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(libc.info)Process Creation Concepts

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Process Creation Concepts
=========================

   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'
(Note: 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 Note: 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.