pthreads with the pthread_mutex_lock function. The only data I need to protect is in one data structure. Is it safe if I apply the lock only when I write to the data structure? Or should I apply the lock whenever I read or write?

I found a question similar to this, but it was for Windows, from that question it would that the answer to mine would be that it is ok. Just want to make sure though.

EDIT

follow up: So I want to pass in a command line argument and on read from it (from different threads). Do I still have to use pthread_mutex_lock?

Answer 1

It is necessary to apply the lock when you read as well unless you can guarantee atomic writes (at which point you don't even need to lock on write). The problem arises from writes that take more than 1 cycle.

Imagine you write 8 bytes as two 4 byte writes. If the other thread kicks off after it has half been written then the read will read invalid data. Its veyr ucommon that this happens but when it does its a hell of a bug to track down.

Answer 2

Yes, you need to be locked for reads as well as writes.

Compilers and CPUs do not necessarily write to a field in a structure atomically. In addition your code may not write atomically, and the structure may at certain points be out of sync with regards to itself.

If all you need to share is a single integer value, you might choose to use atomic integers. GCC has atomic attributes you can use. This is not as portable as using pthreads locks.

Answer 3

You could use a pthreads_rwlock_t to allow "one-writer OR N-readers" concurrency. But if you stick with the general pthread_mutex_lock, it needs to be acquired for ANY access to the shared data structure it's protecting, so you're cutting things down to "one reader-or-writer" concurrency.