MACH-STACK(3)

NAME
stacktrace, localaddr,

SYNOPSIS
#include <u.h>
#include <libc.h>
#include <mach.h>

int    stacktrace(Map *map, Rgetter rget, Tracer trace)

int    localvar(Map *map, char *fn, char *val, Loc *loc)

DESCRIPTION
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 Rgetter function, of the form

    ulong rget(Map *map, char *name)

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 SP and PC, and for the link register (usually LR) on CISC machines.

Given the map and the rgetter, stacktrace unwinds the stack starting at the innermost function. At each level in the trace, it calls the tracer function, which has the form

    int trace(Map *map, ulong pc, ulong callerpc,
    Rgetter rget, Symbol *s)

The tracer is passed the map, the current program counter, the program counter of the caller (zero if the caller is unknown), a new rget function, and a symbol (see 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.

The rgetter passed to the tracer is not the rgetter passed to 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 lget4, etc., when evaluating the locations of local variables.

Localvar uses stacktrace to walk up the stack looking for the innermost instance of a function named fn ; once it finds the function, it looks for the parameter or local variable var, storing the location of the variable in loc.

EXAMPLE
The following code writes a simple stack trace to standard output, stopping after at most 20 stack frames.
    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\n", buf);
    	return depth < 20;
    }
    	if(stacktrace(map, nil, trace) <= 0)
    		print("no stack frame);

SOURCE
/usr/local/plan9/src/libmach

SEE ALSO
mach(3)