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]
1 – 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.
2 – Qualifiers
2.1 /CARRIAGE_CONTROL
/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.
2.2 /CLI
/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$SYSTEM and have the
file type .EXE.
2.3 /INPUT
/INPUT=filespec
Specifies an input file containing one or more DCL commands to
be executed by the spawned subprocess. The file type defaults
to .COM. The asterisk (*) and the percent sign (%) wildcard
characters are not allowed in the file specification. Once
processing of the input file is complete, the subprocess is
terminated. If both a command string and the /INPUT qualifier
are specified, the specified command string executes before
additional commands are obtained from the /INPUT qualifier.
If neither is specified, SYS$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$COMMAND
Noninteractive NRO PPF Null device
Any Any other SYS$INPUT
If SYS$INPUT is a terminal, it cannot have an associated terminal
mailbox.
2.4 /KERNEL_THREAD_LIMIT
/KERNEL_THREAD_LIMIT=n
Sets the limit of the number of kernel threads that can be
created in the new process. If the value specified is higher than
the SYSGEN parameter MULTITHREAD, an error message is returned.
Specifying the value 0 makes the process use the system-wide
limit set by the MULTITHREAD parameter.
2.5 /KEYPAD
/KEYPAD (default)
/NOKEYPAD
Copies keypad key definitions and the current keypad state from
the parent process. By default, if you have established key
definitions or states with the DEFINE/KEY or the SET KEY command,
these settings are copied to the subprocess. Use the /NOKEYPAD
qualifier if you do not want the key settings to be copied.
2.6 /LOG
/LOG (default)
/NOLOG
Displays the assigned subprocess name and any messages indicating
transfer of control between processes.
2.7 /LOGICAL_NAMES
/LOGICAL_NAMES (default)
/NOLOGICAL_NAMES
Copies process logical names and logical name tables to the
subprocess. By default, all process logical names and logical
name tables are copied to the subprocess except those explicitly
marked CONFINE or created in executive or kernel mode.
2.8 /NOTIFY
/NOTIFY
/NONOTIFY (default)
Controls whether a message is broadcast to your terminal
notifying you that your subprocess has completed or aborted.
This qualifier should not be used unless you specify the /NOWAIT
qualifier. The /NOTIFY qualifier cannot be specified when the
SPAWN command is executed from within a noninteractive process.
Note that messages broadcast as a result of using the /NOTIFY
qualifier are considered to be DCL messages. Therefore, if SET
BROADCAST=NODCL is in effect, all such notification messages are
suppressed.
2.9 /OUTPUT
/OUTPUT=filespec
Specifies the output file to which the results of the SPAWN
operation are written. The asterisk (*) and the percent sign (%)
wildcard characters cannot be used in the file specification.
(Do not specify SYS$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.
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$OUTPUT
If you omit the /OUTPUT qualifier, output is written to the
current SYS$OUTPUT device.
2.10 /PRIVILEGES
/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.
2.11 /PROCESS
/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 VSI OpenVMS System Management
Utilities Reference Manual for more information.
2.12 /PROMPT
/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.
2.13 /SYMBOLS
/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.
2.14 /TABLE
/TABLE=command-table
Specifies the name of an alternate command table to be used by
the subprocess.
2.15 /TRUSTED
/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 VSI OpenVMS Guide to System
Security.
2.16 /WAIT
/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:.
3 – Example
$ 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
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.