plan9port/man/man3/mach-stack.3

.TH MACH-STACK 3
.SH NAME
stacktrace, localaddr, unwindframe, windindex, windreglocs \- stack traces
.SH SYNOPSIS
.B #include <u.h>
.br
.B #include <libc.h>
.br
.B #include <mach.h>
.PP
.ft B
.ta \w'\fBxxxxxx'u +\w'\fBxxxxxx'u
int	stacktrace(Map *map, Rgetter rget, Tracer trace)
.PP
.ft B
int	localaddr(Map *map, Regs *regs, char *fn, char *val, ulong *val)
.PP
.ft B
int	unwindframe(Map *map, Regs *regs, ulong *next, Symbol *sym)
.PP
.ft B
int	windindex(char *regname)
.PP
.ft B
Loc*	windreglocs(void)
.SH DESCRIPTION
.I Stacktrace
provides machine-independent
implementations of process stack traces.
They must retrieve data and register contents from an executing
image.  Sometimes the desired registers are not the current
registers but rather a set of saved registers stored elsewhere
in memory.
The caller may specify an initial register set in the form of an
.I Rgetter
function, of the form
.PP
.RS
.B "ulong rget(Map *map, char *name)
.RE
.PP
It returns the contents of a register when given a map
and a register name.
It is usually sufficient for the register function
to return meaningful values only for 
.BR SP
and
.BR PC ,
and for the link register
(usually
.BR LR )
on CISC machines.
.PP
Given the map and the rgetter,
.I stacktrace
unwinds the stack starting at the innermost function.
At each level in the trace, it calls the tracer function, which has the form
.PP
.RS
.B "int trace(Map *map, ulong pc, ulong callerpc,
.br
.B "	Rgetter rget, Symbol *s)
.RE
.PP
The tracer is passed the map, the current program counter,
the program counter of the caller (zero if the caller is unknown),
a new
.I rget
function, and a symbol 
(see
.MR mach-symbol (3) )
describing the current function
(nil if no symbol is known).
The value returned by the tracer
controls whether the stack trace continues:
a zero or negative return value stops the trace,
while a positive return value continues it.
.PP
The rgetter passed to the tracer is not the rgetter
passed to
.B stacktrace
itself.
Instead, it is a function returning the register values
at the time of the call, to the extent that they can be
reconstructed.
The most common use for this rgetter
is as an argument to
.IR lget4 ,
etc., when evaluating the locations of local variables.
.PP
.I Localaddr
uses
.I stacktrace
to walk up the stack looking for the innermost instance of a function named
.I fn ;
once it finds the function,
it looks for the parameter or local variable
.IR var ,
storing the address of the variable in
.IR val .
.PP
.I Unwindframe
is the low-level function on which
.I stacktrace
is built.
Given the current memory image in
.I map
and the current register set in
.I regs ,
.I unwindframe
fills in
.I next
with the values of the register set 
at the time of the call to the function in the current program counter.
.I Sym
should be the symbol corresponding to the current function,
if available.
.PP
The
.I next
array holds only the
.IR "winding registers" ,
typically the caller-save registers and the program counter and stack pointer.
The order of registers in the array is called the
.IR "winding order" .
The winding set can be found in the array
.IB mach -> windreg \fR,
which has
.IB mach -> nwindreg
entries.
.I Windindex
returns the index of the named register
in the winding order.
.I Windreglocs
returns an array of
.I Loc
structures corresponding to the winding registers,
in the winding order.
.SH EXAMPLE
The following code writes a simple stack trace to standard output,
stopping after at most 20 stack frames.
.RS
.ft B
.nf
.ta \w'xxxx'u +\w'xxxx'u +\w'xxxx'u +\w'xxxx'u +\w'xxxx'u
static int
trace(Map *map, ulong pc, ulong callerpc,
	Rgetter rget, Symbol *s, int depth)
{
	char buf[512];
	int i, first;
	u32int v;
	Symbol s2;

	if(sym)
		print("%s+%lx", s->name, pc - loceval(s->loc));
	else
		print("%lux", pc);
	print("(");
	first = 0;
	for(i=0; indexlsym(s, &i, &s2)>=0; i++){
		if(s.class != CPARAM)
			continue;
		if(first++)
			print(", ");
		if(lget4(map, rget, s->loc, &v) >= 0)
			print("%s=%#lux", s->name, (ulong)v);
		else
			print("%s=???", s->name);
	}
	print(") called from ");
	symoff(buf, sizeof buf, callerpc, CTEXT);
	print("%s\en", buf);
	return depth < 20;
}

	if(stacktrace(map, nil, trace) <= 0)
		print("no stack frame\n");
.RE
.SH SOURCE
.B \*9/src/libmach
.SH SEE ALSO
.MR mach (3)
.SH BUGS
Need to talk about Regs