backtrace_prim.c

/**************************************************************************/
/*                                                                        */
/*                                 OCaml                                  */
/*                                                                        */
/*             Xavier Leroy, projet Gallium, INRIA Rocquencourt           */
/*                                                                        */
/*   Copyright 2006 Institut National de Recherche en Informatique et     */
/*     en Automatique.                                                    */
/*                                                                        */
/*   All rights reserved.  This file is distributed under the terms of    */
/*   the GNU Lesser General Public License version 2.1, with the          */
/*   special exception on linking described in the file LICENSE.          */
/*                                                                        */
/**************************************************************************/

#define CAML_INTERNALS

/* Stack backtrace for uncaught exceptions */

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#include "caml/alloc.h"
#include "caml/backtrace.h"
#include "caml/backtrace_prim.h"
#include "caml/memory.h"
#include "caml/misc.h"
#include "caml/mlvalues.h"
#include "caml/stack.h"

/* Returns the next frame descriptor (or NULL if none is available),
   and updates *pc and *sp to point to the following one.  */
frame_descr * caml_next_frame_descriptor(uintnat * pc, char ** sp)
{
  frame_descr * d;
  uintnat h;

  while (1) {
    h = Hash_retaddr(*pc);
    while (1) {
      d = caml_frame_descriptors[h];
      if (d == NULL) return NULL; /* happens if some code compiled without -g */
      if (d->retaddr == *pc) break;
      h = (h+1) & caml_frame_descriptors_mask;
    }
    /* Skip to next frame */
    if (d->frame_size != 0xFFFF) {
      /* Regular frame, update sp/pc and return the frame descriptor */
#ifndef Stack_grows_upwards
      *sp += (d->frame_size & 0xFFFC);
#else
      *sp -= (d->frame_size & 0xFFFC);
#endif
      *pc = Saved_return_address(*sp);
#ifdef Mask_already_scanned
      *pc = Mask_already_scanned(*pc);
#endif
      return d;
    } else {
      /* Special frame marking the top of a stack chunk for an ML callback.
         Skip C portion of stack and continue with next ML stack chunk. */
      struct caml_context * next_context = Callback_link(*sp);
      *sp = next_context->bottom_of_stack;
      *pc = next_context->last_retaddr;
      /* A null sp means no more ML stack chunks; stop here. */
      if (*sp == NULL) return NULL;
    }
  }
}

int caml_alloc_backtrace_buffer(void){
  CAMLassert(caml_backtrace_pos == 0);
  caml_backtrace_buffer =
    caml_stat_alloc_noexc(BACKTRACE_BUFFER_SIZE * sizeof(backtrace_slot));
  if (caml_backtrace_buffer == NULL) return -1;
  return 0;
}

/* Stores the return addresses contained in the given stack fragment
   into the backtrace array ; this version is performance-sensitive as
   it is called at each [raise] in a program compiled with [-g], so we
   preserved the global, statically bounded buffer of the old
   implementation -- before the more flexible
   [caml_get_current_callstack] was implemented. */
void caml_stash_backtrace(value exn, uintnat pc, char * sp, char * trapsp)
{
  if (exn != caml_backtrace_last_exn) {
    caml_backtrace_pos = 0;
    caml_backtrace_last_exn = exn;
  }

  if (caml_backtrace_buffer == NULL && caml_alloc_backtrace_buffer() == -1)
    return;

  /* iterate on each frame  */
  while (1) {
    frame_descr * descr = caml_next_frame_descriptor(&pc, &sp);
    if (descr == NULL) return;
    /* store its descriptor in the backtrace buffer */
    if (caml_backtrace_pos >= BACKTRACE_BUFFER_SIZE) return;
    caml_backtrace_buffer[caml_backtrace_pos++] = (backtrace_slot) descr;

    /* Stop when we reach the current exception handler */
#ifndef Stack_grows_upwards
    if (sp > trapsp) return;
#else
    if (sp < trapsp) return;
#endif
  }
}

/* Stores upto [max_frames_value] frames of the current call stack to
   return to the user. This is used not in an exception-raising
   context, but only when the user requests to save the trace
   (hopefully less often). Instead of using a bounded buffer as
   [caml_stash_backtrace], we first traverse the stack to compute the
   right size, then allocate space for the trace. */
CAMLprim value caml_get_current_callstack(value max_frames_value)
{
  CAMLparam1(max_frames_value);
  CAMLlocal1(trace);

  /* we use `intnat` here because, were it only `int`, passing `max_int`
     from the OCaml side would overflow on 64bits machines. */
  intnat max_frames = Long_val(max_frames_value);
  intnat trace_size;

  /* first compute the size of the trace */
  {
    uintnat pc = caml_last_return_address;
    /* note that [caml_bottom_of_stack] always points to the most recent
     * frame, independently of the [Stack_grows_upwards] setting */
    char * sp = caml_bottom_of_stack;
    char * limitsp = caml_top_of_stack;

    trace_size = 0;
    while (1) {
      frame_descr * descr = caml_next_frame_descriptor(&pc, &sp);
      if (descr == NULL) break;
      if (trace_size >= max_frames) break;
      ++trace_size;

#ifndef Stack_grows_upwards
      if (sp > limitsp) break;
#else
      if (sp < limitsp) break;
#endif
    }
  }

  trace = caml_alloc((mlsize_t) trace_size, 0);

  /* then collect the trace */
  {
    uintnat pc = caml_last_return_address;
    char * sp = caml_bottom_of_stack;
    intnat trace_pos;

    for (trace_pos = 0; trace_pos < trace_size; trace_pos++) {
      frame_descr * descr = caml_next_frame_descriptor(&pc, &sp);
      CAMLassert(descr != NULL);
      Field(trace, trace_pos) = Val_backtrace_slot((backtrace_slot) descr);
    }
  }

  CAMLreturn(trace);
}


debuginfo caml_debuginfo_extract(backtrace_slot slot)
{
  uintnat infoptr;
  frame_descr * d = (frame_descr *)slot;

  if ((d->frame_size & 1) == 0) {
    return NULL;
  }
  /* Recover debugging info */
  infoptr = ((uintnat) d +
             sizeof(char *) + sizeof(short) + sizeof(short) +
             sizeof(short) * d->num_live + sizeof(frame_descr *) - 1)
            & -sizeof(frame_descr *);
  return *((debuginfo*)infoptr);
}

debuginfo caml_debuginfo_next(debuginfo dbg)
{
  uint32_t * infoptr;

  if (dbg == NULL)
    return NULL;

  infoptr = dbg;
  infoptr += 2; /* Two packed info fields */
  return *((debuginfo*)infoptr);
}

/* Extract location information for the given frame descriptor */
void caml_debuginfo_location(debuginfo dbg, /*out*/ struct caml_loc_info * li)
{
  uint32_t info1, info2;

  /* If no debugging information available, print nothing.
     When everything is compiled with -g, this corresponds to
     compiler-inserted re-raise operations. */
  if (dbg == NULL) {
    li->loc_valid = 0;
    li->loc_is_raise = 1;
    li->loc_is_inlined = 0;
    return;
  }
  /* Recover debugging info */
  info1 = ((uint32_t *)dbg)[0];
  info2 = ((uint32_t *)dbg)[1];
  /* Format of the two info words:
       llllllllllllllllllll aaaaaaaa bbbbbbbbbb nnnnnnnnnnnnnnnnnnnnnnnn kk
                          44       36         26                       2  0
                       (32+12)    (32+4)
     k ( 2 bits): 0 if it's a call
                  1 if it's a raise
     n (24 bits): offset (in 4-byte words) of file name relative to dbg
     l (20 bits): line number
     a ( 8 bits): beginning of character range
     b (10 bits): end of character range */
  li->loc_valid = 1;
  li->loc_is_raise = (info1 & 3) == 1;
  li->loc_is_inlined = caml_debuginfo_next(dbg) != NULL;
  li->loc_filename = (char *) dbg + (info1 & 0x3FFFFFC);
  li->loc_lnum = info2 >> 12;
  li->loc_startchr = (info2 >> 4) & 0xFF;
  li->loc_endchr = ((info2 & 0xF) << 6) | (info1 >> 26);
}

CAMLprim value caml_add_debug_info(backtrace_slot start, value size,
                                   value events)
{
  return Val_unit;
}

CAMLprim value caml_remove_debug_info(backtrace_slot start)
{
  return Val_unit;
}

int caml_debug_info_available(void)
{
  return 1;
}