2a4a1db342
current license information and adapted to the FreeBSD build environment before they will build. Approved by: David Taylor <davidt@caldera.com>
191 lines
4.6 KiB
Plaintext
191 lines
4.6 KiB
Plaintext
.\" This module is believed to contain source code proprietary to AT&T.
|
|
.\" Use and redistribution is subject to the Berkeley Software License
|
|
.\" Agreement and your Software Agreement with AT&T (Western Electric).
|
|
.\"
|
|
.\" @(#)p3 8.1 (Berkeley) 6/8/93
|
|
.\"
|
|
.\" $FreeBSD$
|
|
.SH
|
|
V. PROCESSES AND IMAGES
|
|
.PP
|
|
An
|
|
.IT image
|
|
is a computer execution environment.
|
|
It includes a memory image,
|
|
general register values,
|
|
status of open files,
|
|
current directory and the like.
|
|
An image is the current state of a pseudo-computer.
|
|
.PP
|
|
A
|
|
.IT process
|
|
is the execution of an image.
|
|
While the processor is executing on behalf of a process,
|
|
the image must reside in main memory;
|
|
during the execution of other processes it remains in main memory
|
|
unless the appearance of an active, higher-priority
|
|
process
|
|
forces it to be swapped out to the disk.
|
|
.PP
|
|
The user-memory part of an image is divided into three logical segments.
|
|
The program text segment begins at location 0 in the virtual address space.
|
|
During execution, this segment is write-protected
|
|
and a single copy of it is shared among
|
|
all processes executing the same program.
|
|
At the first hardware protection byte boundary above the program text segment in the
|
|
virtual address space begins a non-shared, writable data segment,
|
|
the size of which may be extended by a system call.
|
|
Starting at the highest
|
|
address in the virtual address space is a stack segment,
|
|
which automatically grows downward
|
|
as the stack pointer fluctuates.
|
|
.SH
|
|
5.1 Processes
|
|
.PP
|
|
Except while
|
|
the system
|
|
is bootstrapping itself into operation, a new
|
|
process can come into existence only
|
|
by use of the
|
|
.UL fork
|
|
system call:
|
|
.P1
|
|
processid = fork\|(\|\|)\|
|
|
.P2
|
|
When
|
|
.UL fork
|
|
is executed, the process
|
|
splits into two independently executing processes.
|
|
The two processes have independent
|
|
copies of the original memory image,
|
|
and share all open files.
|
|
The new processes differ only in that one is considered
|
|
the parent process:
|
|
in the parent,
|
|
the returned
|
|
.UL processid
|
|
actually identifies the child process
|
|
and is never 0,
|
|
while in the child,
|
|
the returned value is always 0.
|
|
.PP
|
|
Because the values returned by
|
|
.UL fork
|
|
in the parent and child process are distinguishable,
|
|
each process may determine whether
|
|
it is the parent or child.
|
|
.SH
|
|
5.2 Pipes
|
|
.PP
|
|
Processes may communicate
|
|
with related processes using the same system
|
|
.UL read
|
|
and
|
|
.UL write
|
|
calls that are used for file-system I/O.
|
|
The call:
|
|
.P1
|
|
filep = pipe\|(\|\|)\|
|
|
.P2
|
|
returns a file descriptor
|
|
.UL filep
|
|
and
|
|
creates an inter-process channel called a
|
|
.IT pipe .
|
|
This channel, like other open files, is passed from parent to child process in
|
|
the image by the
|
|
.UL fork
|
|
call.
|
|
A
|
|
.UL read
|
|
using a pipe file descriptor
|
|
waits until another process writes using the
|
|
file descriptor for the same pipe.
|
|
At this point, data are passed between the images of the
|
|
two processes.
|
|
Neither process need know that a pipe,
|
|
rather than an ordinary file,
|
|
is involved.
|
|
.PP
|
|
Although
|
|
inter-process communication
|
|
via pipes is a quite valuable tool
|
|
(see Section 6.2),
|
|
it is not a completely general
|
|
mechanism,
|
|
because the pipe must be set up by a common ancestor
|
|
of the processes involved.
|
|
.SH
|
|
5.3 Execution of programs
|
|
.PP
|
|
Another major system primitive
|
|
is invoked by
|
|
.P1
|
|
execute\|(\|file, arg\*s\d1\u\*n, arg\*s\d2\u\*n, .\|.\|. , arg\*s\dn\u\*n\|)\|
|
|
.P2
|
|
which requests the system to read in and execute the program
|
|
named by
|
|
.UL file ,
|
|
passing it string arguments
|
|
.UL arg\v'.3'\*s1\*n\v'-.3'\| ,
|
|
.UL arg\v'.3'\*s2\*n\v'-.3'\| ,
|
|
.UL .\|.\|.\|\| ,
|
|
.UL arg\v'.3'\*sn\*n\v'-.3' .
|
|
All the code and data in the process invoking
|
|
.UL execute
|
|
is replaced from the
|
|
.UL file ,
|
|
but
|
|
open files, current directory, and
|
|
inter-process relationships are unaltered.
|
|
Only if the call fails, for example
|
|
because
|
|
.UL file
|
|
could not be found or because
|
|
its execute-permission bit was not set, does a return
|
|
take place from the
|
|
.UL execute
|
|
primitive;
|
|
it resembles a ``jump'' machine instruction
|
|
rather than a subroutine call.
|
|
.SH
|
|
5.4 Process synchronization
|
|
.PP
|
|
Another process control system call:
|
|
.P1
|
|
processid = wait\|(\|status\|)\|
|
|
.P2
|
|
causes its caller to suspend
|
|
execution until one of its children has completed execution.
|
|
Then
|
|
.UL wait
|
|
returns the
|
|
.UL processid
|
|
of the terminated process.
|
|
An error return is taken if the calling process has no
|
|
descendants.
|
|
Certain status from the child process
|
|
is also available.
|
|
.SH
|
|
5.5 Termination
|
|
.PP
|
|
Lastly:
|
|
.P1
|
|
exit\|(\|status\|)\|
|
|
.P2
|
|
terminates a process,
|
|
destroys its image,
|
|
closes its open files,
|
|
and generally obliterates it.
|
|
The parent is notified through
|
|
the
|
|
.UL wait
|
|
primitive,
|
|
and
|
|
.UL status
|
|
is made available
|
|
to it.
|
|
Processes may also terminate as a result of
|
|
various illegal actions or user-generated signals
|
|
(Section VII below).
|