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HP OpenVMS DCL Dictionary
SPAWN
Creates a subprocess of the current process. Portions of the current
process context are copied to the subprocess.
The RESOURCE_WAIT state is required to spawn a process.
Requires TMPMBX (temporary mailbox) or PRMMBX (permanent mailbox)
privilege. The SPAWN command does not manage terminal characteristics.
The SPAWN and ATTACH commands cannot be used if your terminal has an
associated mailbox.
Format
SPAWN [command-string]
Parameter
command-string
Specifies a command string of less than 132 characters that is to be
executed in the context of the created subprocess. When the command
completes execution, the subprocess terminates and control returns to
the parent process. If both a command string and the /INPUT qualifier
are specified, the specified command string executes before additional
commands are obtained from the /INPUT qualifier.
If the command string contains apostrophe substitution, the apostrophe
substitution is performed in the context of the parent command line
interpreter (CLI); other phases of substitution are performed in the
context of the subprocess CLI.
Description
The SPAWN command creates a subprocess of your current process with the
following attributes copied from the parent process:
- All global symbols except $RESTART, $SEVERITY, $STATUS
- All local symbols defined for the current command procedure level
- Key definitions
- The current keypad state
- The current prompt string
- All process logical names and logical name tables except those
explicitly marked CONFINE or those created in executive or kernel mode
- Default disk and directory
- Current SET MESSAGE settings
- Current process privileges
- Control and verification states
Note that some attributes, such as the process's current command
tables, are not copied.
When the subprocess is created, the process-permanent open files and
any image or procedure context are not copied from the parent
process. The subprocess is set to command level 0 (DCL level with the
current prompt).
If you do not specify the /PROCESS qualifier, the name of this
subprocess is composed of a portion of the parent process name and a
unique number. For example, if the parent process name is SKONETSKI,
the subprocess names can be SKONETSKI_123, SKONETSKI_4732, and so on.
The LOGIN.COM file of the parent process is not executed for the
subprocess because the context is copied separately, allowing quicker
initialization of the subprocess. When the /WAIT qualifier is in
effect, the parent process remains in hibernation until the subprocess
terminates or returns control to the parent by way of the ATTACH
command.
More than one process simultaneously attempts to use the same input or
output stream in the following circumstances:
- When several processes share that stream and you terminate a
subprocess to which you are not currently attached
- When you terminate a process that is not spawned from the process
to which you are currently attached
You should use the LOGOUT command to terminate the subprocess and
return to the parent process. You can also use the ATTACH command to
transfer control of the terminal to another process in the subprocess
tree, including the parent process. (The SHOW PROCESS/SUBPROCESSES
command displays the processes in the subprocess tree and points to the
current process.)
Note
Because a tree of subprocesses can be established using the SPAWN
command, you must be careful when terminating any process in the tree.
When a process is terminated, all subprocesses below that point in the
tree are terminated automatically.
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Qualifiers used with the SPAWN command must directly follow the command
verb. The command string parameter begins after the last qualifier and
continues to the end of the command line.
Qualifiers
/CARRIAGE_CONTROL
/NOCARRIAGE_CONTROL
Determines whether carriage-return and line-feed characters are
prefixed to the subprocess's prompt string. By default, SPAWN copies
the current setting of the parent process.
/CLI=cli-filespec
/NOCLI
Specifies the name of a command language interpreter (CLI) to be used
by the subprocess. The default CLI is the same as the parent process
(defined in SYSUAF). If you specify the /CLI qualifier, the attributes
of the parent process are copied to the subprocess.
The CLI you specify must be located in SYS$SYSTEM and have the file
type .EXE.
/INPUT=filespec
Specifies an input file containing one or more DCL commands to be
executed by the spawned subprocess. The file type defaults to .COM. The
asterisk (*) and the percent sign (%) wildcard characters are not
allowed in the file specification. Once processing of the input file is
complete, the subprocess is terminated. If both a command string and
the /INPUT qualifier are specified, the specified command string
executes before additional commands are obtained from the /INPUT
qualifier. If neither is specified, SYS$INPUT is assumed (in which case
a SPAWN/NOWAIT command is aborted if Ctrl/Y is pressed to abort
something running in your parent process).
You cannot explicitly specify non-record-oriented, process-permanent
files (NRO PPFs) with the /INPUT qualifier. The system displays an
error message when it encounters such a file as the value for the
/INPUT qualifier.
Note that when NRO PPFs are used as implicit input (that is, the /INPUT
qualifier is not specified and SYS$INPUT is a NRO PPF), the SPAWN
command can succeed, as shown in the following table:
| Process Type |
SYS$INPUT |
Implicit Input |
|
Interactive
|
NRO PPF
|
SYS$COMMAND
|
|
Noninteractive
|
NRO PPF
|
Null device
|
|
Any
|
Any other
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SYS$INPUT
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If SYS$INPUT is a terminal, it cannot have an associated terminal
mailbox.
/KERNEL_THREAD_LIMIT=n
Sets the limit of the number of kernel threads that can be created in
the new process. If the value specified is higher than the SYSGEN
parameter MULTITHREAD, an error message is returned. Specifying the
value 0 makes the process use the systemwide limit set by the
MULTITHREAD parameter.
/KEYPAD (default)
/NOKEYPAD
Copies keypad key definitions and the current keypad state from the
parent process. By default, if you have established key definitions or
states with the DEFINE/KEY or the SET KEY command, these settings are
copied to the subprocess. Use the /NOKEYPAD qualifier if you do not
want the key settings to be copied.
/LOG (default)
/NOLOG
Displays the assigned subprocess name and any messages indicating
transfer of control between processes.
/LOGICAL_NAMES (default)
/NOLOGICAL_NAMES
Copies process logical names and logical name tables to the subprocess.
By default, all process logical names and logical name tables are
copied to the subprocess except those explicitly marked CONFINE or
created in executive or kernel mode.
/NOTIFY
/NONOTIFY (default)
Controls whether a message is broadcast to your terminal notifying you
that your subprocess has completed or aborted. This qualifier should
not be used unless you specify the /NOWAIT qualifier. The /NOTIFY
qualifier cannot be specified when the SPAWN command is executed from
within a noninteractive process.
Note that messages broadcast as a result of using the /NOTIFY qualifier
are considered to be DCL messages. Therefore, if SET BROADCAST=NODCL is
in effect, all such notification messages are suppressed.
/OUTPUT=filespec
Specifies the output file to which the results of the SPAWN operation
are written. The asterisk (*) and the percent sign (%) wildcard
characters cannot be used in the file specification. (Do not specify
SYS$COMMAND as a file specification for the /OUTPUT qualifier when
using the /NOWAIT qualifier; both parent and subprocess output will be
displayed simultaneously on your terminal.)
Caution
If you include a version number in the filespec argument of the /OUTPUT
qualifier, and that version of the specified file already exists, the
existing file is replaced with the new output.
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You cannot explicitly specify non-record-oriented, process-permanent
files (NRO PPFs) with the /OUTPUT qualifier. The system displays an
error message when it encounters such a file as the value for the
/OUTPUT qualifier.
Note that when NRO PPFs are used as implicit output, the SPAWN command
can succeed, as shown in the following table:
| Process Type |
SYS$OUTPUT |
Implicit Output |
|
Any
|
NRO PPF
|
Mailbox transmitting records for parent to write to its current
SYS$OUTPUT device
|
|
Any
|
Any other
|
SYS$OUTPUT
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If you omit the /OUTPUT qualifier, output is written to the current
SYS$OUTPUT device.
/PRIVILEGES={CURRENT|AUTHORIZED}
Determines whether the spawned process inherits the current process'
current or authorized privileges as its authorized privileges. By
default, the authorized privilege mask for the subprocess is taken from
the current privileges of its creator. (This corresponds to
/PRIVILEGES=CURRENT.) If the /PRIVILEGES=AUTHORIZED qualifier is
specified, the subprocess' authorized privileges will be taken from the
caller's authorized privileges.
/PROCESS=subprocess-name
Specifies the name of the subprocess to be created. If you omit the
/PROCESS qualifier, a unique process name is assigned with the same
base name as the parent process and a unique number.
The default subprocess name format is username_n. If you
specify a process name that already exists, an error message is
displayed. If the /LOG qualifier has been specified, the assigned name
of the subprocess is displayed.
Note
Prior to OpenVMS Version 7.3-1, if no process name was supplied, the
system constructed a name by appending _n to the user name,
where n was the next available nonduplicate integer for any
process currently in the system. For example, the first spawned process
from user SYSTEM would be called SYSTEM_1, the second, SYSTEM_2, and so
on. The next available number was chosen as soon as a gap was found.
Starting with OpenVMS Alpha Version 7.3-1, a random number is chosen to
append to the user name. In addition, starting with OpenVMS Version
8.3, the chosen number has, by default, a maximum value of 65535
instead of the former maximum value of 255. Therefore, the first
processes that are spawned from user SYSTEM might be SYSTEM_7376,
SYSTEM_9, SYSTEM_47124, and so on.
Some applications might rely on the prior methods of assigning
subprocess names. The DCL_CTLFLAGS system parameter allows you to
configure the system as necessary.
The rationale for changing from sequential to random number generation
is that determining the next available open sequential number slot is
very expensive in terms of performance. The mechanism attempts to
create the process by incrementing names until it finds one that is
unique. When several subprocesses already exist, the cost of creating
the subprocess in the same OpenVMS group, the cost multiplies because
process names must be unique throughout the group.
The use of random number generation results in a very high probability
of finding a unique name on the first try, because it is unlikely that
the same number is already in use. This greatly reduces the cost of
process creation, and applications that rely on spawned subprocesses
might see a dramatic performance improvement as a result of this change.
For more information, see the HP OpenVMS System Management Utilities Reference Manual for more information.
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/PROMPT[=string]
Specifies the prompt string for DCL to use in the subprocess. The
default is the prompt of the parent process.
The string can consist of more than one character. All valid ASCII
characters can be used in the string. The string must be enclosed in
quotation marks (" ") if it contains spaces, special
characters, or lowercase characters. Otherwise, letters are
automatically converted to uppercase, and leading and trailing spaces
are removed.
If no string is specified, the dollar sign ($) DCL default prompt
string is used for the subprocess.
/SYMBOLS (default)
/NOSYMBOLS
Determines whether global and local symbols (except $RESTART,
$SEVERITY, and $STATUS) are passed to the subprocess. $RESTART,
$SEVERITY, and $STATUS symbols are never passed to the
subprocess.
/TABLE=command-table
Specifies the name of an alternate command table to be used by the
subprocess.
/TRUSTED
/NOTRUSTED
Indicates that the SPAWN command input originates in a trusted command
procedure. SPAWN commands are not allowed in CAPTIVE accounts. The
/TRUSTED qualifier provides a way for properly written captive command
procedures to perform SPAWN operations when the command input
originates in the captive command procedure where it can be trusted.
For more information about trusted command procedures, see the
HP OpenVMS Guide to System Security.
/WAIT (default)
/NOWAIT
Requires that you wait for the subprocess to terminate before you enter
another DCL command. The /NOWAIT qualifier allows you to enter new
commands while the subprocess is running. (Use the /OUTPUT qualifier
with the /NOWAIT qualifier to avoid displaying both parent and
subprocess output on the terminal simultaneously.)
Note that specifying the /NOWAIT qualifier causes both input and output
to be shared with the parent process. If the input device is a
terminal, control characters, such as Ctrl/T or Ctrl/Y, also affect all
subprocesses sharing the input device. The Ctrl/Y control character,
for example, interrupts all such subprocesses.
This problem may be avoided by specifying /INPUT=NL:.
Example
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$ RUN MYPROG
.
.
.
$ [Ctrl/Y]
$ SPAWN MAIL
%DCL-S-SPAWNED, process SKONETSKI_272 spawned
%DCL-S-ATTACHED, terminal now attached to process SKONETSKI_272
MAIL> READ
.
.
.
MAIL> EXIT
%DCL-S-RETURNED, control returned to process SKONETSKI
$ CONTINUE
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The SPAWN command in this example allows you to enter the OpenVMS Mail
utility (MAIL) without terminating the currently running program. After
you exit from Mail, control is returned to the parent process.
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