Newsgroups: comp.sys.atari.st

Newsgroups: comp.sys.atari.st
From: [email protected] (Allan Pratt)
Date: Tue, 18-Aug-87 13:56:59 EDT
Subject: Pexec cookbook!

This is in response to a request from Christian Kaernbach which I got
from BITNET: I can't reply directly to BITNET, but I'm sure other
people will find this interesting, too: it's a preliminary version of
the long-awaited Pexec cookbook!

In broad terms, the things you have to know about Pexec are that it
starts up a process, lets it execute, then returns to the caller when
that process terminates. The "caller" -- the process which used
Pexec in the first place -- has some responsibilities: it has to make
memory available to the OS for allocation to the child, and it has to
build up the argument string for the child.

All GEMDOS programs are started with the largest block of OS memory
allocated to them. Except in very rare circumstances, this block is
the one stretching from the end of the accessories and resident
utilities to the beginning of screen memory. The point is that your
program has probably been allocated ALL of free memory. In order to
make memory available for a child process, you have to SHRINK the
block you own, returning the top part of it to GEMDOS. The time to
do this is when you start up.

If you use Alcyon C (from the developer's kit), you know that you
always link with a file called GEMSTART. If you've been paying
attention, you should have gotten the *new* GEMSTART from Compuserve
(or from somebody else who got it): I wrote that GEMSTART. In
GEMSTART.S, there is a lot of discussion about memory models, and
then a variable you set telling how much memory you want to keep or
give back to the OS. Make your choice (when in doubt, use STACK=1),
assemble GEMSTART.S, call the result GEMSEXEC.O (or something), and
link the programs which Pexec with that file rather than the normal
GEMSTART.

Now here's a discussion of what GEMSTART has to do with respect to
keeping or returning memory:

Your program is invoked with the address of its own basepage as the
argument to a function (that is, at 4(sp).l). In this basepage is
the structure you can find in your documentation. The interesting
fields are HITPA (the address of first byte NOT in your TPA), BSSBASE
(the first address of your bss) and BSSLEN (the length of your BSS).

Your stack pointer starts at HITPA-8 (because 8 is the length of the
basepage argument and the dummy return PC on the stack). The space
from BSSBASE+BSSLEN to your SP is the "stack+heap" space. Library
malloc() calls use this space, moving a pointer called the "break"
(in the variable __break, or the C variable _break if you use Alcyon
C) up as it uses memory. Your stack pointer moves down from the top
as it uses memory, and if the sp and _break ever meet, you're out of
memory. In fact, if they ever come close (within a "chicken factor"
of about 512 bytes or 1K), malloc() will fail because it doesn't want
your stack to overwrite good data.

When a process starts, it gets *all* of memory allocated to it: from
the end of any accessories or resident utilities up to the default
screen memory. If you want to use Pexec, you have to give some
memory back to the OS. You do this with the Mshrink call. Its
arguments are the address of the memory block to shrink (your
basepage address) and the new size to shrink it to. You should be
sure to leave enough room above your BSS for a reasonable stack (at
least 2K) plus any malloc() calls you expect to make. Let's say
you're writing "make" and you want to leave about 32K for malloc()
(for your dependency structures). Also, since make is recursive, you
should leave lots of space for the stack - maybe another 16K. The
new top of memory that your program needs is:

newtop = your bss base address + your bss size + 16K stack + 32K heap

Since your stack pointer is at the top of your CURRENT TPA, and
you're about to shrink that, you'd better move your stack:

move.l newtop,sp

Now you want to compute your new TPA size and call Mshrink:

move.l newtop,d0
sub.l basepage,d0 ; newtop-basepage is desired TPA size
move.l d0,-(sp) ; set up Mshrink(basepage,d0)
move.l basepage,-(sp)
move.w #$4a ; fn code for Mshrink
trap #1
add.l #10,sp ; clean up args

Now that you've shrunk your TPA, the OS can allocate this new memory
to your child. It can also use this memory for Malloc(), which is
used occasionally by GEM VDI for blt buffers, etc. Note that you
only have to do this once, when you start up: after that, you can do
as much Pexec'ing as you want.

When you want to exec a child, you build its complete filespec into
one string, and its arguments into another. The argument string is a
little strange: the first character of the argument string is the
length of the rest of the string!

Here is a simple system call: pass it the name of the file to execute
and the argument string to use.

long system(cmd,args)
char *cmd, *args;
{
char buf[128];

if (strlen(args) > 126) {
printf("argument string too long\n");
return -1;
}
strcpy(buf+1,args); /* copy args to
buffer+1 */
buf[0] = strlen(args); /* set buffer[0] to
len */
return Pexec(0,cmd,buf,0L);
}

The first zero in the Pexec call is the Pexec function code: load and
go. The cmd argument is the full filespec, with the path, file name,
and file type. The third argument is the command-line argument
string, and the fourth argument is the environment pointer. A null
environment pointer means "let the child inherit A COPY OF my
environment."

This call will load the program, pass the arguments and environment
to it, and execute it. When the program terminates, the call returns
the exit code from the program. If the Pexec fails (not enough
memory, file not found, etc.) a negative code is returned, and you
should deal with it accordingly. Note that error returns from Pexec
are always negative LONGS, while return codes from the child will
have zeros in the upper 16 bits.

EXIT CODES:

GEMDOS, like MS-DOS before it, allows programs to return a 16-bit
exit code to their parents when they terminate. This is done with
the Pterm(errcode) call. The value in errcode is passed to the
parent as the return value of the Pexec system call. The C library
function exit(errcode) usually uses this call.

Unfortunately, the people who wrote the startup file for the Alcyon C
compiler didn't use this. The compiler calls exit() with an error
code, and exit() calls _exit(), but _exit always uses Pterm0(), which
returns zero as the exit code. I fixed this by rewriting GEMSTART.S,
the file you link with first when using Alcyon.

Even though new programs return the right exit code, the compiler
itself still doesn't. Well, I have patched the binaries of all the
passes of the compiler so they DO. It isn't hard, and I will post
instructions at a later date for doing it. IF YOU DO THIS, PLEASE
DON'T BOTHER OUR CUSTOMER SUPPORT PEOPLE IF IT DOESN'T WORK. THEY
DON'T KNOW ANYTHING ABOUT IT.

I hope that this little cookbook makes Pexec less mysterious. I
haven't covered such topics as the critical-error and terminate
vectors, even though they are intimately connected with the idea of
exec'ing children. A more complete cookbook should be forthcoming.

If there are any errors or gross omissions in the above text, please
let me know BY MAIL so I can correct them coherently. Landon isn't
here to check my semantics, so I may have missed something. [Landon
is on vacation in France until early September.]

********************************************************************

C. Kaernbach's question was why his accessory, which basically did a
Pexec from a file selector, didn't always work. The answer is that
it works when used within a program which has returned enough memory
to the OS for the child. Why might it bomb? Because if a program
has returned a *little* memory to the OS (only about 2K), a bug in
Pexec shows up that breaks the memory manager. Accessories are
strange beasts anyway, so for the most part combining two strange
beasts (Accessories and Pexec) is bad news.

/----------------------------------------------\
| Opinions expressed above do not necessarily | -- Allan Pratt,
Atari Corp.
| reflect those of Atari Corp. or anyone else. |
...lll-lcc!atari!apratt
\----------------------------------------------/ (APRATT on
GEnie)

Newsgroups: comp.sys.atari.st
From: [email protected]
Date: Wed, 19-Aug-87 12:11:44 EDT
Subject: Re: Pexec cookbook! (w/ Lattice C)

In article <[email protected]> [email protected] (Allan Pratt) writes:
>All GEMDOS programs are started with the largest block of OS memory
>allocated to them... The point is that your
>program has probably been allocated ALL of free memory. In order to
>make memory available for a child process, you have to SHRINK the
>block you own, returning the top part of it to GEMDOS. The time to
>do this is when you start up.

Does anybody know exactly how I do this using Lattice C? The
documentation
(for version 3.04) is less than clear on this point, and everything
I've
tried hasn't worked. I have never really *needed* to give the extra
700K
or so of memory back, but the sheer wastefulness of it annoys me (:-),
and
someday I may want to use that memory. I'd appreciate any pointers on
the
subject.
And while I'm asking advice on Lattice C, is there any special
startup
module needed to link a program used as a desk accessory, as there
(apparently) is with other C's?
--
Joel Plutchak

Arpanet: [email protected]
Bitnet: plutchak@WISCMACC
uucp: Who knows-- it's Eunice; try ...ihnp4!uwvax!uwwircs!plutchak

Newsgroups: comp.sys.atari.st
From: [email protected] (Gert Poletiek)
Date: Thu, 20-Aug-87 12:08:00 EDT
Subject: Re: Pexec cookbook! (w/ Lattice C)

In article <[email protected]> [email protected] writes:
>In article <[email protected]> [email protected] (Allan Pratt) writes:
>>All GEMDOS programs are started with the largest block of OS memory
>>allocated to them... The point is that your
>>program has probably been allocated ALL of free memory. In order to

>>make memory available for a child process, you have to SHRINK the
>>block you own, returning the top part of it to GEMDOS. The time to
>>do this is when you start up.

> Does anybody know exactly how I do this using Lattice C? The
documentation
>(for version 3.04) is less than clear on this point, and everything
I've
>tried hasn't worked. I have never really *needed* to give the extra
700K
>or so of memory back, but the sheer wastefulness of it annoys me
(:-), and
>someday I may want to use that memory. I'd appreciate any pointers
on the
>subject.
> And while I'm asking advice on Lattice C, is there any special
startup
>module needed to link a program used as a desk accessory, as there
>(apparently) is with other C's?
>--
> Joel Plutchak

The Lattice C compiler is a different story. When the Metacomco guys
wrote their
memory management on top of the *not perfect* memory allocation
mechanism
that GemDos offers they made it static. I.e., When a Lattice program
starts
it does a shrink only for the amount:

available - ( (_memneed)L + (_stksiz)L )

When you write a program that needs to execute the Pexec GemDos call
you have to find out how much memory your program requires during its
run. Watch out: The memory allocation mechanism from the Lattice C
library (malloc and the like) all work in the heap that is _memneed
bytes large. As soon as your program requests more than _memneed it
will break. Hence you must know how much heap space your program will
need.

You will need a malloc that allows you to request memory from the
system, not a malloc that gives yu a chuck from a pool that was
requested from the system during program startup. Such a malloc would
request blocks from GemDos using trap#1 call Malloc of about 16
kbytes. (If you request 10 bytes a time using Malloc from GemDos the
40 folder bug will turn out to be a 2 folder bug).

If you want I could post a primitive set of memory allocation
routines that work on 520's 1040' and Mega4's

Newsgroups: comp.sys.atari.st
From: [email protected] (Jim Kaubisch)
Date: Thu, 20-Aug-87 20:15:12 EDT
Subject: Re: Pexec cookbook! (w/ Lattice C)

[ discussion about memory allocation ]

> Does anybody know exactly how I do this using Lattice C? The
documentation
> (for version 3.04) is less than clear on this point, and everything
I've
> tried hasn't worked. I have never really *needed* to give the extra
700K
> or so of memory back, but the sheer wastefulness of it annoys me
(:-), and
> someday I may want to use that memory. I'd appreciate any pointers
on the
> subject.

I've been trying to figure this out for some time. I'm trying to get
the Lattice compiler and programs it generates to run under the
Beckemeyer MT-C shell. Because of Lattice's "taking all memory"
approach, all Lattice progs crash the shell. I haven't been able to
get around it. My current efforts are in the area of trying to build
an alternate STARTUP.BIN. My fall-back position is to throw away
Lattice C.

> And while I'm asking advice on Lattice C, is there any special
startup
> module needed to link a program used as a desk accessory, as there
> (apparently) is with other C's?

You do need a special startup module. The latest version of Lattice
supposedly includes one.

----------------
Jim Kaubisch, ----=---- ACSnet: [email protected]
Austec International Ltd, ---===--- UUCP: ...!munnari!ausmelb.oz!whk

344 St Kilda Rd, --== ==-- ARPA: whk%ausmelb.oz.au
Melbourne 3004 AUSTRALIA -=== ===- Phone: +61 3 699 4511
--
Jim Kaubisch, ----=---- ACSnet: [email protected]
Austec International Ltd, ---===--- UUCP: ...!munnari!ausmelb.oz!whk

344 St Kilda Rd, --== ==-- ARPA: whk%ausmelb.oz.au
Melbourne 3004 AUSTRALIA -=== ===- Phone: +61 3 699 4511
D

Newsgroups: comp.sys.atari.st
From: [email protected] (Robert Hamilton)
Date: Mon, 24-Aug-87 10:10:07 EDT
Subject: Re: Pexec cookbook! (w/ Lattice C)

In article <[email protected]> [email protected] (Jim Kaubisch)
writes:

> [ discussion about memory allocation ]

>> Does anybody know exactly how I do this using Lattice C? The
documentation
>> (for version 3.04) is less than clear on this point, and everything
I've
>> tried hasn't worked. I have never really *needed* to give the
extra 700K
>> or so of memory back, but the sheer wastefulness of it annoys me
(:-), and
>> someday I may want to use that memory. I'd appreciate any pointers
on the
>> subject.

I don't see the problem here... Cant you just set _mneed as described
in the manual???

In fact I'm writing after trying out the latest version of lattice
3.04 Its very strange indeed. Most of the old lattice bugs have been
fixed (20 file bug, redirection problem, malloc etc) and the debugger
is very usable (macros already!)

But I find that very often the linker crashes out !! Its not
consistent either sometimes a bus, sometimes an address error.

But the resultant prog is invariably ok!
Trying a link with the old linker reports an "internal phase error"
Very puzzling. Anybody come across this?
--
JANET: [email protected] | Computing Science
EMAIL: [email protected] | Paisley College
UUCP: ...!seismo!mcvax!ukc!paisley!rh | High St., Paisley
Phone: +44 41 887 1241 Ext. 219 | Scotland , PA12BE

Newsgroups: comp.sys.atari.st
From: [email protected] (John R. Bane)
Date: Tue, 25-Aug-87 12:30:54 EDT
Subject: Re: Pexec cookbook! (w/ Lattice C)

In article <[email protected]>, [email protected] (Robert
Hamilton) writes:
> But I find that very often the linker crashes out !!
> Its not consistent either sometimes a bus, sometimes an address
error.
> But the resultant prog is invariably ok!

The Lattice linker always crashes on me when I link more than one
.BIN file (i.e., whenever I link any significant system). Programs
with only one object module link OK, and even if the linker crashes,
the link succeeds; it produces a .PRG file that runs fine. --
Rene P.S. Bane [email protected] ...!parcvax!bane.UUCP