dbus-string.c 72.8 KB
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/* -*- mode: C; c-file-style: "gnu" -*- */
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/* dbus-string.c String utility class (internal to D-Bus implementation)
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 * 
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 * Copyright (C) 2002, 2003, 2004, 2005 Red Hat, Inc.
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 * Copyright (C) 2006 Ralf Habacker <ralf.habacker@freenet.de>
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 *
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 * Licensed under the Academic Free License version 2.1
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 * 
 * This program 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 2 of the License, or
 * (at your option) any later version.
 *
 * This program 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 this program; if not, write to the Free Software
 * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
 *
 */

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#include "dbus-internals.h"
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#include "dbus-string.h"
/* we allow a system header here, for speed/convenience */
#include <string.h>
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/* for vsnprintf */
#include <stdio.h>
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#define DBUS_CAN_USE_DBUS_STRING_PRIVATE 1
#include "dbus-string-private.h"
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#include "dbus-marshal-basic.h" /* probably should be removed by moving the usage of DBUS_TYPE
                                 * into the marshaling-related files
                                 */
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/* for DBUS_VA_COPY */
#include "dbus-sysdeps.h"
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/**
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 * @defgroup DBusString DBusString class
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 * @ingroup  DBusInternals
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 * @brief DBusString data structure for safer string handling
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 *
 * Types and functions related to DBusString. DBusString is intended
 * to be a string class that makes it hard to mess up security issues
 * (and just in general harder to write buggy code).  It should be
 * used (or extended and then used) rather than the libc stuff in
 * string.h.  The string class is a bit inconvenient at spots because
 * it handles out-of-memory failures and tries to be extra-robust.
 * 
 * A DBusString has a maximum length set at initialization time; this
 * can be used to ensure that a buffer doesn't get too big.  The
 * _dbus_string_lengthen() method checks for overflow, and for max
 * length being exceeded.
 * 
 * Try to avoid conversion to a plain C string, i.e. add methods on
 * the string object instead, only convert to C string when passing
 * things out to the public API. In particular, no sprintf, strcpy,
 * strcat, any of that should be used. The GString feature of
 * accepting negative numbers for "length of string" is also absent,
 * because it could keep us from detecting bogus huge lengths. i.e. if
 * we passed in some bogus huge length it would be taken to mean
 * "current length of string" instead of "broken crack"
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 *
 * @todo #DBusString needs a lot of cleaning up; some of the
 * API is no longer used, and the API is pretty inconsistent.
 * In particular all the "append" APIs, especially those involving
 * alignment but probably lots of them, are no longer used by the
 * marshaling code which always does "inserts" now.
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 */

/**
 * @addtogroup DBusString
 * @{
 */

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static void
fixup_alignment (DBusRealString *real)
{
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  unsigned char *aligned;
  unsigned char *real_block;
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  unsigned int old_align_offset;

  /* we have to have extra space in real->allocated for the align offset and nul byte */
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  _dbus_assert (real->len <= real->allocated - _DBUS_STRING_ALLOCATION_PADDING);
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  old_align_offset = real->align_offset;
  real_block = real->str - old_align_offset;
  
  aligned = _DBUS_ALIGN_ADDRESS (real_block, 8);

  real->align_offset = aligned - real_block;
  real->str = aligned;
  
  if (old_align_offset != real->align_offset)
    {
      /* Here comes the suck */
      memmove (real_block + real->align_offset,
               real_block + old_align_offset,
               real->len + 1);
    }

  _dbus_assert (real->align_offset < 8);
  _dbus_assert (_DBUS_ALIGN_ADDRESS (real->str, 8) == real->str);
}
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static void
undo_alignment (DBusRealString *real)
{
  if (real->align_offset != 0)
    {
      memmove (real->str - real->align_offset,
               real->str,
               real->len + 1);

      real->str = real->str - real->align_offset;
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      real->align_offset = 0;
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    }
}

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/**
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 * Initializes a string that can be up to the given allocation size
 * before it has to realloc. The string starts life with zero length.
 * The string must eventually be freed with _dbus_string_free().
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 * 
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 * @param str memory to hold the string
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 * @param allocate_size amount to preallocate
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 * @returns #TRUE on success, #FALSE if no memory
 */
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dbus_bool_t
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_dbus_string_init_preallocated (DBusString *str,
                                int         allocate_size)
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{
  DBusRealString *real;
  
  _dbus_assert (str != NULL);

  _dbus_assert (sizeof (DBusString) == sizeof (DBusRealString));
  
  real = (DBusRealString*) str;

  /* It's very important not to touch anything
   * other than real->str if we're going to fail,
   * since we also use this function to reset
   * an existing string, e.g. in _dbus_string_steal_data()
   */
  
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  real->str = dbus_malloc (_DBUS_STRING_ALLOCATION_PADDING + allocate_size);
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  if (real->str == NULL)
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    return FALSE;  
  
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  real->allocated = _DBUS_STRING_ALLOCATION_PADDING + allocate_size;
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  real->len = 0;
  real->str[real->len] = '\0';
  
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  real->max_length = _DBUS_STRING_MAX_MAX_LENGTH;
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  real->constant = FALSE;
  real->locked = FALSE;
  real->invalid = FALSE;
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  real->align_offset = 0;
  
  fixup_alignment (real);
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  return TRUE;
}

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/**
 * Initializes a string. The string starts life with zero length.  The
 * string must eventually be freed with _dbus_string_free().
 * 
 * @param str memory to hold the string
 * @returns #TRUE on success, #FALSE if no memory
 */
dbus_bool_t
_dbus_string_init (DBusString *str)
{
  return _dbus_string_init_preallocated (str, 0);
}

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#ifdef DBUS_BUILD_TESTS
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/* The max length thing is sort of a historical artifact
 * from a feature that turned out to be dumb; perhaps
 * we should purge it entirely. The problem with
 * the feature is that it looks like memory allocation
 * failure, but is not a transient or resolvable failure.
 */
static void
set_max_length (DBusString *str,
                int         max_length)
{
  DBusRealString *real;
  
  real = (DBusRealString*) str;

  real->max_length = max_length;
}
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#endif /* DBUS_BUILD_TESTS */
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/**
 * Initializes a constant string. The value parameter is not copied
 * (should be static), and the string may never be modified.
 * It is safe but not necessary to call _dbus_string_free()
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 * on a const string. The string has a length limit of MAXINT - 8.
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 * 
 * @param str memory to use for the string
 * @param value a string to be stored in str (not copied!!!)
 */
void
_dbus_string_init_const (DBusString *str,
                         const char *value)
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{
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  _dbus_assert (value != NULL);
  
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  _dbus_string_init_const_len (str, value,
                               strlen (value));
}

/**
 * Initializes a constant string with a length. The value parameter is
 * not copied (should be static), and the string may never be
 * modified.  It is safe but not necessary to call _dbus_string_free()
 * on a const string.
 * 
 * @param str memory to use for the string
 * @param value a string to be stored in str (not copied!!!)
 * @param len the length to use
 */
void
_dbus_string_init_const_len (DBusString *str,
                             const char *value,
                             int         len)
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{
  DBusRealString *real;
  
  _dbus_assert (str != NULL);
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  _dbus_assert (len == 0 || value != NULL);
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  _dbus_assert (len <= _DBUS_STRING_MAX_MAX_LENGTH);
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  _dbus_assert (len >= 0);
  
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  real = (DBusRealString*) str;
  
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  real->str = (unsigned char*) value;
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  real->len = len;
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  real->allocated = real->len + _DBUS_STRING_ALLOCATION_PADDING; /* a lie, just to avoid special-case assertions... */
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  real->max_length = real->len + 1;
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  real->constant = TRUE;
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  real->locked = TRUE;
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  real->invalid = FALSE;
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  real->align_offset = 0;
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  /* We don't require const strings to be 8-byte aligned as the
   * memory is coming from elsewhere.
   */
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}

/**
 * Frees a string created by _dbus_string_init().
 *
 * @param str memory where the string is stored.
 */
void
_dbus_string_free (DBusString *str)
{
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  DBusRealString *real = (DBusRealString*) str;
  DBUS_GENERIC_STRING_PREAMBLE (real);
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  if (real->constant)
    return;
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  dbus_free (real->str - real->align_offset);
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  real->invalid = TRUE;
}

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#ifdef DBUS_BUILD_TESTS
/* Not using this feature at the moment,
 * so marked DBUS_BUILD_TESTS-only
 */
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/**
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 * Locks a string such that any attempts to change the string will
 * result in aborting the program. Also, if the string is wasting a
 * lot of memory (allocation is sufficiently larger than what the
 * string is really using), _dbus_string_lock() will realloc the
 * string's data to "compact" it.
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 *
 * @param str the string to lock.
 */
void
_dbus_string_lock (DBusString *str)
{  
  DBUS_LOCKED_STRING_PREAMBLE (str); /* can lock multiple times */

  real->locked = TRUE;

  /* Try to realloc to avoid excess memory usage, since
   * we know we won't change the string further
   */
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#define MAX_WASTE 48
  if (real->allocated - MAX_WASTE > real->len)
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    {
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      unsigned char *new_str;
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      int new_allocated;

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      new_allocated = real->len + _DBUS_STRING_ALLOCATION_PADDING;
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      new_str = dbus_realloc (real->str - real->align_offset,
                              new_allocated);
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      if (new_str != NULL)
        {
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          real->str = new_str + real->align_offset;
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          real->allocated = new_allocated;
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          fixup_alignment (real);
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        }
    }
}
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#endif /* DBUS_BUILD_TESTS */
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static dbus_bool_t
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reallocate_for_length (DBusRealString *real,
                       int             new_length)
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{
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  int new_allocated;
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  unsigned char *new_str;
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  /* at least double our old allocation to avoid O(n), avoiding
   * overflow
   */
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  if (real->allocated > (_DBUS_STRING_MAX_MAX_LENGTH + _DBUS_STRING_ALLOCATION_PADDING) / 2)
    new_allocated = _DBUS_STRING_MAX_MAX_LENGTH + _DBUS_STRING_ALLOCATION_PADDING;
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  else
    new_allocated = real->allocated * 2;
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  /* if you change the code just above here, run the tests without
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   * the following assert-only hack before you commit
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   */
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  /* This is keyed off asserts in addition to tests so when you
   * disable asserts to profile, you don't get this destroyer
   * of profiles.
   */
#ifdef DBUS_DISABLE_ASSERT
#else
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#ifdef DBUS_BUILD_TESTS
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  new_allocated = 0; /* ensure a realloc every time so that we go
                      * through all malloc failure codepaths
                      */
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#endif /* DBUS_BUILD_TESTS */
#endif /* !DBUS_DISABLE_ASSERT */

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  /* But be sure we always alloc at least space for the new length */
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  new_allocated = MAX (new_allocated,
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                       new_length + _DBUS_STRING_ALLOCATION_PADDING);
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  _dbus_assert (new_allocated >= real->allocated); /* code relies on this */
  new_str = dbus_realloc (real->str - real->align_offset, new_allocated);
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  if (_DBUS_UNLIKELY (new_str == NULL))
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    return FALSE;
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  real->str = new_str + real->align_offset;
  real->allocated = new_allocated;
  fixup_alignment (real);
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  return TRUE;
}

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static dbus_bool_t
set_length (DBusRealString *real,
            int             new_length)
{
  /* Note, we are setting the length not including nul termination */

  /* exceeding max length is the same as failure to allocate memory */
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  if (_DBUS_UNLIKELY (new_length > real->max_length))
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    return FALSE;
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  else if (new_length > (real->allocated - _DBUS_STRING_ALLOCATION_PADDING) &&
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           _DBUS_UNLIKELY (!reallocate_for_length (real, new_length)))
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    return FALSE;
  else
    {
      real->len = new_length;
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      real->str[new_length] = '\0';
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      return TRUE;
    }
}

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static dbus_bool_t
open_gap (int             len,
          DBusRealString *dest,
          int             insert_at)
{
  if (len == 0)
    return TRUE;

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  if (len > dest->max_length - dest->len)
    return FALSE; /* detected overflow of dest->len + len below */
  
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  if (!set_length (dest, dest->len + len))
    return FALSE;

  memmove (dest->str + insert_at + len, 
           dest->str + insert_at,
           dest->len - len - insert_at);

  return TRUE;
}

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#ifndef _dbus_string_get_data
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/**
 * Gets the raw character buffer from the string.  The returned buffer
 * will be nul-terminated, but note that strings may contain binary
 * data so there may be extra nul characters prior to the termination.
 * This function should be little-used, extend DBusString or add
 * stuff to dbus-sysdeps.c instead. It's an error to use this
 * function on a const string.
 *
 * @param str the string
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 * @returns the data
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 */
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char*
_dbus_string_get_data (DBusString *str)
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{
  DBUS_STRING_PREAMBLE (str);
  
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  return (char*) real->str;
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}
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#endif /* _dbus_string_get_data */
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/* only do the function if we don't have the macro */
#ifndef _dbus_string_get_const_data
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/**
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 * Gets the raw character buffer from a const string.
 *
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 * @param str the string
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 * @returns the string data
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 */
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const char*
_dbus_string_get_const_data (const DBusString  *str)
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{
  DBUS_CONST_STRING_PREAMBLE (str);
  
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  return (const char*) real->str;
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}
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#endif /* _dbus_string_get_const_data */
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/**
 * Gets a sub-portion of the raw character buffer from the
 * string. The "len" field is required simply for error
 * checking, to be sure you don't try to use more
 * string than exists. The nul termination of the
 * returned buffer remains at the end of the entire
 * string, not at start + len.
 *
 * @param str the string
 * @param start byte offset to return
 * @param len length of segment to return
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 * @returns the string data
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 */
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char*
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_dbus_string_get_data_len (DBusString *str,
                           int         start,
                           int         len)
{
  DBUS_STRING_PREAMBLE (str);
  _dbus_assert (start >= 0);
  _dbus_assert (len >= 0);
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  _dbus_assert (start <= real->len);
  _dbus_assert (len <= real->len - start);
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  return (char*) real->str + start;
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}

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/* only do the function if we don't have the macro */
#ifndef _dbus_string_get_const_data_len
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/**
 * const version of _dbus_string_get_data_len().
 *
 * @param str the string
 * @param start byte offset to return
 * @param len length of segment to return
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 * @returns the string data
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 */
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const char*
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_dbus_string_get_const_data_len (const DBusString  *str,
                                 int                start,
                                 int                len)
{
  DBUS_CONST_STRING_PREAMBLE (str);
  _dbus_assert (start >= 0);
  _dbus_assert (len >= 0);
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  _dbus_assert (start <= real->len);
  _dbus_assert (len <= real->len - start);
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  return (const char*) real->str + start;
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}
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#endif /* _dbus_string_get_const_data_len */
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/* only do the function if we don't have the macro */
#ifndef _dbus_string_set_byte
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/**
 * Sets the value of the byte at the given position.
 *
 * @param str the string
 * @param i the position
 * @param byte the new value
 */
void
_dbus_string_set_byte (DBusString    *str,
                       int            i,
                       unsigned char  byte)
{
  DBUS_STRING_PREAMBLE (str);
  _dbus_assert (i < real->len);
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  _dbus_assert (i >= 0);
  
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  real->str[i] = byte;
}
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#endif /* _dbus_string_set_byte */
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/* only have the function if we didn't create a macro */
#ifndef _dbus_string_get_byte
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/**
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 * Gets the byte at the given position. It is
 * allowed to ask for the nul byte at the end of
 * the string.
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 *
 * @param str the string
 * @param start the position
 * @returns the byte at that position
 */
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unsigned char
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_dbus_string_get_byte (const DBusString  *str,
                       int                start)
{
  DBUS_CONST_STRING_PREAMBLE (str);
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  _dbus_assert (start <= real->len);
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  _dbus_assert (start >= 0);
  
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  return real->str[start];
}
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#endif /* _dbus_string_get_byte */
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/**
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 * Inserts a number of bytes of a given value at the
 * given position.
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 *
 * @param str the string
 * @param i the position
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 * @param n_bytes number of bytes
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 * @param byte the value to insert
 * @returns #TRUE on success
 */
dbus_bool_t
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_dbus_string_insert_bytes (DBusString   *str,
			   int           i,
			   int           n_bytes,
			   unsigned char byte)
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{
  DBUS_STRING_PREAMBLE (str);
  _dbus_assert (i <= real->len);
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  _dbus_assert (i >= 0);
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  _dbus_assert (n_bytes >= 0);

  if (n_bytes == 0)
    return TRUE;
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  if (!open_gap (n_bytes, real, i))
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    return FALSE;
  
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  memset (real->str + i, byte, n_bytes);
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  return TRUE;
}

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/**
 * Inserts a single byte at the given position.
 *
 * @param str the string
 * @param i the position
 * @param byte the value to insert
 * @returns #TRUE on success
 */
dbus_bool_t
_dbus_string_insert_byte (DBusString   *str,
			   int           i,
			   unsigned char byte)
{
  DBUS_STRING_PREAMBLE (str);
  _dbus_assert (i <= real->len);
  _dbus_assert (i >= 0);
  
  if (!open_gap (1, real, i))
    return FALSE;

  real->str[i] = byte;

  return TRUE;
}

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/**
 * Like _dbus_string_get_data(), but removes the
 * gotten data from the original string. The caller
 * must free the data returned. This function may
 * fail due to lack of memory, and return #FALSE.
 *
 * @param str the string
 * @param data_return location to return the buffer
 * @returns #TRUE on success
 */
dbus_bool_t
_dbus_string_steal_data (DBusString        *str,
                         char             **data_return)
{
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  int old_max_length;
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  DBUS_STRING_PREAMBLE (str);
  _dbus_assert (data_return != NULL);
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  undo_alignment (real);
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  *data_return = (char*) real->str;
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  old_max_length = real->max_length;
  
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  /* reset the string */
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  if (!_dbus_string_init (str))
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    {
      /* hrm, put it back then */
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      real->str = (unsigned char*) *data_return;
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      *data_return = NULL;
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      fixup_alignment (real);
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      return FALSE;
    }

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  real->max_length = old_max_length;

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  return TRUE;
}

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#ifdef DBUS_BUILD_TESTS
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/**
 * Like _dbus_string_get_data_len(), but removes the gotten data from
 * the original string. The caller must free the data returned. This
 * function may fail due to lack of memory, and return #FALSE.
 * The returned string is nul-terminated and has length len.
 *
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 * @todo this function is broken because on failure it
 * may corrupt the source string.
 * 
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 * @param str the string
 * @param data_return location to return the buffer
 * @param start the start of segment to steal
 * @param len the length of segment to steal
 * @returns #TRUE on success
 */
dbus_bool_t
_dbus_string_steal_data_len (DBusString        *str,
                             char             **data_return,
                             int                start,
                             int                len)
{
  DBusString dest;
  DBUS_STRING_PREAMBLE (str);
  _dbus_assert (data_return != NULL);
  _dbus_assert (start >= 0);
  _dbus_assert (len >= 0);
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  _dbus_assert (start <= real->len);
  _dbus_assert (len <= real->len - start);
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  if (!_dbus_string_init (&dest))
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    return FALSE;

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  set_max_length (&dest, real->max_length);
  
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  if (!_dbus_string_move_len (str, start, len, &dest, 0))
    {
      _dbus_string_free (&dest);
      return FALSE;
    }
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  _dbus_warn ("Broken code in _dbus_string_steal_data_len(), see @todo, FIXME\n");
  if (!_dbus_string_steal_data (&dest, data_return))
    {
      _dbus_string_free (&dest);
      return FALSE;
    }

  _dbus_string_free (&dest);
  return TRUE;
}
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#endif /* DBUS_BUILD_TESTS */
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/**
 * Copies the data from the string into a char*
 *
 * @param str the string
 * @param data_return place to return the data
 * @returns #TRUE on success, #FALSE on no memory
 */
dbus_bool_t
_dbus_string_copy_data (const DBusString  *str,
                        char             **data_return)
{
  DBUS_CONST_STRING_PREAMBLE (str);
  _dbus_assert (data_return != NULL);
  
  *data_return = dbus_malloc (real->len + 1);
  if (*data_return == NULL)
    return FALSE;

  memcpy (*data_return, real->str, real->len + 1);

  return TRUE;
}

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/**
 * Copies the contents of a DBusString into a different
 * buffer. The resulting buffer will be nul-terminated.
 * 
 * @param str a string
 * @param buffer a C buffer to copy data to
 * @param avail_len maximum length of C buffer
 */
void
_dbus_string_copy_to_buffer (const DBusString  *str,
			     char              *buffer,
			     int                avail_len)
{
  int copy_len;
  DBUS_CONST_STRING_PREAMBLE (str);

  _dbus_assert (avail_len >= 0);

  copy_len = MIN (avail_len, real->len+1);
  memcpy (buffer, real->str, copy_len);
  if (avail_len > 0 && avail_len == copy_len)
    buffer[avail_len-1] = '\0';
}

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/**
 * Copies a segment of the string into a char*
 *
 * @param str the string
 * @param data_return place to return the data
 * @param start start index
 * @param len length to copy
 * @returns #FALSE if no memory
 */
dbus_bool_t
_dbus_string_copy_data_len (const DBusString  *str,
                            char             **data_return,
                            int                start,
                            int                len)
{
  DBusString dest;

  DBUS_CONST_STRING_PREAMBLE (str);
  _dbus_assert (data_return != NULL);
  _dbus_assert (start >= 0);
  _dbus_assert (len >= 0);
  _dbus_assert (start <= real->len);
  _dbus_assert (len <= real->len - start);

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  if (!_dbus_string_init (&dest))
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    return FALSE;

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  set_max_length (&dest, real->max_length);

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  if (!_dbus_string_copy_len (str, start, len, &dest, 0))
    {
      _dbus_string_free (&dest);
      return FALSE;
    }

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  if (!_dbus_string_steal_data (&dest, data_return))
    {
      _dbus_string_free (&dest);
      return FALSE;
    }

  _dbus_string_free (&dest);
  return TRUE;
}
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#endif /* DBUS_BUILD_TESTS */
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/* Only have the function if we don't have the macro */
#ifndef _dbus_string_get_length
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/**
 * Gets the length of a string (not including nul termination).
 *
 * @returns the length.
 */
int
_dbus_string_get_length (const DBusString  *str)
{
  DBUS_CONST_STRING_PREAMBLE (str);
  
  return real->len;
}
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#endif /* !_dbus_string_get_length */
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/**
 * Makes a string longer by the given number of bytes.  Checks whether
 * adding additional_length to the current length would overflow an
 * integer, and checks for exceeding a string's max length.
 * The new bytes are not initialized, other than nul-terminating
 * the end of the string. The uninitialized bytes may contain
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 * nul bytes or other junk.
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 *
 * @param str a string
 * @param additional_length length to add to the string.
 * @returns #TRUE on success.
 */
dbus_bool_t
_dbus_string_lengthen (DBusString *str,
                       int         additional_length)
{
  DBUS_STRING_PREAMBLE (str);  
  _dbus_assert (additional_length >= 0);
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  if (_DBUS_UNLIKELY (additional_length > real->max_length - real->len))
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    return FALSE; /* would overflow */
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  return set_length (real,
                     real->len + additional_length);
}

/**
 * Makes a string shorter by the given number of bytes.
 *
 * @param str a string
 * @param length_to_remove length to remove from the string.
 */
void
_dbus_string_shorten (DBusString *str,
                      int         length_to_remove)
{
  DBUS_STRING_PREAMBLE (str);
  _dbus_assert (length_to_remove >= 0);
  _dbus_assert (length_to_remove <= real->len);

  set_length (real,
              real->len - length_to_remove);
}

/**
 * Sets the length of a string. Can be used to truncate or lengthen
 * the string. If the string is lengthened, the function may fail and
 * return #FALSE. Newly-added bytes are not initialized, as with
 * _dbus_string_lengthen().
 *
 * @param str a string
 * @param length new length of the string.
 * @returns #FALSE on failure.
 */
dbus_bool_t
_dbus_string_set_length (DBusString *str,
                         int         length)
{
  DBUS_STRING_PREAMBLE (str);
  _dbus_assert (length >= 0);

  return set_length (real, length);
}

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align_insert_point_then_open_gap (DBusString *str,
                                  int        *insert_at_p,
                                  int         alignment,
                                  int         gap_size)
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{
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  unsigned long new_len; /* ulong to avoid _DBUS_ALIGN_VALUE overflow */
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  unsigned long gap_pos;
  int insert_at;
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  int delta;
  DBUS_STRING_PREAMBLE (str);
  _dbus_assert (alignment >= 1);
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  _dbus_assert (alignment <= 8); /* it has to be a bug if > 8 */
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  insert_at = *insert_at_p;

  _dbus_assert (insert_at <= real->len);
  
  gap_pos = _DBUS_ALIGN_VALUE (insert_at, alignment);
  new_len = real->len + (gap_pos - insert_at) + gap_size;
  
  if (_DBUS_UNLIKELY (new_len > (unsigned long) real->max_length))
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    return FALSE;
  
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  delta = new_len - real->len;
  _dbus_assert (delta >= 0);

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  if (delta == 0) /* only happens if gap_size == 0 and insert_at is aligned already */
    {
      _dbus_assert (((unsigned long) *insert_at_p) == gap_pos);
      return TRUE;
    }
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  if (_DBUS_UNLIKELY (!open_gap (new_len - real->len,
                                 real, insert_at)))
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    return FALSE;

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  /* nul the padding if we had to add any padding */
  if (gap_size < delta)
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    {
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      memset (&real->str[insert_at], '\0',
              gap_pos - insert_at);
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    }
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  *insert_at_p = gap_pos;
  
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  return TRUE;
}

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static dbus_bool_t
align_length_then_lengthen (DBusString *str,
                            int         alignment,
                            int         then_lengthen_by)
{
  int insert_at;

  insert_at = _dbus_string_get_length (str);
  
  return align_insert_point_then_open_gap (str,
                                           &insert_at,
                                           alignment, then_lengthen_by);
}

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/**
 * Align the length of a string to a specific alignment (typically 4 or 8)
 * by appending nul bytes to the string.
 *
 * @param str a string
 * @param alignment the alignment
 * @returns #FALSE if no memory
 */
dbus_bool_t
_dbus_string_align_length (DBusString *str,
                           int         alignment)
{
  return align_length_then_lengthen (str, alignment, 0);
}

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/**
 * Preallocate extra_bytes such that a future lengthening of the
 * string by extra_bytes is guaranteed to succeed without an out of
 * memory error.
 *
 * @param str a string
 * @param extra_bytes bytes to alloc
 * @returns #FALSE if no memory
 */
dbus_bool_t
_dbus_string_alloc_space (DBusString        *str,
                          int                extra_bytes)
{
  if (!_dbus_string_lengthen (str, extra_bytes))
    return FALSE;
  _dbus_string_shorten (str, extra_bytes);

  return TRUE;
}

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static dbus_bool_t
append (DBusRealString *real,
        const char     *buffer,
        int             buffer_len)
{
  if (buffer_len == 0)
    return TRUE;

  if (!_dbus_string_lengthen ((DBusString*)real, buffer_len))
    return FALSE;

  memcpy (real->str + (real->len - buffer_len),
          buffer,
          buffer_len);

  return TRUE;
}

/**
 * Appends a nul-terminated C-style string to a DBusString.
 *
 * @param str the DBusString
 * @param buffer the nul-terminated characters to append
 * @returns #FALSE if not enough memory.
 */
dbus_bool_t
_dbus_string_append (DBusString *str,
                     const char *buffer)
{
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  unsigned long buffer_len;
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  DBUS_STRING_PREAMBLE (str);
  _dbus_assert (buffer != NULL);
  
  buffer_len = strlen (buffer);
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  if (buffer_len > (unsigned long) real->max_length)
    return FALSE;
  
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  return append (real, buffer, buffer_len);
}

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/** assign 2 bytes from one string to another */
#define ASSIGN_2_OCTETS(p, octets) \
  *((dbus_uint16_t*)(p)) = *((dbus_uint16_t*)(octets));
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/** assign 4 bytes from one string to another */
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#define ASSIGN_4_OCTETS(p, octets) \
  *((dbus_uint32_t*)(p)) = *((dbus_uint32_t*)(octets));

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#define ASSIGN_8_OCTETS(p, octets) \
  *((dbus_uint64_t*)(p)) = *((dbus_uint64_t*)(octets));
#else
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/** assign 8 bytes from one string to another */
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#define ASSIGN_8_OCTETS(p, octets)              \
do {                                            \
  unsigned char *b;                             \
                                                \
  b = p;                                        \
                                                \
  *b++ = octets[0];                             \
  *b++ = octets[1];                             \
  *b++ = octets[2];                             \
  *b++ = octets[3];                             \
  *b++ = octets[4];                             \
  *b++ = octets[5];                             \
  *b++ = octets[6];                             \
  *b++ = octets[7];                             \
  _dbus_assert (b == p + 8);                    \
} while (0)
#endif /* DBUS_HAVE_INT64 */

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/**
 * Appends 4 bytes aligned on a 4 byte boundary
 * with any alignment padding initialized to 0.
 *
 * @param str the DBusString
 * @param octets 4 bytes to append
 * @returns #FALSE if not enough memory.
 */
dbus_bool_t
_dbus_string_append_4_aligned (DBusString         *str,
                               const unsigned char octets[4])
{
  DBUS_STRING_PREAMBLE (str);
  
  if (!align_length_then_lengthen (str, 4, 4))
    return FALSE;

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  ASSIGN_4_OCTETS (real->str + (real->len - 4), octets);
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  return TRUE;
}
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#endif /* DBUS_BUILD_TESTS */
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/**
 * Appends 8 bytes aligned on an 8 byte boundary
 * with any alignment padding initialized to 0.
 *
 * @param str the DBusString
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 * @param octets 8 bytes to append
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 * @returns #FALSE if not enough memory.
 */
dbus_bool_t
_dbus_string_append_8_aligned (DBusString         *str,
                               const unsigned char octets[8])
{
  DBUS_STRING_PREAMBLE (str);
  
  if (!align_length_then_lengthen (str, 8, 8))
    return FALSE;

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  ASSIGN_8_OCTETS (real->str + (real->len - 8), octets);

  return TRUE;
}
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#endif /* DBUS_BUILD_TESTS */
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/**
 * Inserts 2 bytes aligned on a 2 byte boundary
 * with any alignment padding initialized to 0.
 *
 * @param str the DBusString
 * @param insert_at where to insert
 * @param octets 2 bytes to insert
 * @returns #FALSE if not enough memory.
 */
dbus_bool_t
_dbus_string_insert_2_aligned (DBusString         *str,
                               int                 insert_at,
                               const unsigned char octets[4])
{
  DBUS_STRING_PREAMBLE (str);
  
  if (!align_insert_point_then_open_gap (str, &insert_at, 2, 2))
    return FALSE;

  ASSIGN_2_OCTETS (real->str + insert_at, octets);

  return TRUE;
}

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/**
 * Inserts 4 bytes aligned on a 4 byte boundary
 * with any alignment padding initialized to 0.
 *
 * @param str the DBusString
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 * @param insert_at where to insert
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 * @param octets 4 bytes to insert
 * @returns #FALSE if not enough memory.
 */
dbus_bool_t
_dbus_string_insert_4_aligned (DBusString         *str,
                               int                 insert_at,
                               const unsigned char octets[4])
{
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  DBUS_STRING_PREAMBLE (str);
  
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  if (!align_insert_point_then_open_gap (str, &insert_at, 4, 4))
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    return FALSE;

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  ASSIGN_4_OCTETS (real->str + insert_at, octets);

  return TRUE;
}

/**
 * Inserts 8 bytes aligned on an 8 byte boundary
 * with any alignment padding initialized to 0.
 *
 * @param str the DBusString
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 * @param octets 8 bytes to insert
 * @returns #FALSE if not enough memory.
 */
dbus_bool_t
_dbus_string_insert_8_aligned (DBusString         *str,
                               int                 insert_at,
                               const unsigned char octets[8])
{
  DBUS_STRING_PREAMBLE (str);
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  if (!align_insert_point_then_open_gap (str, &insert_at, 8, 8))
    return FALSE;

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  _dbus_assert (_DBUS_ALIGN_VALUE (insert_at, 8) == (unsigned) insert_at);
  
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  ASSIGN_8_OCTETS (real->str + insert_at, octets);
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  return TRUE;
}

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/**
 * Inserts padding at *insert_at such to align it to the given
 * boundary. Initializes the padding to nul bytes. Sets *insert_at
 * to the aligned position.
 *
 * @param str the DBusString
 * @param insert_at location to be aligned
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 * @returns #FALSE if not enough memory.
 */
dbus_bool_t
_dbus_string_insert_alignment (DBusString        *str,
                               int               *insert_at,
                               int                alignment)
{
  DBUS_STRING_PREAMBLE (str);
  
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    return FALSE;

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  _dbus_assert (_DBUS_ALIGN_VALUE (*insert_at, alignment) == (unsigned) *insert_at);
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  return TRUE;
}

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/**
 * Appends a printf-style formatted string
 * to the #DBusString.
 *
 * @param str the string
 * @param format printf format
 * @param args variable argument list
 * @returns #FALSE if no memory
 */
dbus_bool_t
_dbus_string_append_printf_valist  (DBusString        *str,
                                    const char        *format,
                                    va_list            args)
{
  int len;
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  va_list args_copy;

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  DBUS_STRING_PREAMBLE (str);
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  DBUS_VA_COPY (args_copy, args);

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  /* Measure the message length without terminating nul */
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  len = _dbus_printf_string_upper_bound (format, args);
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  if (!_dbus_string_lengthen (str, len))
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    {
      /* don't leak the copy */
      va_end (args_copy);
      return FALSE;
    }
  
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  vsprintf ((char*) (real->str + (real->len - len)),
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            format, args_copy);

  va_end (args_copy);
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  return TRUE;
}

/**
 * Appends a printf-style formatted string
 * to the #DBusString.
 *
 * @param str the string
 * @param format printf format
 * @returns #FALSE if no memory
 */
dbus_bool_t
_dbus_string_append_printf (DBusString        *str,
                            const char        *format,
                            ...)
{
  va_list args;
  dbus_bool_t retval;
  
  va_start (args, format);
  retval = _dbus_string_append_printf_valist (str, format, args);
  va_end (args);

  return retval;
}

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/**
 * Appends block of bytes with the given length to a DBusString.
 *
 * @param str the DBusString
 * @param buffer the bytes to append
 * @param len the number of bytes to append
 * @returns #FALSE if not enough memory.
 */
dbus_bool_t
_dbus_string_append_len (DBusString *str,
                         const char *buffer,
                         int         len)
{
  DBUS_STRING_PREAMBLE (str);
  _dbus_assert (buffer != NULL);
  _dbus_assert (len >= 0);

  return append (real, buffer, len);
}

/**
 * Appends a single byte to the string, returning #FALSE
 * if not enough memory.
 *
 * @param str the string
 * @param byte the byte to append
 * @returns #TRUE on success
 */
dbus_bool_t
_dbus_string_append_byte (DBusString    *str,
                          unsigned char  byte)
{
  DBUS_STRING_PREAMBLE (str);

  if (!set_length (real, real->len + 1))
    return FALSE;

  real->str[real->len-1] = byte;

  return TRUE;
}

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#ifdef DBUS_BUILD_TESTS
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/**
 * Appends a single Unicode character, encoding the character
 * in UTF-8 format.
 *
 * @param str the string
 * @param ch the Unicode character
 */
dbus_bool_t
_dbus_string_append_unichar (DBusString    *str,
                             dbus_unichar_t ch)
{
  int len;
  int first;
  int i;
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  unsigned char *out;
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  DBUS_STRING_PREAMBLE (str);

  /* this code is from GLib but is pretty standard I think */
  
  len = 0;
  
  if (ch < 0x80)
    {
      first = 0;
      len = 1;
    }
  else if (ch < 0x800)
    {
      first = 0xc0;
      len = 2;
    }
  else if (ch < 0x10000)
    {
      first = 0xe0;
      len = 3;
    }
   else if (ch < 0x200000)
    {
      first = 0xf0;
      len = 4;
    }
  else if (ch < 0x4000000)
    {
      first = 0xf8;
      len = 5;
    }
  else
    {
      first = 0xfc;
      len = 6;
    }

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  if (len > (real->max_length - real->len))
    return FALSE; /* real->len + len would overflow */
  
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  if (!set_length (real, real->len + len))
    return FALSE;

  out = real->str + (real->len - len);
  
  for (i = len - 1; i > 0; --i)
    {
      out[i] = (ch & 0x3f) | 0x80;
      ch >>= 6;
    }
  out[0] = ch | first;

  return TRUE;
}
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#endif /* DBUS_BUILD_TESTS */
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static void
delete (DBusRealString *real,
        int             start,
        int             len)
{
  if (len == 0)
    return;
  
  memmove (real->str + start, real->str + start + len, real->len - (start + len));
  real->len -= len;
  real->str[real->len] = '\0';
}

/**
 * Deletes a segment of a DBusString with length len starting at
 * start. (Hint: to clear an entire string, setting length to 0
 * with _dbus_string_set_length() is easier.)
 *
 * @param str the DBusString
 * @param start where to start deleting
 * @param len the number of bytes to delete
 */
void
_dbus_string_delete (DBusString       *str,
                     int               start,
                     int               len)
{
  DBUS_STRING_PREAMBLE (str);
  _dbus_assert (start >= 0);
  _dbus_assert (len >= 0);
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  _dbus_assert (start <= real->len);
  _dbus_assert (len <= real->len - start);
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  delete (real, start, len);
}

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static dbus_bool_t
copy (DBusRealString *source,
      int             start,
      int             len,
      DBusRealString *dest,
      int             insert_at)
{
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  if (len == 0)
    return TRUE;

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  if (!open_gap (len, dest, insert_at))
    return FALSE;
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  memmove (dest->str + insert_at,
           source->str + start,
           len);
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  return TRUE;
}

/**
 * Checks assertions for two strings we're copying a segment between,
 * and declares real_source/real_dest variables.
 *
 * @param source the source string
 * @param start the starting offset
 * @param dest the dest string
 * @param insert_at where the copied segment is inserted
 */
#define DBUS_STRING_COPY_PREAMBLE(source, start, dest, insert_at)       \
  DBusRealString *real_source = (DBusRealString*) source;               \
  DBusRealString *real_dest = (DBusRealString*) dest;                   \
  _dbus_assert ((source) != (dest));                                    \
  DBUS_GENERIC_STRING_PREAMBLE (real_source);                           \
  DBUS_GENERIC_STRING_PREAMBLE (real_dest);                             \
  _dbus_assert (!real_dest->constant);                                  \
  _dbus_assert (!real_dest->locked);                                    \
  _dbus_assert ((start) >= 0);                                          \
  _dbus_assert ((start) <= real_source->len);                           \
  _dbus_assert ((insert_at) >= 0);                                      \
  _dbus_assert ((insert_at) <= real_dest->len)

/**
 * Moves the end of one string into another string. Both strings
 * must be initialized, valid strings.
 *
 * @param source the source string
 * @param start where to chop off the source string
 * @param dest the destination string
 * @param insert_at where to move the chopped-off part of source string
 * @returns #FALSE if not enough memory
 */
dbus_bool_t
_dbus_string_move (DBusString       *source,
                   int               start,
                   DBusString       *dest,
                   int               insert_at)
{
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  DBusRealString *real_source = (DBusRealString*) source;
  _dbus_assert (start <= real_source->len);
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  return _dbus_string_move_len (source, start,
                                real_source->len - start,
                                dest, insert_at);
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}

/**
 * Like _dbus_string_move(), but does not delete the section
 * of the source string that's copied to the dest string.
 *
 * @param source the source string
 * @param start where to start copying the source string
 * @param dest the destination string
 * @param insert_at where to place the copied part of source string
 * @returns #FALSE if not enough memory
 */
dbus_bool_t
_dbus_string_copy (const DBusString *source,
                   int               start,
                   DBusString       *dest,
                   int               insert_at)
{
  DBUS_STRING_COPY_PREAMBLE (source, start, dest, insert_at);

  return copy (real_source, start,
               real_source->len - start,
               real_dest,
               insert_at);
}

/**
 * Like _dbus_string_move(), but can move a segment from
 * the middle of the source string.
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 *
 * @todo this doesn't do anything with max_length field.
 * we should probably just kill the max_length field though.
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 * 
 * @param source the source string
 * @param start first byte of source string to move
 * @param len length of segment to move
 * @param dest the destination string
 * @param insert_at where to move the bytes from the source string
 * @returns #FALSE if not enough memory
 */
dbus_bool_t
_dbus_string_move_len (DBusString       *source,
                       int               start,
                       int               len,
                       DBusString       *dest,
                       int               insert_at)

{
  DBUS_STRING_COPY_PREAMBLE (source, start, dest, insert_at);
  _dbus_assert (len >= 0);
  _dbus_assert ((start + len) <= real_source->len);


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  if (len == 0)
    {
      return TRUE;
    }
  else if (start == 0 &&
           len == real_source->len &&
           real_dest->len == 0)
    {
      /* Short-circuit moving an entire existing string to an empty string
       * by just swapping the buffers.
       */
      /* we assume ->constant doesn't matter as you can't have
       * a constant string involved in a move.
       */
#define ASSIGN_DATA(a, b) do {                  \
        (a)->str = (b)->str;                    \
        (a)->len = (b)->len;                    \
        (a)->allocated = (b)->allocated;        \
        (a)->align_offset = (b)->align_offset;  \
      } while (0)
      
      DBusRealString tmp;
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