erlang message passing architecture

Question

In erlang if two processes A and B are sending message to a process C simultaneously. Will there be a race condition?

• C ! {very large message} sent by A
• C ! {very large message} sent by B

Will C receive the complete message from A and then proceed for the message from B? or is it that C is likely be going to receive chunks of A's message along with chunks of B's message?

Answer 1

Message receiving is an atomic operation.

If you are interested how it is done, read the source code of VM. If I simplify it, the sending process is doing those steps:

1. Allocate a target memory space in the sending process (it's called environment).
2. Copy the message to that memory space
3. Take the external lock on the target process
4. Link message into the mailbox linked list
5. Release the external lock on the target process

As you can see, copying is done outside (before) critical section and the critical section is pretty fast. It is just juggling with few pointers.