Overlay (operating system)
Encyclopedia
In operating system
s, an overlay is when a process
replaces itself with the code of another program. On Unix-like
systems, this is accomplished with the exec
system call.
In Unix
, the only way to run new programs is to fork
the running process, and then overlay the new program on top of the child
. This is known as the fork-exec
technique.
Operating system
An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...
s, an overlay is when a process
Process (computing)
In computing, a process is an instance of a computer program that is being executed. It contains the program code and its current activity. Depending on the operating system , a process may be made up of multiple threads of execution that execute instructions concurrently.A computer program is a...
replaces itself with the code of another program. On Unix-like
Unix-like
A Unix-like operating system is one that behaves in a manner similar to a Unix system, while not necessarily conforming to or being certified to any version of the Single UNIX Specification....
systems, this is accomplished with the exec
Exec (operating system)
The exec collection of functions of Unix-like operating systems cause the running process to be completely replaced by the program passed as an argument to the function...
system call.
In Unix
Unix
Unix is a multitasking, multi-user computer operating system originally developed in 1969 by a group of AT&T employees at Bell Labs, including Ken Thompson, Dennis Ritchie, Brian Kernighan, Douglas McIlroy, and Joe Ossanna...
, the only way to run new programs is to fork
Fork (operating system)
In computing, when a process forks, it creates a copy of itself. More generally, a fork in a multithreading environment means that a thread of execution is duplicated, creating a child thread from the parent thread....
the running process, and then overlay the new program on top of the child
Child process
A child process in computing is a process created by another process .A child process inherits most of its attributes, such as open files, from its parent. In UNIX, a child process is in fact created as a copy of the parent...
. This is known as the fork-exec
Fork-exec
Fork-exec is a commonly used technique in Unix whereby an executing process spawns a new program. fork is the name of the system call that the parent process uses to "divide" itself . After calling fork, the created child process is an exact copy of the parent except for the return value...
technique.