What is a fork process?

The fork() system call will spawn a new child process which is an identical process to the parent except that has a new system process ID.

The process is copied in memory from the parent and a new process structure is assigned by the kernel.

The return value of the function is which discriminates the two threads of execution. A zero is returned by the fork function in the child's process.

The environment, resource limits, umask, controlling terminal, current working directory, root directory, signal masks and other process resources are also duplicated from the parent in the forked child process.

The two processes obviously have two different process id.s. (pid). In a C program process id.s are conveniently represented by variables of pid_t type, the type being defined in the sys/types.h header.

In UNIX the PCB of a process contains the id of the process's parent, hence the child's PCB will contain as parent id (ppid) the pid of the process that called fork(), while the caller will have as ppid the pid of the process that spawned it.

The child process has its own copy of the parent's file descriptors. These descriptors reference the same under-lying objects, so that files are shared between the child and the parent. This makes sense, since other processes might access those files as well, and having them already open in the child is a time-saver.