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5. Processes and images
.es
An
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image
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is a computer execution environment.
It includes a core image,
general register values,
status of open files,
current directory and the like.
An image is the current state of a pseudo-computer.
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A
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process
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is the execution of an image.
While the processor is executing on behalf of a process,
the image must reside in core;
during the execution of other processes it remains in core
unless the appearance of an active, higher-priority
process
forces it to be swapped out to the fixed-head disk.
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The user-core 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 8K 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 hardware's stack pointer fluctuates.
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5.1 Processes
.es
Except while \*sUNIX\*n is bootstrapping itself into operation, a new
process can come into existence only
by use of the \fIfork\fR system call:
.dc
processid = fork\|(\|label\|)\|
.ec
When \fIfork\fR is executed by a process, it
splits into two independently executing processes.
The two processes have independent
copies of the original core image,
and share any open files.
The new processes differ only in that one is considered
the parent process:
in the parent,
control returns directly from the \fIfork\fR,
while in the child, control is passed
to location
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label.
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The \fIprocessid\fR returned by the \fIfork\fR call is the identification
of the other process.
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Because the return points in the parent and child process
are not the same, each image existing after a
\fIfork\fR may determine whether it is the parent or child process.
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5.2 Pipes
.es
Processes may communicate
with related processes using the same system
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read
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and
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write
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calls that are used for file system I/O.
The call
.dc
filep = pipe\|(\|\|)\|
.ec
returns a file descriptor \fIfilep\fR and
creates an inter-process channel called a \fIpipe\fR.
This channel, like other open files, is passed from parent to child process in
the image by the \fIfork\fR call.
A \fIread\fR 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.
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Although
inter-process communication
via pipes is a quite valuable tool
(see \(sc6.2),
it is not a completely general
mechanism,
since the pipe must be set up by a common ancestor
of the processes involved.
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5.3 Execution of programs
.es
Another major system primitive
is invoked by
.dc
execute\|(\|file, arg\*t\d1\u\*n, arg\*t\d2\u\*n, .\|.\|. , arg\*t\dn\u\*n\|)\|
.ec
which requests the system to read in and execute the program
named by \fIfile\fR, passing it string arguments
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arg\v'.3'\*t1\*n\v'-.3'\|, arg\v'.3'\*t2\*n\v'-.3'\|, .\|.\|.\|\|, arg\v'.3'\*tn\*n\v'-.3'.
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All the code and data in the process using \fIexecute\fR
is replaced from the \fIfile\fR,
but
open files, current directory, and
inter-process relationships are unaltered.
Only if the call fails, for example
because \fIfile\fR could not be found or because
its execute-permission bit was not set, does a return
take place from the \fIexecute\fR
primitive;
it resembles a ``jump'' machine instruction
rather than a subroutine call.
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5.4 Process synchronization
.es
Another process control system call
.dc
processid = wait\|(\|\|)\|
.ec
causes its caller to suspend
execution until one of its children has completed execution.
Then \fIwait\fR returns the \fIprocessid\fR 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.
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5.5 Termination
.es
Lastly,
.dc
exit\|(\|status\|)\|
.ec
terminates a process,
destroys its image,
closes its open files,
and generally obliterates it.
When the parent is notified through
the \fIwait\fR primitive,
the indicated \fIstatus\fR is available
to the parent.
Processes may also terminate as a result of
various illegal actions or user-generated signals
(\(sc7 below).