lread.c

/* Lisp parsing and input streams.

Copyright (C) 1985-1989, 1993-1995, 1997-2013 Free Software Foundation,
Inc.

This file is part of GNU Emacs.

GNU Emacs is free software: you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation, either version 3 of the License, or
(at your option) any later version.

GNU Emacs is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
GNU General Public License for more details.

You should have received a copy of the GNU General Public License
along with GNU Emacs.  If not, see <http://www.gnu.org/licenses/>.  */


#include <config.h>
#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <sys/file.h>
#include <errno.h>
#include <limits.h>	/* For CHAR_BIT.  */
#include <stat-time.h>
#include "lisp.h"
#include "intervals.h"
#include "character.h"
#include "buffer.h"
#include "charset.h"
#include "coding.h"
#include <epaths.h>
#include "commands.h"
#include "keyboard.h"
#include "frame.h"
#include "termhooks.h"
#include "coding.h"
#include "blockinput.h"

#ifdef MSDOS
#include "msdos.h"
#endif

#ifdef HAVE_NS
#include "nsterm.h"
#endif

#include <unistd.h>

#ifdef HAVE_SETLOCALE
#include <locale.h>
#endif /* HAVE_SETLOCALE */

#include <fcntl.h>

#ifdef HAVE_FSEEKO
#define file_offset off_t
#define file_tell ftello
#else
#define file_offset long
#define file_tell ftell
#endif

/* Hash table read constants.  */
static Lisp_Object Qhash_table, Qdata;
static Lisp_Object Qtest, Qsize;
static Lisp_Object Qweakness;
static Lisp_Object Qrehash_size;
static Lisp_Object Qrehash_threshold;

static Lisp_Object Qread_char, Qget_file_char, Qcurrent_load_list;
Lisp_Object Qstandard_input;
Lisp_Object Qvariable_documentation;
static Lisp_Object Qascii_character, Qload, Qload_file_name;
Lisp_Object Qbackquote, Qcomma, Qcomma_at, Qcomma_dot, Qfunction;
static Lisp_Object Qinhibit_file_name_operation;
static Lisp_Object Qeval_buffer_list;
Lisp_Object Qlexical_binding;
static Lisp_Object Qfile_truename, Qdo_after_load_evaluation; /* ACM 2006/5/16 */

/* Used instead of Qget_file_char while loading *.elc files compiled
   by Emacs 21 or older.  */
static Lisp_Object Qget_emacs_mule_file_char;

static Lisp_Object Qload_force_doc_strings;

static Lisp_Object Qload_in_progress;

/* The association list of objects read with the #n=object form.
   Each member of the list has the form (n . object), and is used to
   look up the object for the corresponding #n# construct.
   It must be set to nil before all top-level calls to read0.  */
static Lisp_Object read_objects;

/* List of descriptors now open for Fload.  */
static Lisp_Object load_descriptor_list;

/* File for get_file_char to read from.  Use by load.  */
static FILE *instream;

/* For use within read-from-string (this reader is non-reentrant!!)  */
static ptrdiff_t read_from_string_index;
static ptrdiff_t read_from_string_index_byte;
static ptrdiff_t read_from_string_limit;

/* Number of characters read in the current call to Fread or
   Fread_from_string.  */
static EMACS_INT readchar_count;

/* This contains the last string skipped with #@.  */
static char *saved_doc_string;
/* Length of buffer allocated in saved_doc_string.  */
static ptrdiff_t saved_doc_string_size;
/* Length of actual data in saved_doc_string.  */
static ptrdiff_t saved_doc_string_length;
/* This is the file position that string came from.  */
static file_offset saved_doc_string_position;

/* This contains the previous string skipped with #@.
   We copy it from saved_doc_string when a new string
   is put in saved_doc_string.  */
static char *prev_saved_doc_string;
/* Length of buffer allocated in prev_saved_doc_string.  */
static ptrdiff_t prev_saved_doc_string_size;
/* Length of actual data in prev_saved_doc_string.  */
static ptrdiff_t prev_saved_doc_string_length;
/* This is the file position that string came from.  */
static file_offset prev_saved_doc_string_position;

/* True means inside a new-style backquote
   with no surrounding parentheses.
   Fread initializes this to false, so we need not specbind it
   or worry about what happens to it when there is an error.  */
static bool new_backquote_flag;
static Lisp_Object Qold_style_backquotes;

/* A list of file names for files being loaded in Fload.  Used to
   check for recursive loads.  */

static Lisp_Object Vloads_in_progress;

static int read_emacs_mule_char (int, int (*) (int, Lisp_Object),
                                 Lisp_Object);

static void readevalloop (Lisp_Object, FILE *, Lisp_Object, bool,
                          Lisp_Object, Lisp_Object,
                          Lisp_Object, Lisp_Object);
static Lisp_Object load_unwind (Lisp_Object);
static Lisp_Object load_descriptor_unwind (Lisp_Object);

/* Functions that read one byte from the current source READCHARFUN
   or unreads one byte.  If the integer argument C is -1, it returns
   one read byte, or -1 when there's no more byte in the source.  If C
   is 0 or positive, it unreads C, and the return value is not
   interesting.  */

static int readbyte_for_lambda (int, Lisp_Object);
static int readbyte_from_file (int, Lisp_Object);
static int readbyte_from_string (int, Lisp_Object);

/* Handle unreading and rereading of characters.
   Write READCHAR to read a character,
   UNREAD(c) to unread c to be read again.

   These macros correctly read/unread multibyte characters.  */

#define READCHAR readchar (readcharfun, NULL)
#define UNREAD(c) unreadchar (readcharfun, c)

/* Same as READCHAR but set *MULTIBYTE to the multibyteness of the source.  */
#define READCHAR_REPORT_MULTIBYTE(multibyte) readchar (readcharfun, multibyte)

/* When READCHARFUN is Qget_file_char, Qget_emacs_mule_file_char,
   Qlambda, or a cons, we use this to keep an unread character because
   a file stream can't handle multibyte-char unreading.  The value -1
   means that there's no unread character.  */
static int unread_char;

static int
readchar (Lisp_Object readcharfun, bool *multibyte)
{
  Lisp_Object tem;
  register int c;
  int (*readbyte) (int, Lisp_Object);
  unsigned char buf[MAX_MULTIBYTE_LENGTH];
  int i, len;
  bool emacs_mule_encoding = 0;

  if (multibyte)
    *multibyte = 0;

  readchar_count++;

  if (BUFFERP (readcharfun))
    {
      register struct buffer *inbuffer = XBUFFER (readcharfun);

      ptrdiff_t pt_byte = BUF_PT_BYTE (inbuffer);

      if (! BUFFER_LIVE_P (inbuffer))
	return -1;

      if (pt_byte >= BUF_ZV_BYTE (inbuffer))
	return -1;

      if (! NILP (BVAR (inbuffer, enable_multibyte_characters)))
	{
	  /* Fetch the character code from the buffer.  */
	  unsigned char *p = BUF_BYTE_ADDRESS (inbuffer, pt_byte);
	  BUF_INC_POS (inbuffer, pt_byte);
	  c = STRING_CHAR (p);
	  if (multibyte)
	    *multibyte = 1;
	}
      else
	{
	  c = BUF_FETCH_BYTE (inbuffer, pt_byte);
	  if (! ASCII_BYTE_P (c))
	    c = BYTE8_TO_CHAR (c);
	  pt_byte++;
	}
      SET_BUF_PT_BOTH (inbuffer, BUF_PT (inbuffer) + 1, pt_byte);

      return c;
    }
  if (MARKERP (readcharfun))
    {
      register struct buffer *inbuffer = XMARKER (readcharfun)->buffer;

      ptrdiff_t bytepos = marker_byte_position (readcharfun);

      if (bytepos >= BUF_ZV_BYTE (inbuffer))
	return -1;

      if (! NILP (BVAR (inbuffer, enable_multibyte_characters)))
	{
	  /* Fetch the character code from the buffer.  */
	  unsigned char *p = BUF_BYTE_ADDRESS (inbuffer, bytepos);
	  BUF_INC_POS (inbuffer, bytepos);
	  c = STRING_CHAR (p);
	  if (multibyte)
	    *multibyte = 1;
	}
      else
	{
	  c = BUF_FETCH_BYTE (inbuffer, bytepos);
	  if (! ASCII_BYTE_P (c))
	    c = BYTE8_TO_CHAR (c);
	  bytepos++;
	}

      XMARKER (readcharfun)->bytepos = bytepos;
      XMARKER (readcharfun)->charpos++;

      return c;
    }

  if (EQ (readcharfun, Qlambda))
    {
      readbyte = readbyte_for_lambda;
      goto read_multibyte;
    }

  if (EQ (readcharfun, Qget_file_char))
    {
      readbyte = readbyte_from_file;
      goto read_multibyte;
    }

  if (STRINGP (readcharfun))
    {
      if (read_from_string_index >= read_from_string_limit)
	c = -1;
      else if (STRING_MULTIBYTE (readcharfun))
	{
	  if (multibyte)
	    *multibyte = 1;
	  FETCH_STRING_CHAR_ADVANCE_NO_CHECK (c, readcharfun,
					      read_from_string_index,
					      read_from_string_index_byte);
	}
      else
	{
	  c = SREF (readcharfun, read_from_string_index_byte);
	  read_from_string_index++;
	  read_from_string_index_byte++;
	}
      return c;
    }

  if (CONSP (readcharfun))
    {
      /* This is the case that read_vector is reading from a unibyte
	 string that contains a byte sequence previously skipped
	 because of #@NUMBER.  The car part of readcharfun is that
	 string, and the cdr part is a value of readcharfun given to
	 read_vector.  */
      readbyte = readbyte_from_string;
      if (EQ (XCDR (readcharfun), Qget_emacs_mule_file_char))
	emacs_mule_encoding = 1;
      goto read_multibyte;
    }

  if (EQ (readcharfun, Qget_emacs_mule_file_char))
    {
      readbyte = readbyte_from_file;
      emacs_mule_encoding = 1;
      goto read_multibyte;
    }

  tem = call0 (readcharfun);

  if (NILP (tem))
    return -1;
  return XINT (tem);

 read_multibyte:
  if (unread_char >= 0)
    {
      c = unread_char;
      unread_char = -1;
      return c;
    }
  c = (*readbyte) (-1, readcharfun);
  if (c < 0)
    return c;
  if (multibyte)
    *multibyte = 1;
  if (ASCII_BYTE_P (c))
    return c;
  if (emacs_mule_encoding)
    return read_emacs_mule_char (c, readbyte, readcharfun);
  i = 0;
  buf[i++] = c;
  len = BYTES_BY_CHAR_HEAD (c);
  while (i < len)
    {
      c = (*readbyte) (-1, readcharfun);
      if (c < 0 || ! TRAILING_CODE_P (c))
	{
	  while (--i > 1)
	    (*readbyte) (buf[i], readcharfun);
	  return BYTE8_TO_CHAR (buf[0]);
	}
      buf[i++] = c;
    }
  return STRING_CHAR (buf);
}

#define FROM_FILE_P(readcharfun)			\
  (EQ (readcharfun, Qget_file_char)			\
   || EQ (readcharfun, Qget_emacs_mule_file_char))

static void
skip_dyn_bytes (Lisp_Object readcharfun, ptrdiff_t n)
{
  if (FROM_FILE_P (readcharfun))
    {
      block_input ();		/* FIXME: Not sure if it's needed.  */
      fseek (instream, n, SEEK_CUR);
      unblock_input ();
    }
  else
    { /* We're not reading directly from a file.  In that case, it's difficult
	 to reliably count bytes, since these are usually meant for the file's
	 encoding, whereas we're now typically in the internal encoding.
	 But luckily, skip_dyn_bytes is used to skip over a single
	 dynamic-docstring (or dynamic byte-code) which is always quoted such
	 that \037 is the final char.  */
      int c;
      do {
	c = READCHAR;
      } while (c >= 0 && c != '\037');
    }
}

/* Unread the character C in the way appropriate for the stream READCHARFUN.
   If the stream is a user function, call it with the char as argument.  */

static void
unreadchar (Lisp_Object readcharfun, int c)
{
  readchar_count--;
  if (c == -1)
    /* Don't back up the pointer if we're unreading the end-of-input mark,
       since readchar didn't advance it when we read it.  */
    ;
  else if (BUFFERP (readcharfun))
    {
      struct buffer *b = XBUFFER (readcharfun);
      ptrdiff_t charpos = BUF_PT (b);
      ptrdiff_t bytepos = BUF_PT_BYTE (b);

      if (! NILP (BVAR (b, enable_multibyte_characters)))
	BUF_DEC_POS (b, bytepos);
      else
	bytepos--;

      SET_BUF_PT_BOTH (b, charpos - 1, bytepos);
    }
  else if (MARKERP (readcharfun))
    {
      struct buffer *b = XMARKER (readcharfun)->buffer;
      ptrdiff_t bytepos = XMARKER (readcharfun)->bytepos;

      XMARKER (readcharfun)->charpos--;
      if (! NILP (BVAR (b, enable_multibyte_characters)))
	BUF_DEC_POS (b, bytepos);
      else
	bytepos--;

      XMARKER (readcharfun)->bytepos = bytepos;
    }
  else if (STRINGP (readcharfun))
    {
      read_from_string_index--;
      read_from_string_index_byte
	= string_char_to_byte (readcharfun, read_from_string_index);
    }
  else if (CONSP (readcharfun))
    {
      unread_char = c;
    }
  else if (EQ (readcharfun, Qlambda))
    {
      unread_char = c;
    }
  else if (FROM_FILE_P (readcharfun))
    {
      unread_char = c;
    }
  else
    call1 (readcharfun, make_number (c));
}

static int
readbyte_for_lambda (int c, Lisp_Object readcharfun)
{
  return read_bytecode_char (c >= 0);
}


static int
readbyte_from_file (int c, Lisp_Object readcharfun)
{
  if (c >= 0)
    {
      block_input ();
      ungetc (c, instream);
      unblock_input ();
      return 0;
    }

  block_input ();
  c = getc (instream);

  /* Interrupted reads have been observed while reading over the network.  */
  while (c == EOF && ferror (instream) && errno == EINTR)
    {
      unblock_input ();
      QUIT;
      block_input ();
      clearerr (instream);
      c = getc (instream);
    }

  unblock_input ();

  return (c == EOF ? -1 : c);
}

static int
readbyte_from_string (int c, Lisp_Object readcharfun)
{
  Lisp_Object string = XCAR (readcharfun);

  if (c >= 0)
    {
      read_from_string_index--;
      read_from_string_index_byte
	= string_char_to_byte (string, read_from_string_index);
    }

  if (read_from_string_index >= read_from_string_limit)
    c = -1;
  else
    FETCH_STRING_CHAR_ADVANCE (c, string,
			       read_from_string_index,
			       read_from_string_index_byte);
  return c;
}


/* Read one non-ASCII character from INSTREAM.  The character is
   encoded in `emacs-mule' and the first byte is already read in
   C.  */

static int
read_emacs_mule_char (int c, int (*readbyte) (int, Lisp_Object), Lisp_Object readcharfun)
{
  /* Emacs-mule coding uses at most 4-byte for one character.  */
  unsigned char buf[4];
  int len = emacs_mule_bytes[c];
  struct charset *charset;
  int i;
  unsigned code;

  if (len == 1)
    /* C is not a valid leading-code of `emacs-mule'.  */
    return BYTE8_TO_CHAR (c);

  i = 0;
  buf[i++] = c;
  while (i < len)
    {
      c = (*readbyte) (-1, readcharfun);
      if (c < 0xA0)
	{
	  while (--i > 1)
	    (*readbyte) (buf[i], readcharfun);
	  return BYTE8_TO_CHAR (buf[0]);
	}
      buf[i++] = c;
    }

  if (len == 2)
    {
      charset = CHARSET_FROM_ID (emacs_mule_charset[buf[0]]);
      code = buf[1] & 0x7F;
    }
  else if (len == 3)
    {
      if (buf[0] == EMACS_MULE_LEADING_CODE_PRIVATE_11
	  || buf[0] == EMACS_MULE_LEADING_CODE_PRIVATE_12)
	{
	  charset = CHARSET_FROM_ID (emacs_mule_charset[buf[1]]);
	  code = buf[2] & 0x7F;
	}
      else
	{
	  charset = CHARSET_FROM_ID (emacs_mule_charset[buf[0]]);
	  code = ((buf[1] << 8) | buf[2]) & 0x7F7F;
	}
    }
  else
    {
      charset = CHARSET_FROM_ID (emacs_mule_charset[buf[1]]);
      code = ((buf[2] << 8) | buf[3]) & 0x7F7F;
    }
  c = DECODE_CHAR (charset, code);
  if (c < 0)
    Fsignal (Qinvalid_read_syntax,
	     Fcons (build_string ("invalid multibyte form"), Qnil));
  return c;
}


static Lisp_Object read_internal_start (Lisp_Object, Lisp_Object,
                                        Lisp_Object);
static Lisp_Object read0 (Lisp_Object);
static Lisp_Object read1 (Lisp_Object, int *, bool);

static Lisp_Object read_list (bool, Lisp_Object);
static Lisp_Object read_vector (Lisp_Object, bool);

static Lisp_Object substitute_object_recurse (Lisp_Object, Lisp_Object,
                                              Lisp_Object);
static void substitute_object_in_subtree (Lisp_Object,
                                          Lisp_Object);
static void substitute_in_interval (INTERVAL, Lisp_Object);


/* Get a character from the tty.  */

/* Read input events until we get one that's acceptable for our purposes.

   If NO_SWITCH_FRAME, switch-frame events are stashed
   until we get a character we like, and then stuffed into
   unread_switch_frame.

   If ASCII_REQUIRED, check function key events to see
   if the unmodified version of the symbol has a Qascii_character
   property, and use that character, if present.

   If ERROR_NONASCII, signal an error if the input we
   get isn't an ASCII character with modifiers.  If it's false but
   ASCII_REQUIRED is true, just re-read until we get an ASCII
   character.

   If INPUT_METHOD, invoke the current input method
   if the character warrants that.

   If SECONDS is a number, wait that many seconds for input, and
   return Qnil if no input arrives within that time.  */

static Lisp_Object
read_filtered_event (bool no_switch_frame, bool ascii_required,
		     bool error_nonascii, bool input_method, Lisp_Object seconds)
{
  Lisp_Object val, delayed_switch_frame;
  EMACS_TIME end_time;

#ifdef HAVE_WINDOW_SYSTEM
  if (display_hourglass_p)
    cancel_hourglass ();
#endif

  delayed_switch_frame = Qnil;

  /* Compute timeout.  */
  if (NUMBERP (seconds))
    {
      double duration = extract_float (seconds);
      EMACS_TIME wait_time = EMACS_TIME_FROM_DOUBLE (duration);
      end_time = add_emacs_time (current_emacs_time (), wait_time);
    }

  /* Read until we get an acceptable event.  */
 retry:
  do
    val = read_char (0, Qnil, (input_method ? Qnil : Qt), 0,
		     NUMBERP (seconds) ? &end_time : NULL);
  while (INTEGERP (val) && XINT (val) == -2); /* wrong_kboard_jmpbuf */

  if (BUFFERP (val))
    goto retry;

  /* `switch-frame' events are put off until after the next ASCII
     character.  This is better than signaling an error just because
     the last characters were typed to a separate minibuffer frame,
     for example.  Eventually, some code which can deal with
     switch-frame events will read it and process it.  */
  if (no_switch_frame
      && EVENT_HAS_PARAMETERS (val)
      && EQ (EVENT_HEAD_KIND (EVENT_HEAD (val)), Qswitch_frame))
    {
      delayed_switch_frame = val;
      goto retry;
    }

  if (ascii_required && !(NUMBERP (seconds) && NILP (val)))
    {
      /* Convert certain symbols to their ASCII equivalents.  */
      if (SYMBOLP (val))
	{
	  Lisp_Object tem, tem1;
	  tem = Fget (val, Qevent_symbol_element_mask);
	  if (!NILP (tem))
	    {
	      tem1 = Fget (Fcar (tem), Qascii_character);
	      /* Merge this symbol's modifier bits
		 with the ASCII equivalent of its basic code.  */
	      if (!NILP (tem1))
		XSETFASTINT (val, XINT (tem1) | XINT (Fcar (Fcdr (tem))));
	    }
	}

      /* If we don't have a character now, deal with it appropriately.  */
      if (!INTEGERP (val))
	{
	  if (error_nonascii)
	    {
	      Vunread_command_events = Fcons (val, Qnil);
	      error ("Non-character input-event");
	    }
	  else
	    goto retry;
	}
    }

  if (! NILP (delayed_switch_frame))
    unread_switch_frame = delayed_switch_frame;

#if 0

#ifdef HAVE_WINDOW_SYSTEM
  if (display_hourglass_p)
    start_hourglass ();
#endif

#endif

  return val;
}

DEFUN ("read-char", Fread_char, Sread_char, 0, 3, 0,
       doc: /* Read a character from the command input (keyboard or macro).
It is returned as a number.
If the character has modifiers, they are resolved and reflected to the
character code if possible (e.g. C-SPC -> 0).

If the user generates an event which is not a character (i.e. a mouse
click or function key event), `read-char' signals an error.  As an
exception, switch-frame events are put off until non-character events
can be read.
If you want to read non-character events, or ignore them, call
`read-event' or `read-char-exclusive' instead.

If the optional argument PROMPT is non-nil, display that as a prompt.
If the optional argument INHERIT-INPUT-METHOD is non-nil and some
input method is turned on in the current buffer, that input method
is used for reading a character.
If the optional argument SECONDS is non-nil, it should be a number
specifying the maximum number of seconds to wait for input.  If no
input arrives in that time, return nil.  SECONDS may be a
floating-point value.  */)
  (Lisp_Object prompt, Lisp_Object inherit_input_method, Lisp_Object seconds)
{
  Lisp_Object val;

  if (! NILP (prompt))
    message_with_string ("%s", prompt, 0);
  val = read_filtered_event (1, 1, 1, ! NILP (inherit_input_method), seconds);

  return (NILP (val) ? Qnil
	  : make_number (char_resolve_modifier_mask (XINT (val))));
}

DEFUN ("read-event", Fread_event, Sread_event, 0, 3, 0,
       doc: /* Read an event object from the input stream.
If the optional argument PROMPT is non-nil, display that as a prompt.
If the optional argument INHERIT-INPUT-METHOD is non-nil and some
input method is turned on in the current buffer, that input method
is used for reading a character.
If the optional argument SECONDS is non-nil, it should be a number
specifying the maximum number of seconds to wait for input.  If no
input arrives in that time, return nil.  SECONDS may be a
floating-point value.  */)
  (Lisp_Object prompt, Lisp_Object inherit_input_method, Lisp_Object seconds)
{
  if (! NILP (prompt))
    message_with_string ("%s", prompt, 0);
  return read_filtered_event (0, 0, 0, ! NILP (inherit_input_method), seconds);
}

DEFUN ("read-char-exclusive", Fread_char_exclusive, Sread_char_exclusive, 0, 3, 0,
       doc: /* Read a character from the command input (keyboard or macro).
It is returned as a number.  Non-character events are ignored.
If the character has modifiers, they are resolved and reflected to the
character code if possible (e.g. C-SPC -> 0).

If the optional argument PROMPT is non-nil, display that as a prompt.
If the optional argument INHERIT-INPUT-METHOD is non-nil and some
input method is turned on in the current buffer, that input method
is used for reading a character.
If the optional argument SECONDS is non-nil, it should be a number
specifying the maximum number of seconds to wait for input.  If no
input arrives in that time, return nil.  SECONDS may be a
floating-point value.  */)
  (Lisp_Object prompt, Lisp_Object inherit_input_method, Lisp_Object seconds)
{
  Lisp_Object val;

  if (! NILP (prompt))
    message_with_string ("%s", prompt, 0);

  val = read_filtered_event (1, 1, 0, ! NILP (inherit_input_method), seconds);

  return (NILP (val) ? Qnil
	  : make_number (char_resolve_modifier_mask (XINT (val))));
}

DEFUN ("get-file-char", Fget_file_char, Sget_file_char, 0, 0, 0,
       doc: /* Don't use this yourself.  */)
  (void)
{
  register Lisp_Object val;
  block_input ();
  XSETINT (val, getc (instream));
  unblock_input ();
  return val;
}




/* Return true if the lisp code read using READCHARFUN defines a non-nil
   `lexical-binding' file variable.  After returning, the stream is
   positioned following the first line, if it is a comment or #! line,
   otherwise nothing is read.  */

static bool
lisp_file_lexically_bound_p (Lisp_Object readcharfun)
{
  int ch = READCHAR;

  if (ch == '#')
    {
      ch = READCHAR;
      if (ch != '!')
        {
          UNREAD (ch);
          UNREAD ('#');
          return 0;
        }
      while (ch != '\n' && ch != EOF)
        ch = READCHAR;
      if (ch == '\n') ch = READCHAR;
      /* It is OK to leave the position after a #! line, since
         that is what read1 does.  */
    }

  if (ch != ';')
    /* The first line isn't a comment, just give up.  */
    {
      UNREAD (ch);
      return 0;
    }
  else
    /* Look for an appropriate file-variable in the first line.  */
    {
      bool rv = 0;
      enum {
	NOMINAL, AFTER_FIRST_DASH, AFTER_ASTERIX,
      } beg_end_state = NOMINAL;
      bool in_file_vars = 0;

#define UPDATE_BEG_END_STATE(ch)				\
  if (beg_end_state == NOMINAL)					\
    beg_end_state = (ch == '-' ? AFTER_FIRST_DASH : NOMINAL);	\
  else if (beg_end_state == AFTER_FIRST_DASH)			\
    beg_end_state = (ch == '*' ? AFTER_ASTERIX : NOMINAL);	\
  else if (beg_end_state == AFTER_ASTERIX)			\
    {								\
      if (ch == '-')						\
	in_file_vars = !in_file_vars;				\
      beg_end_state = NOMINAL;					\
    }

      /* Skip until we get to the file vars, if any.  */
      do
	{
	  ch = READCHAR;
	  UPDATE_BEG_END_STATE (ch);
	}
      while (!in_file_vars && ch != '\n' && ch != EOF);

      while (in_file_vars)
	{
	  char var[100], val[100];
	  unsigned i;

	  ch = READCHAR;

	  /* Read a variable name.  */
	  while (ch == ' ' || ch == '\t')
	    ch = READCHAR;

	  i = 0;
	  while (ch != ':' && ch != '\n' && ch != EOF && in_file_vars)
	    {
	      if (i < sizeof var - 1)
		var[i++] = ch;
	      UPDATE_BEG_END_STATE (ch);
	      ch = READCHAR;
	    }

	  /* Stop scanning if no colon was found before end marker.  */
	  if (!in_file_vars || ch == '\n' || ch == EOF)
	    break;

	  while (i > 0 && (var[i - 1] == ' ' || var[i - 1] == '\t'))
	    i--;
	  var[i] = '\0';

	  if (ch == ':')
	    {
	      /* Read a variable value.  */
	      ch = READCHAR;

	      while (ch == ' ' || ch == '\t')
		ch = READCHAR;

	      i = 0;
	      while (ch != ';' && ch != '\n' && ch != EOF && in_file_vars)
		{
		  if (i < sizeof val - 1)
		    val[i++] = ch;
		  UPDATE_BEG_END_STATE (ch);
		  ch = READCHAR;
		}
	      if (! in_file_vars)
		/* The value was terminated by an end-marker, which remove.  */
		i -= 3;
	      while (i > 0 && (val[i - 1] == ' ' || val[i - 1] == '\t'))
		i--;
	      val[i] = '\0';

	      if (strcmp (var, "lexical-binding") == 0)
		/* This is it...  */
		{
		  rv = (strcmp (val, "nil") != 0);
		  break;
		}
	    }
	}

      while (ch != '\n' && ch != EOF)
	ch = READCHAR;

      return rv;
    }
}

/* Value is a version number of byte compiled code if the file
   associated with file descriptor FD is a compiled Lisp file that's
   safe to load.  Only files compiled with Emacs are safe to load.
   Files compiled with XEmacs can lead to a crash in Fbyte_code
   because of an incompatible change in the byte compiler.  */

static int
safe_to_load_version (int fd)
{
  char buf[512];
  int nbytes, i;
  int version = 1;

  /* Read the first few bytes from the file, and look for a line
     specifying the byte compiler version used.  */
  nbytes = emacs_read (fd, buf, sizeof buf);
  if (nbytes > 0)
    {
      /* Skip to the next newline, skipping over the initial `ELC'
	 with NUL bytes following it, but note the version.  */
      for (i = 0; i < nbytes && buf[i] != '\n'; ++i)
	if (i == 4)
	  version = buf[i];

      if (i >= nbytes
	  || fast_c_string_match_ignore_case (Vbytecomp_version_regexp,