Allan Pratt's 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.]
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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.
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| Opinions expressed above do not necessarily | -- Allan Pratt, Atari Corp.
| reflect those of Atari Corp. or anyone else. | ...lll-lcc!atari!apratt
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