To examine the fork() system call with code and through a process graph. An Overview of Processes in Linux:
Linux is a "multitasking" operating system. This allows large, complex programs to be split into a number of smaller, simpler programs, called processes, which run concurrently or "in parallel".
However, the processes don’t actually run concurrently because there’s a single, or at most, a small number of processors. It is the role of the operating system to switch these processes in and out of the processor in such a way that they appear to be running concurrently.
Processes are created by the fork() system call. The process calling fork() is called the "parent" and the newly created process is called the "child". The child is an almost identical copy in terms of the parent: code, data, stack, open file descriptors, etc.
A fork() call returns the PID of the child to the parent and returns 0 to the child. Note that although fork() is called once, it returns twice, once to the parent and once to the child.
int x = 2; int main() if (fork() == 0) printf ("Child process: x = d\n", x++): else printf ("Parent process: x = 3d\n", X--); \n", ++ x) if (fork() == 0) /* Second fork () */ printf ("Secondary Child process: x d else printf ("Secondary Parent process: x = d\n", --x); return 0;