This section gives an overview of processes and of the steps involved in creating a process and making it run another program.
Each process is named by a process ID number. A unique process ID is allocated to each process when it is created. The lifetime of a process ends when its termination is reported to its parent process; at that time, all of the process resources, including its process ID, are freed.
Processes are created with the fork system call (so the operation
of creating a new process is sometimes called forking a process).
The child process created by fork is a copy of the original
parent process, except that it has its own process ID.
After forking a child process, both the parent and child processes
continue to execute normally. If you want your program to wait for a
child process to finish executing before continuing, you must do this
explicitly after the fork operation, by calling wait or
waitpid (see Process Completion). These functions give you
limited information about why the child terminated—for example, its
exit status code.
A newly forked child process continues to execute the same program as
its parent process, at the point where the fork call returns.
You can use the return value from fork to tell whether the program
is running in the parent process or the child.
Having several processes run the same program is only occasionally
useful. But the child can execute another program using one of the
exec functions; see Executing a File. The program that the
process is executing is called its process image. Starting
execution of a new program causes the process to forget all about its
previous process image; when the new program exits, the process exits
too, instead of returning to the previous process image.