xref: /netbsd-src/external/gpl3/gcc.old/dist/libgfortran/io/unit.c (revision 4c3eb207d36f67d31994830c0a694161fc1ca39b)

/* Copyright (C) 2002-2020 Free Software Foundation, Inc.
   Contributed by Andy Vaught
   F2003 I/O support contributed by Jerry DeLisle

This file is part of the GNU Fortran runtime library (libgfortran).

Libgfortran 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, or (at your option)
any later version.

Libgfortran 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.

Under Section 7 of GPL version 3, you are granted additional
permissions described in the GCC Runtime Library Exception, version
3.1, as published by the Free Software Foundation.

You should have received a copy of the GNU General Public License and
a copy of the GCC Runtime Library Exception along with this program;
see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
<http://www.gnu.org/licenses/>.  */

#include "io.h"
#include "fbuf.h"
#include "format.h"
#include "unix.h"
#include "async.h"
#include <string.h>
#include <assert.h>


/* IO locking rules:
   UNIT_LOCK is a master lock, protecting UNIT_ROOT tree and UNIT_CACHE.
   Concurrent use of different units should be supported, so
   each unit has its own lock, LOCK.
   Open should be atomic with its reopening of units and list_read.c
   in several places needs find_unit another unit while holding stdin
   unit's lock, so it must be possible to acquire UNIT_LOCK while holding
   some unit's lock.  Therefore to avoid deadlocks, it is forbidden
   to acquire unit's private locks while holding UNIT_LOCK, except
   for freshly created units (where no other thread can get at their
   address yet) or when using just trylock rather than lock operation.
   In addition to unit's private lock each unit has a WAITERS counter
   and CLOSED flag.  WAITERS counter must be either only
   atomically incremented/decremented in all places (if atomic builtins
   are supported), or protected by UNIT_LOCK in all places (otherwise).
   CLOSED flag must be always protected by unit's LOCK.
   After finding a unit in UNIT_CACHE or UNIT_ROOT with UNIT_LOCK held,
   WAITERS must be incremented to avoid concurrent close from freeing
   the unit between unlocking UNIT_LOCK and acquiring unit's LOCK.
   Unit freeing is always done under UNIT_LOCK.  If close_unit sees any
   WAITERS, it doesn't free the unit but instead sets the CLOSED flag
   and the thread that decrements WAITERS to zero while CLOSED flag is
   set is responsible for freeing it (while holding UNIT_LOCK).
   flush_all_units operation is iterating over the unit tree with
   increasing UNIT_NUMBER while holding UNIT_LOCK and attempting to
   flush each unit (and therefore needs the unit's LOCK held as well).
   To avoid deadlocks, it just trylocks the LOCK and if unsuccessful,
   remembers the current unit's UNIT_NUMBER, unlocks UNIT_LOCK, acquires
   unit's LOCK and after flushing reacquires UNIT_LOCK and restarts with
   the smallest UNIT_NUMBER above the last one flushed.

   If find_unit/find_or_create_unit/find_file/get_unit routines return
   non-NULL, the returned unit has its private lock locked and when the
   caller is done with it, it must call either unlock_unit or close_unit
   on it.  unlock_unit or close_unit must be always called only with the
   private lock held.  */



/* Table of allocated newunit values.  A simple solution would be to
   map OS file descriptors (fd's) to unit numbers, e.g. with newunit =
   -fd - 2, however that doesn't work since Fortran allows an existing
   unit number to be reassociated with a new file. Thus the simple
   approach may lead to a situation where we'd try to assign a
   (negative) unit number which already exists. Hence we must keep
   track of allocated newunit values ourselves. This is the purpose of
   the newunits array. The indices map to newunit values as newunit =
   -index + NEWUNIT_FIRST. E.g. newunits[0] having the value true
   means that a unit with number NEWUNIT_FIRST exists. Similar to
   POSIX file descriptors, we always allocate the lowest (in absolute
   value) available unit number.
 */
static bool *newunits;
static int newunit_size; /* Total number of elements in the newunits array.  */
/* Low water indicator for the newunits array. Below the LWI all the
   units are allocated, above and equal to the LWI there may be both
   allocated and free units. */
static int newunit_lwi;

/* Unit numbers assigned with NEWUNIT start from here.  */
#define NEWUNIT_START -10

#define CACHE_SIZE 3
static gfc_unit *unit_cache[CACHE_SIZE];

gfc_offset max_offset;
gfc_offset default_recl;

gfc_unit *unit_root;
#ifdef __GTHREAD_MUTEX_INIT
__gthread_mutex_t unit_lock = __GTHREAD_MUTEX_INIT;
#else
__gthread_mutex_t unit_lock;
#endif

/* We use these filenames for error reporting.  */

static char stdin_name[] = "stdin";
static char stdout_name[] = "stdout";
static char stderr_name[] = "stderr";


#ifdef HAVE_NEWLOCALE
locale_t c_locale;
#else
/* If we don't have POSIX 2008 per-thread locales, we need to use the
   traditional setlocale().  To prevent multiple concurrent threads
   doing formatted I/O from messing up the locale, we need to store a
   global old_locale, and a counter keeping track of how many threads
   are currently doing formatted I/O.  The first thread saves the old
   locale, and the last one restores it.  */
char *old_locale;
int old_locale_ctr;
#ifdef __GTHREAD_MUTEX_INIT
__gthread_mutex_t old_locale_lock = __GTHREAD_MUTEX_INIT;
#else
__gthread_mutex_t old_locale_lock;
#endif
#endif


/* This implementation is based on Stefan Nilsson's article in the
   July 1997 Doctor Dobb's Journal, "Treaps in Java". */

/* pseudo_random()-- Simple linear congruential pseudorandom number
   generator.  The period of this generator is 44071, which is plenty
   for our purposes.  */

static int
pseudo_random (void)
{
  static int x0 = 5341;

  x0 = (22611 * x0 + 10) % 44071;
  return x0;
}


/* rotate_left()-- Rotate the treap left */

static gfc_unit *
rotate_left (gfc_unit *t)
{
  gfc_unit *temp;

  temp = t->right;
  t->right = t->right->left;
  temp->left = t;

  return temp;
}


/* rotate_right()-- Rotate the treap right */

static gfc_unit *
rotate_right (gfc_unit *t)
{
  gfc_unit *temp;

  temp = t->left;
  t->left = t->left->right;
  temp->right = t;

  return temp;
}


static int
compare (int a, int b)
{
  if (a < b)
    return -1;
  if (a > b)
    return 1;

  return 0;
}


/* insert()-- Recursive insertion function.  Returns the updated treap. */

static gfc_unit *
insert (gfc_unit *new, gfc_unit *t)
{
  int c;

  if (t == NULL)
    return new;

  c = compare (new->unit_number, t->unit_number);

  if (c < 0)
    {
      t->left = insert (new, t->left);
      if (t->priority < t->left->priority)
	t = rotate_right (t);
    }

  if (c > 0)
    {
      t->right = insert (new, t->right);
      if (t->priority < t->right->priority)
	t = rotate_left (t);
    }

  if (c == 0)
    internal_error (NULL, "insert(): Duplicate key found!");

  return t;
}


/* insert_unit()-- Create a new node, insert it into the treap.  */

static gfc_unit *
insert_unit (int n)
{
  gfc_unit *u = xcalloc (1, sizeof (gfc_unit));
  u->unit_number = n;
  u->internal_unit_kind = 0;
#ifdef __GTHREAD_MUTEX_INIT
  {
    __gthread_mutex_t tmp = __GTHREAD_MUTEX_INIT;
    u->lock = tmp;
  }
#else
  __GTHREAD_MUTEX_INIT_FUNCTION (&u->lock);
#endif
  LOCK (&u->lock);
  u->priority = pseudo_random ();
  unit_root = insert (u, unit_root);
  return u;
}


/* destroy_unit_mutex()-- Destroy the mutex and free memory of unit.  */

static void
destroy_unit_mutex (gfc_unit *u)
{
  __gthread_mutex_destroy (&u->lock);
  free (u);
}


static gfc_unit *
delete_root (gfc_unit *t)
{
  gfc_unit *temp;

  if (t->left == NULL)
    return t->right;
  if (t->right == NULL)
    return t->left;

  if (t->left->priority > t->right->priority)
    {
      temp = rotate_right (t);
      temp->right = delete_root (t);
    }
  else
    {
      temp = rotate_left (t);
      temp->left = delete_root (t);
    }

  return temp;
}


/* delete_treap()-- Delete an element from a tree.  The 'old' value
   does not necessarily have to point to the element to be deleted, it
   must just point to a treap structure with the key to be deleted.
   Returns the new root node of the tree. */

static gfc_unit *
delete_treap (gfc_unit *old, gfc_unit *t)
{
  int c;

  if (t == NULL)
    return NULL;

  c = compare (old->unit_number, t->unit_number);

  if (c < 0)
    t->left = delete_treap (old, t->left);
  if (c > 0)
    t->right = delete_treap (old, t->right);
  if (c == 0)
    t = delete_root (t);

  return t;
}


/* delete_unit()-- Delete a unit from a tree */

static void
delete_unit (gfc_unit *old)
{
  unit_root = delete_treap (old, unit_root);
}


/* get_gfc_unit()-- Given an integer, return a pointer to the unit
   structure.  Returns NULL if the unit does not exist,
   otherwise returns a locked unit. */

static gfc_unit *
get_gfc_unit (int n, int do_create)
{
  gfc_unit *p;
  int c, created = 0;

  NOTE ("Unit n=%d, do_create = %d", n, do_create);
  LOCK (&unit_lock);

retry:
  for (c = 0; c < CACHE_SIZE; c++)
    if (unit_cache[c] != NULL && unit_cache[c]->unit_number == n)
      {
	p = unit_cache[c];
	goto found;
      }

  p = unit_root;
  while (p != NULL)
    {
      c = compare (n, p->unit_number);
      if (c < 0)
	p = p->left;
      if (c > 0)
	p = p->right;
      if (c == 0)
	break;
    }

  if (p == NULL && do_create)
    {
      p = insert_unit (n);
      created = 1;
    }

  if (p != NULL)
    {
      for (c = 0; c < CACHE_SIZE - 1; c++)
	unit_cache[c] = unit_cache[c + 1];

      unit_cache[CACHE_SIZE - 1] = p;
    }

  if (created)
    {
      /* Newly created units have their lock held already
	 from insert_unit.  Just unlock UNIT_LOCK and return.  */
      UNLOCK (&unit_lock);
      return p;
    }

found:
  if (p != NULL && (p->child_dtio == 0))
    {
      /* Fast path.  */
      if (! TRYLOCK (&p->lock))
	{
	  /* assert (p->closed == 0); */
	  UNLOCK (&unit_lock);
	  return p;
	}

      inc_waiting_locked (p);
    }


  UNLOCK (&unit_lock);

  if (p != NULL && (p->child_dtio == 0))
    {
      LOCK (&p->lock);
      if (p->closed)
	{
	  LOCK (&unit_lock);
	  UNLOCK (&p->lock);
	  if (predec_waiting_locked (p) == 0)
	    destroy_unit_mutex (p);
	  goto retry;
	}

      dec_waiting_unlocked (p);
    }
  return p;
}


gfc_unit *
find_unit (int n)
{
  return get_gfc_unit (n, 0);
}


gfc_unit *
find_or_create_unit (int n)
{
  return get_gfc_unit (n, 1);
}


/* Helper function to check rank, stride, format string, and namelist.
   This is used for optimization. You can't trim out blanks or shorten
   the string if trailing spaces are significant.  */
static bool
is_trim_ok (st_parameter_dt *dtp)
{
  /* Check rank and stride.  */
  if (dtp->internal_unit_desc)
    return false;
  /* Format strings cannot have 'BZ' or '/'.  */
  if (dtp->common.flags & IOPARM_DT_HAS_FORMAT)
    {
      char *p = dtp->format;
      if (dtp->common.flags & IOPARM_DT_HAS_BLANK)
	return false;
      for (gfc_charlen_type i = 0; i < dtp->format_len; i++)
	{
	  if (p[i] == '/') return false;
	  if (p[i] == 'b' || p[i] == 'B')
	    if (p[i+1] == 'z' || p[i+1] == 'Z')
	      return false;
	}
    }
  if (dtp->u.p.ionml) /* A namelist.  */
    return false;
  return true;
}


gfc_unit *
set_internal_unit (st_parameter_dt *dtp, gfc_unit *iunit, int kind)
{
  gfc_offset start_record = 0;

  iunit->unit_number = dtp->common.unit;
  iunit->recl = dtp->internal_unit_len;
  iunit->internal_unit = dtp->internal_unit;
  iunit->internal_unit_len = dtp->internal_unit_len;
  iunit->internal_unit_kind = kind;

  /* As an optimization, adjust the unit record length to not
     include trailing blanks. This will not work under certain conditions
     where trailing blanks have significance.  */
  if (dtp->u.p.mode == READING && is_trim_ok (dtp))
    {
      int len;
      if (kind == 1)
	  len = string_len_trim (iunit->internal_unit_len,
						   iunit->internal_unit);
      else
	  len = string_len_trim_char4 (iunit->internal_unit_len,
			      (const gfc_char4_t*) iunit->internal_unit);
      iunit->internal_unit_len = len;
      iunit->recl = iunit->internal_unit_len;
    }

  /* Set up the looping specification from the array descriptor, if any.  */

  if (is_array_io (dtp))
    {
      iunit->rank = GFC_DESCRIPTOR_RANK (dtp->internal_unit_desc);
      iunit->ls = (array_loop_spec *)
	xmallocarray (iunit->rank, sizeof (array_loop_spec));
      iunit->internal_unit_len *=
	init_loop_spec (dtp->internal_unit_desc, iunit->ls, &start_record);

      start_record *= iunit->recl;
    }

  /* Set initial values for unit parameters.  */
  if (kind == 4)
    iunit->s = open_internal4 (iunit->internal_unit - start_record,
				 iunit->internal_unit_len, -start_record);
  else
    iunit->s = open_internal (iunit->internal_unit - start_record,
			      iunit->internal_unit_len, -start_record);

  iunit->bytes_left = iunit->recl;
  iunit->last_record=0;
  iunit->maxrec=0;
  iunit->current_record=0;
  iunit->read_bad = 0;
  iunit->endfile = NO_ENDFILE;

  /* Set flags for the internal unit.  */

  iunit->flags.access = ACCESS_SEQUENTIAL;
  iunit->flags.action = ACTION_READWRITE;
  iunit->flags.blank = BLANK_NULL;
  iunit->flags.form = FORM_FORMATTED;
  iunit->flags.pad = PAD_YES;
  iunit->flags.status = STATUS_UNSPECIFIED;
  iunit->flags.sign = SIGN_PROCDEFINED;
  iunit->flags.decimal = DECIMAL_POINT;
  iunit->flags.delim = DELIM_UNSPECIFIED;
  iunit->flags.encoding = ENCODING_DEFAULT;
  iunit->flags.async = ASYNC_NO;
  iunit->flags.round = ROUND_PROCDEFINED;

  /* Initialize the data transfer parameters.  */

  dtp->u.p.advance_status = ADVANCE_YES;
  dtp->u.p.seen_dollar = 0;
  dtp->u.p.skips = 0;
  dtp->u.p.pending_spaces = 0;
  dtp->u.p.max_pos = 0;
  dtp->u.p.at_eof = 0;
  return iunit;
}


/* get_unit()-- Returns the unit structure associated with the integer
   unit or the internal file.  */

gfc_unit *
get_unit (st_parameter_dt *dtp, int do_create)
{
  gfc_unit *unit;

  if ((dtp->common.flags & IOPARM_DT_HAS_INTERNAL_UNIT) != 0)
    {
      int kind;
      if (dtp->common.unit == GFC_INTERNAL_UNIT)
        kind = 1;
      else if (dtp->common.unit == GFC_INTERNAL_UNIT4)
        kind = 4;
      else
	internal_error (&dtp->common, "get_unit(): Bad internal unit KIND");

      dtp->u.p.unit_is_internal = 1;
      dtp->common.unit = newunit_alloc ();
      unit = get_gfc_unit (dtp->common.unit, do_create);
      set_internal_unit (dtp, unit, kind);
      fbuf_init (unit, 128);
      return unit;
    }

  /* Has to be an external unit.  */
  dtp->u.p.unit_is_internal = 0;
  dtp->internal_unit = NULL;
  dtp->internal_unit_desc = NULL;

  /* For an external unit with unit number < 0 creating it on the fly
     is not allowed, such units must be created with
     OPEN(NEWUNIT=...).  */
  if (dtp->common.unit < 0)
    {
      if (dtp->common.unit > NEWUNIT_START) /* Reserved units.  */
	return NULL;
      return get_gfc_unit (dtp->common.unit, 0);
    }

  return get_gfc_unit (dtp->common.unit, do_create);
}


/*************************/
/* Initialize everything.  */

void
init_units (void)
{
  gfc_unit *u;

#ifdef HAVE_NEWLOCALE
  c_locale = newlocale (0, "C", 0);
#else
#ifndef __GTHREAD_MUTEX_INIT
  __GTHREAD_MUTEX_INIT_FUNCTION (&old_locale_lock);
#endif
#endif

#ifndef __GTHREAD_MUTEX_INIT
  __GTHREAD_MUTEX_INIT_FUNCTION (&unit_lock);
#endif

  if (sizeof (max_offset) == 8)
    {
      max_offset = GFC_INTEGER_8_HUGE;
      /* Why this weird value? Because if the recl specifier in the
	 inquire statement is a 4 byte value, u->recl is truncated,
	 and this trick ensures it becomes HUGE(0) rather than -1.
	 The full 8 byte value of default_recl is still 0.99999999 *
	 max_offset which is large enough for all practical
	 purposes.  */
      default_recl = max_offset & ~(1LL<<31);
    }
  else if (sizeof (max_offset) == 4)
    max_offset = default_recl = GFC_INTEGER_4_HUGE;
  else
    internal_error (NULL, "sizeof (max_offset) must be 4 or 8");

  if (options.stdin_unit >= 0)
    {				/* STDIN */
      u = insert_unit (options.stdin_unit);
      u->s = input_stream ();

      u->flags.action = ACTION_READ;

      u->flags.access = ACCESS_SEQUENTIAL;
      u->flags.form = FORM_FORMATTED;
      u->flags.status = STATUS_OLD;
      u->flags.blank = BLANK_NULL;
      u->flags.pad = PAD_YES;
      u->flags.position = POSITION_ASIS;
      u->flags.sign = SIGN_PROCDEFINED;
      u->flags.decimal = DECIMAL_POINT;
      u->flags.delim = DELIM_UNSPECIFIED;
      u->flags.encoding = ENCODING_DEFAULT;
      u->flags.async = ASYNC_NO;
      u->flags.round = ROUND_PROCDEFINED;
      u->flags.share = SHARE_UNSPECIFIED;
      u->flags.cc = CC_LIST;

      u->recl = default_recl;
      u->endfile = NO_ENDFILE;

      u->filename = strdup (stdin_name);

      fbuf_init (u, 0);

      UNLOCK (&u->lock);
    }

  if (options.stdout_unit >= 0)
    {				/* STDOUT */
      u = insert_unit (options.stdout_unit);
      u->s = output_stream ();

      u->flags.action = ACTION_WRITE;

      u->flags.access = ACCESS_SEQUENTIAL;
      u->flags.form = FORM_FORMATTED;
      u->flags.status = STATUS_OLD;
      u->flags.blank = BLANK_NULL;
      u->flags.position = POSITION_ASIS;
      u->flags.sign = SIGN_PROCDEFINED;
      u->flags.decimal = DECIMAL_POINT;
      u->flags.delim = DELIM_UNSPECIFIED;
      u->flags.encoding = ENCODING_DEFAULT;
      u->flags.async = ASYNC_NO;
      u->flags.round = ROUND_PROCDEFINED;
      u->flags.share = SHARE_UNSPECIFIED;
      u->flags.cc = CC_LIST;

      u->recl = default_recl;
      u->endfile = AT_ENDFILE;

      u->filename = strdup (stdout_name);

      fbuf_init (u, 0);

      UNLOCK (&u->lock);
    }

  if (options.stderr_unit >= 0)
    {				/* STDERR */
      u = insert_unit (options.stderr_unit);
      u->s = error_stream ();

      u->flags.action = ACTION_WRITE;

      u->flags.access = ACCESS_SEQUENTIAL;
      u->flags.form = FORM_FORMATTED;
      u->flags.status = STATUS_OLD;
      u->flags.blank = BLANK_NULL;
      u->flags.position = POSITION_ASIS;
      u->flags.sign = SIGN_PROCDEFINED;
      u->flags.decimal = DECIMAL_POINT;
      u->flags.encoding = ENCODING_DEFAULT;
      u->flags.async = ASYNC_NO;
      u->flags.round = ROUND_PROCDEFINED;
      u->flags.share = SHARE_UNSPECIFIED;
      u->flags.cc = CC_LIST;

      u->recl = default_recl;
      u->endfile = AT_ENDFILE;

      u->filename = strdup (stderr_name);

      fbuf_init (u, 256);  /* 256 bytes should be enough, probably not doing
                              any kind of exotic formatting to stderr.  */

      UNLOCK (&u->lock);
    }
  /* The default internal units.  */
  u = insert_unit (GFC_INTERNAL_UNIT);
  UNLOCK (&u->lock);
  u = insert_unit (GFC_INTERNAL_UNIT4);
  UNLOCK (&u->lock);
}


static int
close_unit_1 (gfc_unit *u, int locked)
{
  int i, rc;

  if (ASYNC_IO && u->au)
    async_close (u->au);

  /* If there are previously written bytes from a write with ADVANCE="no"
     Reposition the buffer before closing.  */
  if (u->previous_nonadvancing_write)
    finish_last_advance_record (u);

  rc = (u->s == NULL) ? 0 : sclose (u->s) == -1;

  u->closed = 1;
  if (!locked)
    LOCK (&unit_lock);

  for (i = 0; i < CACHE_SIZE; i++)
    if (unit_cache[i] == u)
      unit_cache[i] = NULL;

  delete_unit (u);

  free (u->filename);
  u->filename = NULL;

  free_format_hash_table (u);
  fbuf_destroy (u);

  if (u->unit_number <= NEWUNIT_START)
    newunit_free (u->unit_number);

  if (!locked)
    UNLOCK (&u->lock);

  /* If there are any threads waiting in find_unit for this unit,
     avoid freeing the memory, the last such thread will free it
     instead.  */
  if (u->waiting == 0)
    destroy_unit_mutex (u);

  if (!locked)
    UNLOCK (&unit_lock);

  return rc;
}

void
unlock_unit (gfc_unit *u)
{
  if (u)
    {
      NOTE ("unlock_unit = %d", u->unit_number);
      UNLOCK (&u->lock);
      NOTE ("unlock_unit done");
    }
}

/* close_unit()-- Close a unit.  The stream is closed, and any memory
   associated with the stream is freed.  Returns nonzero on I/O error.
   Should be called with the u->lock locked. */

int
close_unit (gfc_unit *u)
{
  return close_unit_1 (u, 0);
}


/* close_units()-- Delete units on completion.  We just keep deleting
   the root of the treap until there is nothing left.
   Not sure what to do with locking here.  Some other thread might be
   holding some unit's lock and perhaps hold it indefinitely
   (e.g. waiting for input from some pipe) and close_units shouldn't
   delay the program too much.  */

void
close_units (void)
{
  LOCK (&unit_lock);
  while (unit_root != NULL)
    close_unit_1 (unit_root, 1);
  UNLOCK (&unit_lock);

  free (newunits);

#ifdef HAVE_FREELOCALE
  freelocale (c_locale);
#endif
}


/* High level interface to truncate a file, i.e. flush format buffers,
   and generate an error or set some flags.  Just like POSIX
   ftruncate, returns 0 on success, -1 on failure.  */

int
unit_truncate (gfc_unit *u, gfc_offset pos, st_parameter_common *common)
{
  int ret;

  /* Make sure format buffer is flushed.  */
  if (u->flags.form == FORM_FORMATTED)
    {
      if (u->mode == READING)
	pos += fbuf_reset (u);
      else
	fbuf_flush (u, u->mode);
    }

  /* struncate() should flush the stream buffer if necessary, so don't
     bother calling sflush() here.  */
  ret = struncate (u->s, pos);

  if (ret != 0)
    generate_error (common, LIBERROR_OS, NULL);
  else
    {
      u->endfile = AT_ENDFILE;
      u->flags.position = POSITION_APPEND;
    }

  return ret;
}


/* filename_from_unit()-- If the unit_number exists, return a pointer to the
   name of the associated file, otherwise return the empty string.  The caller
   must free memory allocated for the filename string.  */

char *
filename_from_unit (int n)
{
  gfc_unit *u;
  int c;

  /* Find the unit.  */
  u = unit_root;
  while (u != NULL)
    {
      c = compare (n, u->unit_number);
      if (c < 0)
	u = u->left;
      if (c > 0)
	u = u->right;
      if (c == 0)
	break;
    }

  /* Get the filename.  */
  if (u != NULL && u->filename != NULL)
    return strdup (u->filename);
  else
    return (char *) NULL;
}

void
finish_last_advance_record (gfc_unit *u)
{

  if (u->saved_pos > 0)
    fbuf_seek (u, u->saved_pos, SEEK_CUR);

  if (!(u->unit_number == options.stdout_unit
	|| u->unit_number == options.stderr_unit))
    {
#ifdef HAVE_CRLF
      const int len = 2;
#else
      const int len = 1;
#endif
      char *p = fbuf_alloc (u, len);
      if (!p)
	os_error ("Completing record after ADVANCE_NO failed");
#ifdef HAVE_CRLF
      *(p++) = '\r';
#endif
      *p = '\n';
    }

  fbuf_flush (u, u->mode);
}


/* Assign a negative number for NEWUNIT in OPEN statements or for
   internal units.  */
int
newunit_alloc (void)
{
  LOCK (&unit_lock);
  if (!newunits)
    {
      newunits = xcalloc (16, 1);
      newunit_size = 16;
    }

  /* Search for the next available newunit.  */
  for (int ii = newunit_lwi; ii < newunit_size; ii++)
    {
      if (!newunits[ii])
        {
          newunits[ii] = true;
          newunit_lwi = ii + 1;
	  UNLOCK (&unit_lock);
          return -ii + NEWUNIT_START;
        }
    }

  /* Search failed, bump size of array and allocate the first
     available unit.  */
  int old_size = newunit_size;
  newunit_size *= 2;
  newunits = xrealloc (newunits, newunit_size);
  memset (newunits + old_size, 0, old_size);
  newunits[old_size] = true;
  newunit_lwi = old_size + 1;
    UNLOCK (&unit_lock);
  return -old_size + NEWUNIT_START;
}


/* Free a previously allocated newunit= unit number.  unit_lock must
   be held when calling.  */

void
newunit_free (int unit)
{
  int ind = -unit + NEWUNIT_START;
  assert(ind >= 0 && ind < newunit_size);
  newunits[ind] = false;
  if (ind < newunit_lwi)
    newunit_lwi = ind;
}