jack1/libjack/shm.c

/* This module provides a set of abstract shared memory interfaces
 * with support using both System V and POSIX shared memory
 * implementations.  The code is divided into three sections:
 *
 *	- common (interface-independent) code
 *	- POSIX implementation
 *	- System V implementation
 *
 * The implementation used is determined by whether USE_POSIX_SHM was
 * set in the ./configure step.
 */

/*
 * Copyright (C) 2003 Paul Davis
 * Copyright (C) 2004 Jack O'Quin
 * 
 * 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., 675 Mass Ave, Cambridge, MA 02139, USA.
 *
 * $Id$
 */
#include <config.h>

#include <unistd.h>
#include <fcntl.h>
#include <stdio.h>
#include <string.h>
#include <signal.h>
#include <limits.h>
#include <errno.h>
#include <dirent.h>
#include <sys/mman.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <sysdeps/ipc.h>
#include <sys/shm.h>
#include <sys/sem.h>
#include <sysdeps/ipc.h>

#include <jack/shm.h>
#include <jack/internal.h>
#include <jack/version.h>

#ifdef USE_POSIX_SHM
static jack_shmtype_t jack_shmtype = shm_POSIX;
#else
static jack_shmtype_t jack_shmtype = shm_SYSV;
#endif

/* interface-dependent forward declarations */
static int	jack_access_registry (jack_shm_info_t *ri);
static int	jack_create_registry (jack_shm_info_t *ri);
static void	jack_remove_shm (jack_shm_id_t *id);

/* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
 * common interface-independent section
 * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * */

/* The JACK SHM registry is a chunk of memory for keeping track of the
 * shared memory used by each active JACK server.  This allows the
 * server to clean up shared memory when it exits.  To avoid memory
 * leakage due to kill -9, crashes or debugger-driven exits, this
 * cleanup is also done when a new instance of that server starts.
 */

/* per-process global data for the SHM interfaces */
static jack_shm_id_t   registry_id;	/* SHM id for the registry */
static jack_shm_info_t registry_info = { /* SHM info for the registry */
	.index = JACK_SHM_NULL_INDEX,
	.attached_at = MAP_FAILED
};

/* pointers to registry header and array */
static jack_shm_header_t   *jack_shm_header = NULL;
static jack_shm_registry_t *jack_shm_registry = NULL;
static char jack_shm_server_prefix[JACK_SERVER_NAME_SIZE] = "";

/* jack_shm_lock_registry() serializes updates to the shared memory
 * segment JACK uses to keep track of the SHM segements allocated to
 * all its processes, including multiple servers.
 *
 * This is not a high-contention lock, but it does need to work across
 * multiple processes.  High transaction rates and realtime safety are
 * not required.  Any solution needs to at least be portable to POSIX
 * and POSIX-like systems.
 *
 * We must be particularly careful to ensure that the lock be released
 * if the owning process terminates abnormally.  Otherwise, a segfault
 * or kill -9 at the wrong moment could prevent JACK from ever running
 * again on that machine until after a reboot.
 */

#define JACK_SEMAPHORE_KEY 0x282929
#ifndef USE_POSIX_SHM
#define JACK_SHM_REGISTRY_KEY JACK_SEMAPHORE_KEY
#endif

static int semid = -1;

/* all semaphore errors are fatal -- issue message, but do not return */
static void
semaphore_error (char *msg)
{
	jack_error ("Fatal JACK semaphore error: %s (%s)",
		    msg, strerror (errno));
	abort ();
}

static void
semaphore_init ()
{
	key_t semkey = JACK_SEMAPHORE_KEY;
	struct sembuf sbuf;
	int create_flags = IPC_CREAT | IPC_EXCL
		| S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP | S_IROTH | S_IWOTH;

	/* Get semaphore ID associated with this key. */
	if ((semid = semget(semkey, 0, 0)) == -1) {

		/* Semaphore does not exist - Create. */
		if ((semid = semget(semkey, 1, create_flags)) != -1) {

			/* Initialize the semaphore, allow one owner. */
			sbuf.sem_num = 0;
			sbuf.sem_op = 1;
			sbuf.sem_flg = 0;
			if (semop(semid, &sbuf, 1) == -1) {
				semaphore_error ("semop");
			}

		} else if (errno == EEXIST) {
			if ((semid = semget(semkey, 0, 0)) == -1) {
				semaphore_error ("semget");
			}

		} else {
			semaphore_error ("semget creation"); 
		}
	}
}

static inline void
semaphore_add (int value)
{
	struct sembuf sbuf;

	sbuf.sem_num = 0;
	sbuf.sem_op = value;
	sbuf.sem_flg = SEM_UNDO;
	if (semop(semid, &sbuf, 1) == -1) {
		semaphore_error ("semop");
	}
}

static void 
jack_shm_lock_registry (void)
{
	if (semid == -1)
		semaphore_init ();

	semaphore_add (-1);
}

static void 
jack_shm_unlock_registry (void)
{
	semaphore_add (1);
}

static void
jack_shm_init_registry ()
{
	/* registry must be locked */
	int i;

	memset (jack_shm_header, 0, JACK_SHM_REGISTRY_SIZE);

	jack_shm_header->magic = JACK_SHM_MAGIC;
	jack_shm_header->protocol = jack_protocol_version;
	jack_shm_header->type = jack_shmtype;
	jack_shm_header->size = JACK_SHM_REGISTRY_SIZE;
	jack_shm_header->hdr_len = sizeof (jack_shm_header_t);
	jack_shm_header->entry_len = sizeof (jack_shm_registry_t);

	for (i = 0; i < MAX_SHM_ID; ++i) {
		jack_shm_registry[i].index = i;
	}
}

static int
jack_shm_validate_registry ()
{
	/* registry must be locked */

	if ((jack_shm_header->magic == JACK_SHM_MAGIC)
	    && (jack_shm_header->protocol == jack_protocol_version)
	    && (jack_shm_header->type == jack_shmtype)
	    && (jack_shm_header->size == JACK_SHM_REGISTRY_SIZE)
	    && (jack_shm_header->hdr_len == sizeof (jack_shm_header_t))
	    && (jack_shm_header->entry_len == sizeof (jack_shm_registry_t))) {

		return 0;		/* registry OK */
	}

	return -1;
}

/* set a unique per-user, per-server shm prefix string
 *
 * According to the POSIX standard:
 *
 *   "The name argument conforms to the construction rules for a
 *   pathname. If name begins with the slash character, then processes
 *   calling shm_open() with the same value of name refer to the same
 *   shared memory object, as long as that name has not been
 *   removed. If name does not begin with the slash character, the
 *   effect is implementation-defined. The interpretation of slash
 *   characters other than the leading slash character in name is
 *   implementation-defined."
 *
 * Since the Linux implementation does not allow slashes *within* the
 * name, in the interest of portability we use colons instead.
 */
static void
jack_set_server_prefix (const char *server_name)
{
	snprintf (jack_shm_server_prefix, sizeof (jack_shm_server_prefix),
		  "/jack-%d:%s:", getuid (), server_name);
}

/* gain server addressability to shared memory registration segment
 *
 * returns: 0 if successful
 */
static int
jack_server_initialize_shm (void)
{
	int rc;

	if (jack_shm_header)
		return 0;		/* already initialized */

	jack_shm_lock_registry ();

	rc = jack_access_registry (&registry_info);

	switch (rc) {
	case ENOENT:			/* registry does not exist */
		rc = jack_create_registry (&registry_info);
		break;
	case 0:				/* existing registry */
		if (jack_shm_validate_registry () == 0)
			break;
		/* else it was invalid, so fall through */
	case EINVAL:			/* bad registry */
		/* Apparently, this registry was created by an older
		 * JACK version.  Delete it so we can try again. */
		jack_release_shm (&registry_info);
		jack_remove_shm (&registry_id);
		if ((rc = jack_create_registry (&registry_info)) != 0) {
			jack_error ("incompatible shm registry (%s)",
				    strerror (errno));
#ifndef USE_POSIX_SHM
			jack_error ("to delete, use `ipcrm -M 0x%0.8x'",
				    JACK_SHM_REGISTRY_KEY);
#endif
		}
		break;
	default:			/* failure return code */
		break;
	}

	jack_shm_unlock_registry ();
	return rc;
}

/* gain client addressability to shared memory registration segment
 *
 * NOTE: this function is no longer used for server initialization,
 * instead it calls jack_register_server().
 *
 * returns: 0 if successful
 */
int
jack_initialize_shm (const char *server_name)
{
	int rc;

	if (jack_shm_header)
		return 0;		/* already initialized */

	jack_set_server_prefix (server_name);

	jack_shm_lock_registry ();
	if ((rc = jack_access_registry (&registry_info)) == 0) {
		if ((rc = jack_shm_validate_registry ()) != 0) {
			jack_error ("Incompatible shm registry, "
				    "are jackd and libjack in sync?");
		}
	}
	jack_shm_unlock_registry ();

	return rc;
}

void
jack_destroy_shm (jack_shm_info_t* si)
{
	/* must NOT have the registry locked */
	if (si->index == JACK_SHM_NULL_INDEX)
		return;			/* segment not allocated */

	jack_remove_shm (&jack_shm_registry[si->index].id);
	jack_release_shm_info (si->index);
}

jack_shm_registry_t *
jack_get_free_shm_info ()
{
	/* registry must be locked */
	jack_shm_registry_t* si = NULL;
	int i;

	for (i = 0; i < MAX_SHM_ID; ++i) {
		if (jack_shm_registry[i].size == 0) {
			break;
		}
	}
	
	if (i < MAX_SHM_ID) {
		si = &jack_shm_registry[i];
	}

	return si;
}

static inline void
jack_release_shm_entry (jack_shm_registry_index_t index)
{
	/* the registry must be locked */
	jack_shm_registry[index].size = 0;
	jack_shm_registry[index].allocator = 0;
	memset (&jack_shm_registry[index].id, 0,
		sizeof (jack_shm_registry[index].id));
}

void
jack_release_shm_info (jack_shm_registry_index_t index)
{
	/* must NOT have the registry locked */
	if (jack_shm_registry[index].allocator == getpid()) {
		jack_shm_lock_registry ();
		jack_release_shm_entry (index);
		jack_shm_unlock_registry ();
	}
}

/* Claim server_name for this process.  
 *
 * returns 0 if successful
 *	   EEXIST if server_name was already active for this user
 *	   ENOSPC if server registration limit reached
 *	   ENOMEM if unable to access shared memory registry
 */
int
jack_register_server (const char *server_name)
{
	int i;
	pid_t my_pid = getpid ();

	jack_set_server_prefix (server_name);

	fprintf (stderr, "JACK compiled with %s SHM support.\n", JACK_SHM_TYPE);

	if (jack_server_initialize_shm ())
		return ENOMEM;

	jack_shm_lock_registry ();

	/* See if server_name already registered.  Since server names
	 * are per-user, we register the unique server prefix string.
	 */
	for (i = 0; i < MAX_SERVERS; i++) {

		if (strncmp (jack_shm_header->server[i].name,
			     jack_shm_server_prefix,
			     JACK_SERVER_NAME_SIZE) != 0)
			continue;	/* no match */

		if (jack_shm_header->server[i].pid == my_pid)
			return 0;	/* it's me */

		/* see if server still exists */
		if (kill (jack_shm_header->server[i].pid, 0) == 0) {
			return EEXIST;	/* other server running */
		}

		/* it's gone, reclaim this entry */
		memset (&jack_shm_header->server[i], 0,
			sizeof (jack_shm_server_t));
	}

	/* find a free entry */
	for (i = 0; i < MAX_SERVERS; i++) {
		if (jack_shm_header->server[i].pid == 0)
			break;
	}

	if (i >= MAX_SERVERS)
		return ENOSPC;		/* out of space */

	/* claim it */
	jack_shm_header->server[i].pid = my_pid;
	strncpy (jack_shm_header->server[i].name,
		 jack_shm_server_prefix,
		 JACK_SERVER_NAME_SIZE);

	jack_shm_unlock_registry ();

	return 0;
}

/* release server_name registration */
void
jack_unregister_server (const char *server_name /* unused */)
{
	int i;
	pid_t my_pid = getpid ();

	jack_shm_lock_registry ();

	for (i = 0; i < MAX_SERVERS; i++) {
		if (jack_shm_header->server[i].pid == my_pid) {
			memset (&jack_shm_header->server[i], 0,
				sizeof (jack_shm_server_t));
		}
	}

	jack_shm_unlock_registry ();
}

/* called for server startup and termination */
int
jack_cleanup_shm ()
{
	int i;
	int destroy;
	jack_shm_info_t copy;
	pid_t my_pid = getpid ();

	jack_shm_lock_registry ();
		
	for (i = 0; i < MAX_SHM_ID; i++) {
		jack_shm_registry_t* r;

		r = &jack_shm_registry[i];
		memcpy (&copy, r, sizeof (jack_shm_info_t));
		destroy = FALSE;

		/* ignore unused entries */
		if (r->allocator == 0)
			continue;

		/* is this my shm segment? */
		if (r->allocator == my_pid) {

			/* allocated by this process, so unattach 
			   and destroy. */
			jack_release_shm (&copy);
			destroy = TRUE;

		} else {

			/* see if allocator still exists */
			if (kill (r->allocator, 0)) {
				if (errno == ESRCH) {
					/* allocator no longer exists,
					 * so destroy */
					destroy = TRUE;
				}
			}
		}

		if (destroy) {

			int index = copy.index;

			if ((index >= 0)  && (index < MAX_SHM_ID)) {
				jack_remove_shm (&jack_shm_registry[index].id);
				jack_release_shm_entry (index);
			}
			r->size = 0;
			r->allocator = 0;
		}
	}

	jack_shm_unlock_registry ();

	return TRUE;
}

/* resize a shared memory segment
 *
 * There is no way to resize a System V shm segment.  Resizing is
 * possible with POSIX shm, but not with the non-conformant Mac OS X
 * implementation.  Since POSIX shm is mainly used on that platform,
 * it's simpler to treat them both the same.
 *
 * So, we always resize by deleting and reallocating.  This is
 * tricky, because the old segment will not disappear until
 * all the clients have released it.  We only do what we can
 * from here.
 *
 * This is not done under a single lock.  I don't even want to think
 * about all the things that could possibly go wrong if multple
 * processes tried to resize the same segment concurrently.  That
 * probably doesn't happen.
 */
int
jack_resize_shm (jack_shm_info_t* si, jack_shmsize_t size)
{
	jack_shm_id_t id;

	/* The underlying type of `id' differs for SYSV and POSIX */
	memcpy (&id, &jack_shm_registry[si->index].id, sizeof (id));

	jack_release_shm (si);
	jack_destroy_shm (si);

	if (jack_shmalloc ((char *) id, size, si)) {
		return -1;
	}

	return jack_attach_shm (si);
}

#ifdef USE_POSIX_SHM

/* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
 * POSIX interface-dependent functions
 * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * */

/* gain addressability to existing SHM registry segment
 *
 * sets up global registry pointers, if successful
 *
 * returns: 0 if existing registry accessed successfully
 *          ENOENT if registry does not exist
 *          EINVAL if registry exists, but has the wrong size
 */
static int
jack_access_registry (jack_shm_info_t *ri)
{
	/* registry must be locked */
	int shm_fd;

	strncpy (registry_id, "/jack-shm-registry", sizeof (registry_id));

	/* try to open an existing segment */
	if ((shm_fd = shm_open (registry_id, O_RDWR, 0666)) < 0) {
		int rc = errno;
		if (errno != ENOENT) {
			jack_error ("cannot open existing shm registry segment"
				    " (%s)", strerror (errno));
		}
		close (shm_fd);
		return rc;
	}

	if ((ri->attached_at = mmap (0, JACK_SHM_REGISTRY_SIZE,
				     PROT_READ|PROT_WRITE,
				     MAP_SHARED, shm_fd, 0)) == MAP_FAILED) {
		jack_error ("cannot mmap shm registry segment (%s)",
			    strerror (errno));
		close (shm_fd);
		return EINVAL;
	}

	/* set up global pointers */
	ri->index = JACK_SHM_REGISTRY_INDEX;
	jack_shm_header = ri->attached_at;
	jack_shm_registry = (jack_shm_registry_t *) (jack_shm_header + 1);

	return 0;
}

/* create a new SHM registry segment
 *
 * sets up global registry pointers, if successful
 *
 * returns: 0 if registry created successfully
 *          nonzero error code if unable to allocate a new registry
 */
static int
jack_create_registry (jack_shm_info_t *ri)
{
	/* registry must be locked */
	int shm_fd;

	strncpy (registry_id, "/jack-shm-registry", sizeof (registry_id));

	if ((shm_fd = shm_open (registry_id, O_RDWR|O_CREAT, 0666)) < 0) {
		int rc = errno;
		jack_error ("cannot create shm registry segment (%s)",
			    strerror (errno));
		return rc;
	}

	/* Set the desired segment size.  NOTE: the non-conformant Mac
	 * OS X POSIX shm only allows ftruncate() on segment creation.
	 */
	if (ftruncate (shm_fd, JACK_SHM_REGISTRY_SIZE) < 0) {
		int rc = errno;
		jack_error ("cannot set registry size (%s)", strerror (errno));
		jack_remove_shm (&registry_id);
		close (shm_fd);
		return rc;
	}

	if ((ri->attached_at = mmap (0, JACK_SHM_REGISTRY_SIZE,
				     PROT_READ|PROT_WRITE,
				     MAP_SHARED, shm_fd, 0)) == MAP_FAILED) {
		jack_error ("cannot mmap shm registry segment (%s)",
			    strerror (errno));
		jack_remove_shm (&registry_id);
		close (shm_fd);
		return EINVAL;
	}

	/* set up global pointers */
	ri->index = JACK_SHM_REGISTRY_INDEX;
	jack_shm_header = ri->attached_at;
	jack_shm_registry = (jack_shm_registry_t *) (jack_shm_header + 1);

	/* initialize registry contents */
	jack_shm_init_registry ();

	return 0;
}

static void
jack_remove_shm (jack_shm_id_t *id)
{
	/* registry may or may not be locked */
	shm_unlink ((char *) id);
}

void
jack_release_shm (jack_shm_info_t* si)
{
	/* registry may or may not be locked */
	if (si->attached_at != MAP_FAILED) {
		munmap (si->attached_at, jack_shm_registry[si->index].size);
	}
}

/* allocate a POSIX shared memory segment */
int
jack_shmalloc (const char *shm_name, jack_shmsize_t size, jack_shm_info_t* si)
{
	jack_shm_registry_t* registry;
	int shm_fd;
	int rc = -1;
	char name[SHM_NAME_MAX+1];

	jack_shm_lock_registry ();

	if ((registry = jack_get_free_shm_info ()) == NULL) {
		jack_error ("shm registry full");
		goto unlock;
	}

	/* On Mac OS X, the maximum length of a shared memory segment
	 * name is SHM_NAME_MAX (instead of NAME_MAX or PATH_MAX as
	 * defined by the standard).  Unfortunately, Apple sets this
	 * value so small (about 31 bytes) that it is useless for
	 * actual names.  So, we construct a short name from the
	 * registry index for uniqueness and ignore the shm_name
	 * parameter.  Bah!
	 */
	snprintf (name, sizeof (name), "/jack-%d", registry->index);

	if (strlen (name) >= sizeof (registry->id)) {
		jack_error ("shm segment name too long %s", name);
		goto unlock;
	}

	if ((shm_fd = shm_open (name, O_RDWR|O_CREAT, 0666)) < 0) {
		jack_error ("cannot create shm segment %s (%s)",
			    name, strerror (errno));
		goto unlock;
	}

	if (ftruncate (shm_fd, size) < 0) {
		jack_error ("cannot set size of engine shm "
			    "registry 0 (%s)",
			    strerror (errno));
		close (shm_fd);
		goto unlock;
	}

	close (shm_fd);
	registry->size = size;
	strncpy (registry->id, name, sizeof (registry->id));
	registry->allocator = getpid();
	si->index = registry->index;
	si->attached_at = MAP_FAILED;	/* not attached */
	rc = 0;				/* success */

 unlock:
	jack_shm_unlock_registry ();
	return rc;
}

int
jack_attach_shm (jack_shm_info_t* si)
{
	int shm_fd;
	jack_shm_registry_t *registry = &jack_shm_registry[si->index];

	if ((shm_fd = shm_open (registry->id,
				O_RDWR, 0666)) < 0) {
		jack_error ("cannot open shm segment %s (%s)", registry->id,
			    strerror (errno));
		return -1;
	}

	if ((si->attached_at = mmap (0, registry->size, PROT_READ|PROT_WRITE,
				     MAP_SHARED, shm_fd, 0)) == MAP_FAILED) {
		jack_error ("cannot mmap shm segment %s (%s)", 
			    registry->id,
			    strerror (errno));
		close (shm_fd);
		return -1;
	}

	close (shm_fd);

	return 0;
}

#else

/* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
 * System V interface-dependent functions
 * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * */

/* gain addressability to existing SHM registry segment
 *
 * sets up global registry pointers, if successful
 *
 * returns: 0 if existing registry accessed successfully
 *          ENOENT if registry does not exist
 *          EINVAL if registry exists, but has the wrong size
 *          other nonzero error code if unable to access registry
 */
static int
jack_access_registry (jack_shm_info_t *ri)
{
	/* registry must be locked */

	/* try without IPC_CREAT to get existing segment */
	if ((registry_id = shmget (JACK_SHM_REGISTRY_KEY,
				   JACK_SHM_REGISTRY_SIZE, 0666)) < 0) {

		switch (errno) {

		case ENOENT:		/* segment does not exist */
			return ENOENT;

		case EINVAL:		/* segment exists, but too small */
			/* attempt minimum size access */
			registry_id = shmget (JACK_SHM_REGISTRY_KEY, 1, 0666);
			return EINVAL;

		default:		/* or other error */
			jack_error ("unable to access shm registry (%s)",
				    strerror (errno));
			return errno;
		}
	}

	if ((ri->attached_at = shmat (registry_id, 0, 0)) < 0) {
		jack_error ("cannot attach shm registry segment (%s)",
			    strerror (errno));
		return EINVAL;
	}

	/* set up global pointers */
	ri->index = JACK_SHM_REGISTRY_INDEX;
	jack_shm_header = ri->attached_at;
	jack_shm_registry = (jack_shm_registry_t *) (jack_shm_header + 1);

	return 0;
}

/* create a new SHM registry segment
 *
 * sets up global registry pointers, if successful
 *
 * returns: 0 if registry created successfully
 *          nonzero error code if unable to allocate a new registry
 */
static int
jack_create_registry (jack_shm_info_t *ri)
{
	/* registry must be locked */
	if ((registry_id = shmget (JACK_SHM_REGISTRY_KEY,
				   JACK_SHM_REGISTRY_SIZE,
				   0666|IPC_CREAT)) < 0) {
		jack_error ("cannot create shm registry segment (%s)",
			    strerror (errno));
		return errno;
	}

	if ((ri->attached_at = shmat (registry_id, 0, 0)) < 0) {
		jack_error ("cannot attach shm registry segment (%s)",
			    strerror (errno));
		return EINVAL;
	}

	/* set up global pointers */
	ri->index = JACK_SHM_REGISTRY_INDEX;
	jack_shm_header = ri->attached_at;
	jack_shm_registry = (jack_shm_registry_t *) (jack_shm_header + 1);

	/* initialize registry contents */
	jack_shm_init_registry ();

	return 0;
}

static void
jack_remove_shm (jack_shm_id_t *id)
{
	/* registry may or may not be locked */
	shmctl (*id, IPC_RMID, NULL);
}

void
jack_release_shm (jack_shm_info_t* si)
{
	/* registry may or may not be locked */
	if (si->attached_at != MAP_FAILED) {
		shmdt (si->attached_at);
	}
}

int
jack_shmalloc (const char* name_not_used, jack_shmsize_t size,
	       jack_shm_info_t* si) 
{
	int shmflags;
	int shmid;
	int rc = -1;
	jack_shm_registry_t* registry;

	jack_shm_lock_registry ();

	if ((registry = jack_get_free_shm_info ())) {

		shmflags = 0666 | IPC_CREAT | IPC_EXCL;

		if ((shmid = shmget (IPC_PRIVATE, size, shmflags)) >= 0) {

			registry->size = size;
			registry->id = shmid;
			registry->allocator = getpid();
			si->index = registry->index;
			si->attached_at = MAP_FAILED; /* not attached */
			rc = 0;

		} else {
			jack_error ("cannot create shm segment %s (%s)",
				    name_not_used, strerror (errno));
		}
	}

	jack_shm_unlock_registry ();

	return rc;
}

int
jack_attach_shm (jack_shm_info_t* si)
{
	if ((si->attached_at = shmat (jack_shm_registry[si->index].id,
				      0, 0)) < 0) {
		jack_error ("cannot attach shm segment (%s)",
			    strerror (errno));
		jack_release_shm_info (si->index);
		return -1;
	}
	return 0;
}

#endif /* !USE_POSIX_SHM */