Confusion as to how fork() and exec() work

Question

Consider the following:

Where I'm getting confused is in the step "child duplicate of parent". If you're running a process such as say, skype, if it forks, is it copying skype, then overwriting that process copy with some other program? Moreover, what if the child process has memory requirements far different from the parent process? Wouldn't assigning the same address space as the parent be a problem?

I feel like I'm thinking about this all wrong, perhaps because I'm imagining the processes to be entire programs in execution rather than some simple instruction like "copy data from X to Y".

Answer 1

fork(2) is difficult to understand. It is explained a lot, read also fork (system call) wikipage and several chapters of Advanced Linux Programming. Notice that fork does not copy the running program (i.e. the /usr/bin/skype ELF executable file), but it is lazily copying (using copy-on-write techniques - by configuring the MMU) the address space (in virtual memory) of the forking process. Each process has its address space (but might share some segments with some other processes, see mmap(2) and execve(2) ....). Since each process has its own address space, changes in the address space of one process does not (usually) affect the parent process. However, processes may have shared memory but then need to synchronize: see shm_overview(7) & sem_overview(7)...

By definition of fork, just after the fork syscall the parent and child processes have nearly equal state (in particular the address space of the child is a copy of the address space of the parent). The only difference being the return value of fork.

And execve is overwriting the address space and registers of the current process.

Notice that on Linux all processes (with a few exceptions, like kernel started processes such as /sbin/modprobe etc) are obtained by fork-ing -from the initial /sbin/init process of pid 1.

At last, system calls -listed in syscalls(2)- like fork are an elementary operation from the application's point of view, since the real processing is done inside the Linux kernel. Play with strace(1). See also this answer and that one.

A process is often some machine state (registers) + its address space + some kernel state (e.g. file descriptors), etc... (but read about zombie processes).

^{Take time to follow all the links I gave you.}

Answer 2

All modern Unix implementations use virtual memory. That allows them to get away with not actually copying much when forking. Instead, their memory map contains pointers to the parent's memory until they start modifying it.

When a child process exec's a program, that program is copied into memory (if it wasn't already there) and the process's memory map is updated to point to the new program.