system() function

Syntax

system(num.exp)

Parameter(s)

Example(s)

When performing an execute of a UNIX shell command, the UNIX command aborts if it gets a break signal (the normal action in UNIX). The code below determines that a break has occurred and re-executes the UNIX instruction, giving the appearance of a standard D3 break-and-continue capability:

do.unix:
* Clear the interrupt value
x = system(37)
execute "!exec find / -name ap - print"
* Break - return, reexecute
if system(37) = 10 then goto do.unix

Another useful application is for a FlashBASIC or BASIC program to mimic the behavior of the Update Processor’s interrupt handling and redraw the screen background if it senses an interrupt. For example:

gosub refresh
open "data"
select
loop
   readnext ID else exit
   read rec from ID
   name = rec<1>
   balance = rec<2>
   balance = balance + 50
   print @(20,2):name
   print @(20,3):balance
   if system(37) then gosub refresh; * Screen display might be changed
repeat
stop
refresh:
* Draw the background
print @(-1)
print @(10,1):"Processing Accounts"
print @(10,2):"Name:"
print @(10,3):"Balance:"
return

By using system(37) in conjunction with the break off statement, a FlashBASIC or BASIC program can implement its own signal handler. For example, assume a user wants to update many items in a file, but wants this done atomically. Either all the items in the file must be updated or none of the items must be updated. This is a form of transaction bracketing and can be implemented on previous releases by turning off the BREAK key. However, this method has the disadvantage of not being able to interrupt the process. The system(37) capability provides the ability to offer a controlled interrupt allowing the user to continue, or back out of all changes that have been made. This routine could be expanded to allow other operations provided it did not turn the BREAK key on (which causes the system to catch the break and push a level, allowing the user to type end). For example:

break off
* clear previous signals
x = system(37)
max = 2000
dim xx(max)
for i = 1 to max
   readu xx(i) from i
   writeu "abc" on i
   * break key hit
   if system(37) = 10 then gosub interrupt
next i
* Release the locks
for i = 1 to max
   release i
next i
stop
interrupt:
print "Interrupted"
print "Processed ":i:" items"
print "C)ontinue or Q)uit without modifying file:":
in x
print
x = char(x)
if x = "q" then
   * Change the items back to the way they were
   max = i
   for i = 1 to max
      * note that write releases the locks along the way
      write xx(i) on i
   next i
   stop
end
return

Finally, system(37) increases the capabilities of D3 in the area of interprocess communications. For example, a batch job can poll system(37) repeatedly, and when it senses an interrupt, it can either send a message indicating its progress, or perform some other, more complex action by reading a command item from a file. Note that using system(37) is much more efficient that reading statements and writing status output for every item. For example:

* clear status
xx = system(37)
open "data"
for i = 1 to 100000
   * In a real program, there should be more work done in the main loop. 
   * Otherwise, the system(37) becomes comparatively more expensive.
   write "abc" on i
   if system(37) then execute "msg !0 Processed ":i:" items."
next i
stop
* "z" the preceding program and then type the following
* tcl (line#) (user) who
*
* The actual command executed is unimportant. The who command is a good candidate
* because it's cheap and has no side effects.
*
* Note that the msg does not work with phantoms, but the "tcl" command does.

include dm,bp,includes sysID.inc
imp=system(38)
if imp<sys$host>=sys$unix then
   * Running on UNIX implementation
   n=%open("myfile, O$RDONLY)
   ...
end

crt system(100)
D3UNIX:RS6000;AIX;pick0:prod0;2;3;000047311000;6.0.0.m0;27 May 2001

See also

%gethostid() function, %putenv() function, Active list, casing statement, execute statement (UNIX), footing statement, BASIC functions, heading statement, Locking scheme, matread statement, onerr clause, page statement, Port number, readnext statement, readt statement, readtx statement, readv statement, rewind statement, Statements and functions, status() function, str() function, term command, u713c user exit, ue070 user exit, User exits, weof statement, who command, writet statement