libdtrace buffered output

Question

I'm trying to use dtrace through libdtrace (on Snow Leopard; 10.6.4). I want to catch the printed output of my dtrace script within my own program. One way to do so would be to have the output go to a temporary file and read it from there. However, libdtrace supports a callback function to catch the output directly which I'd prefer.

I assumed the callback would just pass me formatted strings that I could consume, but this does not seem to be the case. For example, in the below test program, I'd expect the printout to be "process pid = 86138". However, it always prints out "process pid = 1" (the dtrace script works fine, when ran with 'dtrace -n').

What am I doing wrong ? How should I be consuming the data passed to the buffer handler ? (Specifically, data from printf and tracemem actions are what I'm interested in).

#include <dtrace.h>
#include <stdio.h>
#include <stdlib.h>
#include <mach/mach.h>
#include <mach-o/loader.h>
#include <mach-o/dyld.h>
#include <mach-o/fat.h>
#include <sys/sysctl.h>
#include <signal.h>

static const char *g_prog =
  "pid86138::write:entry"
  "{"
  "  printf(\"process pid = %d\\n\", pid);"
  "}";

static int dcmdbuffered(const dtrace_bufdata_t *bufdata, void *arg) {
  if((bufdata->dtbda_recdesc != NULL) && (bufdata->dtbda_recdesc->dtrd_action == DTRACEACT_PRINTF))
    printf("BUF: %s\n", bufdata->dtbda_buffered);

  return DTRACE_HANDLE_OK;
}

static int chew(const dtrace_probedata_t *data, void *arg) {
  return DTRACE_CONSUME_THIS;
}

static int chewrec(const dtrace_probedata_t *data, const dtrace_recdesc_t *rec, void *arg) {
  if(rec == NULL)
    return (DTRACE_CONSUME_NEXT);
  return (DTRACE_CONSUME_THIS);
}

int main(int argc, char **argv) {
  int err, done = 0;
  dtrace_proginfo_t info;

  dtrace_hdl_t *g_dtp = dtrace_open(DTRACE_VERSION, DTRACE_O_ILP32, &err);
  dtrace_prog_t *prog = dtrace_program_strcompile(g_dtp, g_prog, DTRACE_PROBESPEC_NAME, 0, 0, NULL);
  dtrace_handle_buffered(g_dtp, dcmdbuffered, NULL);
  dtrace_program_exec(g_dtp, prog, &info);
  dtrace_setopt(g_dtp, "strsize", "4096");
  dtrace_setopt(g_dtp, "bufsize", "4m");
  dtrace_setopt(g_dtp, "arch", "x86_64");
  dtrace_go(g_dtp);

  while(dtrace_work(g_dtp, NULL, chew, chewrec, NULL) == DTRACE_WORKSTATUS_OKAY)
    dtrace_sleep(g_dtp);

  dtrace_stop(g_dtp);
  dtrace_close(g_dtp);
  return 0;
}