Are function callback and interprocess communication are same?

Question

Few days back, in an interview one question was asked to me as,

Are  function callback in C and interprocess communication are same?

I was bit neutral in that question.Because I was in a dilemma. In IPC we communicate between process to process at the end if we shall see process is nothing but a function. Here one function calls other function. In function callback we use function pointer again that is one function calls other function with address. So just wanted to know though they are not completely same, so what the difference?

Answer 1

No, not really.

Function callback is passing the address (or some other identifier) of a function to another function so that it can call back for some reason.

The classic example is the C standard library qsort function. You pass in a pointer to the array (and its sizes), along with a comparison function. The qsort function then uses that comparison function (the callback) to decide which of any two elements in the list is greater so that it can arrange them in order.

IPC, on the other hand, is a means for processes to communicate with each other, such as shared memory, pipes, semaphores and so on.

Some IPC mechanisms may use callbacks but it's by no means necessary. For example, sockets don't use callbacks, they simply allow a user to call read and write.

Similarly with shared memory, you attach to the memory blocks and simply access them much the same as "regular" memory, it's just that the effects are felt across all processes attached to that memory.

One that does sort of use callbacks is ONC RPC (or Sun RPC). It runs a server which awaits client calls over the network and passes them to a pre-configured client function. More detail can be found here.

But, even then, I'd be hesitant to call it a callback since it's really configured at compile-time. "Proper" callbacks tend to be done at run-time.

Answer 2

A function callback means caller and callee are in the same thread. IPC is in different processes. a process is comprised of one or more threads. Ergo, callbacks are not IPC.