Tag: spawn

Understanding spawn and _BPX_SHAREAS

You can use spawn() to create another thread to do work. It may be able to run in the same address space as the originator, or it may run in its own address space.

It is cheaper to run in the requester’s address space, as it just creates a new TCB. If it runs in a different address space, in one of the pool of OMVS BPXAS address spaces, there is additional overhead.

I set up a shell script to call a Rexx script which did a spawn of another shell script.

I used the Rexx script to display information about the threads.

With _BPX_SHAREAS=YES – share the address space

the output was

jobname  asid    ppid    pid    threadid  tcb  cmdline
COLIN    21        1 50397218 212A80003 8BEA50 OMVS                    
COLIN    21 50397218 16842787 212A68002 8B9C90 -sh                     
COLIN2   4C 16842787 50397295 212AA8000 8FB2F8 sh kk.sh                
COLIN2   4C 50397295 33620080 212AB0000 8D6A88 ./r.rexx YES

We can see the following

  1. The top level process in OMVS (parent process id) 1 invoked a program OMVS with process id(pid) 50397218, in address space 0x21
  2. This process invoked a shell (-sh) with pid 16842787 in address space 0x21
  3. This executed a command “sh kk.sh” (my test script) with a process id in address space 0x4c, jobname COLIN2, and TCB 8FB2F8.
  4. This invoked shell script invoked a command “./r.rexx YES” in the same address space 0x4c, jobname COLIN2 with a different TCB 8D6A88. This is sharing the address space.

With _BPX_SHAREAS=NO – do not share the address space

the output was similar to the _BPX_SHAREAS=YES, but different

jobname  asid    ppid    pid    threadid  tcb    cmdline
COLIN    21        1 50397218 212A80003 8BEA50 OMVS                        
COLIN    21 50397218 16842787 212A68002 8B9C90 -sh                         
COLIN9   4B 16842787 83951726 212AA8000 8FB380 sh kk.sh                    
COLIN1   4D 83951726 16842864 212AB0000 8FB2F8 ./r.rexx NO
  • The sh k.sh ran in a different address space 0x4B, with a different jobname COLIN9.
  • Because _BPX_SHARESAS=NO, the command “./r.rexx YES” executed in a different address space 0x4d, jobname COLIN1.

Comparison between the two scenarios

  1. With _BPX_SHAREAS=YES, one address space was shared, with two (lightweight) TCBs in it.
  2. With _BPX_SHAREAS=NO, the address spaces were not shared, and one of the pool of BPXAS address spaces were used.

When do you get not shared, even when _BPX_SHAREAS=YES was specified?

There are several cases when the system will not run a program in a shared address space.

Integrity

If there is a mismatch between APF states, for example

  • the caller is APF authorised; you do not want an unauthorised program access the memory in the shared address space of the APF authorised thread.
  • the caller is not APF authorised, but you are calling an APF authorised program.

The called program may change the userid or the group.

If your program changes the userid or group it is running under, for example a web server doing work for different userids. You can set a flag (s) on a file chmod to indicate that this program may change userid or group. See set-user-ID and Set-group-ID in chmod. You can use the ls -ltr command

-rwsr-sr-x   1 OMVSKERN ZWEADMIN    1336 Feb 26 16:41 r.rexx

Where the first s is for set-user-ID. The second s is for set-group-ID.

When the file had u+s, I got the following error message

FSUM9209 cannot execute: reason code = 0b1b0473: EDC5157I An internal error has occurred.