Are memmove and malloc thread safe?

Question

I have some code that moves bytes in a buffer using memmove(). The buffer is accessed by multiple threads. I get a very weird behavior; sometimes the buffer it's not what it should be and I was thinking if memmove() or/and malloc() are thread safe. I'm working on iOS (in case this is platform dependent).

Answer 1

In an implementation that provides threads, malloc will normally be thread safe (i.e., it will take steps to assure the heap doesn't get corrupted, even if malloc gets called from multiple threads). The exact way it will do that varies: some use a single heap, and internal synchronization to ensure against corruption. Others will use multiple heaps, so different threads can allocate memory simultaneously without collisions.

memmove will normally be just like if you did assignments in your own code -- if you're sharing a buffer across threads, it's your responsibility to synchronize access to that data.

Answer 2

You should be using a mutex (NSLock) as a protective barrier around accessing your buffer. Take a look at Synchronization in Apple's Threading Programming Guide.

Answer 3

Malloc may be thread-safe. The standard doesn't require it, but many C compilers are used in systems whose applications requires thread safety, and your particular compiler's library may be thread safe, or offer a thread safe option. I don't know about iOS.

Memmove (or any other kind of block move) is not thread safe, any more than an assignment statement is thread safe.