Re: [9fans] a jmp_buf in APE question

[erik quanstrom <quanstro@quanstro.net>]

On Mon Jan  7 00:47:14 EST 2013, staal1978@gmail.com wrote:
> Hi all. Perhaps I have been looking at completely the wrong places and perhaps
> I am just not grasping it at all.
>
> For a package that I want to build under APE, I need to put in the stack
> pointer (sp) and the program counter (pc) part of jmp_buf specific for Plan9
> (since APE does not expose any stack-related functions that could have been
> used)
>
> according to /sys/include/ape/setjmp.h, jmp_buf is an array of 10 int:s.
>
> I then went to check out /sys/src/ape/lib/ap/$objtype/setjmp.s
> but I could not really understand anything in those files

as you point out, the interface is opaque.

but if you look under the covers, PC and SP are stored in first two
"entries" of the jmp_buf blob.  obviously PC and SP are not properly
ints, but rather uintptrs, so the absolute offset will depend on the
pointer size (0 and 4 for 386).  a better definition of jmp_buf might be
	typedef uchar jmp_buf[4 * sizeof uintptr];
note, a structure will not do, since a structure would be passed in by
value and therefore any values set would disappear on return from setjmp.

for amd64 the setjmp code is (comments changed assuming that jmp_buf is an
array of uintptrs)

TEXT	setjmp(SB), $0		/* RARG is set to &jmp_buf[0] */
	MOVQ	SP, 0(RARG)	/* store sp in RARG offset 0; jmp_buf[0] = SP */
	MOVQ	0(SP), BX	/* return pc */
	MOVQ	BX, 8(RARG)	/* stored at offset 8; jmp_buf[1] = PC */
	MOVL	$0, AX		/* return 0 */
	RET

note that in this case the absolute offsets are 0 and 8, since pointers on
amd64 take 8 bytes.

> The second question would be, is the position of those two values in jmp_buf
> architecture-specific so that I should make a note of that it is working on
> i386 only?

the structure within jmp_buf is entirely determined by the code in setjmp.s

i don't think it would be wise to rely on the structure of jmp_buf.  it's
not exposing any structure, so it seems unwise to assume it.  the typical
way around this would be to create $objtype.s for each $objtype you wish
to support and store the PC and SP as you like them.  i think what you want
is

	enum {
		Spidx,
		Pcidx,
		Nregsave,
	};
	typedef	uintptr	myjmpbuf[Nregsave];
then it should be a simple matter of reappropriating the code from setjmp.

- erik