rajeev@hpnmdla.HP.COM (Rajeev Menon) (09/15/90)
I am running HP-UX A.B3.10 on an HP 9000/855.
I am have a little program that creates three more processes, such that P0
the original process is parent of P1, which is parent of P2, etc. After
creating the processes, I ask each process to print out its PID and PPID
into a file called "junk". I am in no way synchronizing the processes, so
the order they write to this file is "random". But it doesn't seem random...
And sometimes it seems hard to explain. Can someone help ???
Here is the program called "fork.c"
--------------------------Cut here---------------------------------------
#include <stdio.h>
int cprocs(nprocs)
int nprocs;
{
int i, pid;
for ( i=0; i<nprocs; i++) {
if ((pid = fork()) == -1)
_exit(127);
else if (pid)
break;
}
return i;
}
main()
{
int i, pid;
FILE *fp;
if ( (fp = fopen("junk", "w")) == NULL )
perror("Error in fopen");
i = cprocs(3);
fprintf( fp, "i=%d, pid = %d, ppid = %d.\n", i, getpid(), getppid());
fclose(fp);
}
--------------------------Cut here---------------------------------------
Here is a sequence that I cannot understand :
% cc fork.c ; a.out ; cat junk
i=0, pid = 10593, ppid = 20487.
i=2, pid = 10598, ppid = 10597.
i=3, pid = 10599, ppid = 10598.
i=1, pid = 10597, ppid = 10593.
% cc fork.c ; a.out ; cat junk
i=0, pid = 10779, ppid = 20487.
i=2, pid = 10781, ppid = 10780.
% cc fork.c ; a.out ; cat junk
i=3, pid = 11122, ppid = 11121.
i=2, pid = 11121, ppid = 11120.
i=0, pid = 11119, ppid = 20487.
i=1, pid = 11120, ppid = 11119.
Sometimes (as in case 2),the file "junk" does not contain output from all
the processes. Why ? Now I do
% a.out ; cat junk
i=0, pid = 11189, ppid = 20487.
i=1, pid = 11190, ppid = 1.
i=2, pid = 11191, ppid = 1.
i=3, pid = 11192, ppid = 1.
% a.out ; cat junk
i=2, pid = 11251, ppid = 11250.
i=3, pid = 11252, ppid = 1.
i=0, pid = 11249, ppid = 20487.
i=1, pid = 11250, ppid = 11249.
Some PPIDs show up as 1, showing that they have been orphaned, which could
be expected since I am not synchonizing the processes. But it really puzzles
me as to why this happens when I do "a.out ; cat junk" and not when I do
"cc fork.c ; a.out ; cat junk". Can someone explain what is going on ?
Thanks
Rajeev Menon
Rajeev Menon
Hewlett Packard Company e-mail: rajeev@hpnmd.hp.com
1400 Fountaingrove Pkwy
Santa Rosa CA 95403cpcahil@virtech.uucp (Conor P. Cahill) (09/15/90)
In article <2140001@hpnmdla.HP.COM> rajeev@hpnmdla.HP.COM (Rajeev Menon) writes: > [ questions about pids, ppids, etc with forking processes deleted] The key to your problem is that you can't be sure what process will run first following a fork (maybe the parent, maybe the child). >Sometimes (as in case 2),the file "junk" does not contain output from all >the processes. Why ? Now I do The problem you had with missing output is that you have several files opening & writing to the file at the same time, but you did not set append mode on the file. >Some PPIDs show up as 1, showing that they have been orphaned, which could >be expected since I am not synchonizing the processes. But it really puzzles >me as to why this happens when I do "a.out ; cat junk" and not when I do >"cc fork.c ; a.out ; cat junk". Can someone explain what is going on ? Pure chance (or actually scheduling priorities in the kernel & the effect of other processes on cpu availability for your processes). -- Conor P. Cahill (703)430-9247 Virtual Technologies, Inc., uunet!virtech!cpcahil 46030 Manekin Plaza, Suite 160 Sterling, VA 22170
maart@cs.vu.nl (Maarten Litmaath) (09/18/90)
In article <1990Sep15.133135.28744@virtech.uucp>, cpcahil@virtech.uucp (Conor P. Cahill) writes: )In article <2140001@hpnmdla.HP.COM> ) rajeev@hpnmdla.HP.COM (Rajeev Menon) writes: )> [ questions about pids, ppids, etc with forking processes deleted] ) )The key to your problem is that you can't be sure what process will )run first following a fork (maybe the parent, maybe the child). Right. )>Sometimes (as in case 2),the file "junk" does not contain output from all )>the processes. Why ? Now I do ) )The problem you had with missing output is that you have several )files opening & writing to the file at the same time, but you did not )set append mode on the file. Wrong. The file was only opened once. The forked processes shared a _common_ file offset (resulting in automatic append mode). The command sequences Rajeev used were: % cc fork.c ; a.out ; cat junk i=0, pid = 10779, ppid = 20487. i=2, pid = 10781, ppid = 10780. % cc fork.c ; a.out ; cat junk i=3, pid = 11122, ppid = 11121. i=2, pid = 11121, ppid = 11120. i=0, pid = 11119, ppid = 20487. i=1, pid = 11120, ppid = 11119. I suggest he try the following command sequence: % cc fork.c ; a.out ; sleep 10 ; cat junk Get my point? -- "That's right, you wanted the "Self-Destruct" button, to your left." (John Woods)