no more dither when converting from float to 24 bit values - i am not sure how this...

[jack.git] / 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.
 *
 */
#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 (int new_registry)
{
        int rc;

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

        jack_shm_lock_registry ();

        rc = jack_access_registry (&registry_info);

        if (new_registry) {
                jack_remove_shm (&registry_id);
                rc = ENOENT;
        }

        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 new_registry)
{
        int i;
        pid_t my_pid = getpid ();

        jack_set_server_prefix (server_name);

        jack_info ("JACK compiled with %s SHM support.", JACK_SHM_TYPE);

        if (jack_server_initialize_shm (new_registry))
                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_release_shm (si);
        jack_destroy_shm (si);

        if (jack_shmalloc (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 (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.
         */
        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 (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)",
                                    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 */