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.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).