path: root/sys-utils/ipc.info

This is Info file ipc.info, produced by Makeinfo-1.47 from the input
file ipc.texi.

   This file documents the System V style inter process communication
primitives available under linux.

   Copyright (C) 1992  krishna balasubramanian

   Permission is granted to use this material and the accompanying
programs within the terms of the GNU GPL.


File: ipc.info,  Node: top,  Next: Overview,  Prev: Notes,  Up: (dir)

System V IPC.
*************

   These facilities are provided to maintain compatibility with
programs developed on system V unix systems and others that rely on
these system V mechanisms to accomplish inter process communication
(IPC).

   The specifics described here are applicable to the Linux
implementation. Other implementations may do things slightly
differently.

* Menu:

* Overview::  		What is system V ipc? Overall mechanisms.
* Messages::  		System calls for message passing.
* Semaphores:: 		System calls for semaphores.
* Shared Memory:: 	System calls for shared memory access.
* Notes:: 		Miscellaneous notes.


File: ipc.info,  Node: Overview,  Next: example,  Prev: top,  Up: top

Overview
========

System V IPC consists of three mechanisms:

   * Messages : exchange messages with any process or server.

   * Semaphores : allow unrelated processes to synchronize execution.

   * Shared memory : allow unrelated processes to share memory.

* Menu:

* example:: 	Using shared memory.
* perms:: 	Description of access permissions.
* syscalls::    Overview of ipc system calls.

   Access to all resources is permitted on the basis of permissions set
up when the resource was created.

   A resource here consists of message queue, a semaphore set (array)
or a shared memory segment.

   A resource must first be allocated by a creator before it is used.
The creator can assign a different owner. After use the resource must
be explicitly destroyed by the creator or owner.

   A resource is identified by a numeric ID. Typically a creator
defines a KEY that may be used to access the resource. The user process
may then use this KEY in the "get" system call to obtain the ID for the
corresponding resource. This ID is then used for all further access. A
library call "ftok" is provided to translate pathnames or strings to
numeric keys.

   There are system and implementation defined limits on the number and
sizes of resources of any given type. Some of these are imposed by the
implementation and others by the system administrator when configuring
the kernel (*Note msglimits::, *Note semlimits::, *Note shmlimits::).

   There is an `msqid_ds', `semid_ds' or `shmid_ds' struct associated
with each message queue, semaphore array or shared segment. Each ipc
resource has an associated `ipc_perm' struct which defines the creator,
owner, access perms ..etc.., for the resource. These structures are
detailed in the following sections.


File: ipc.info,  Node: example,  Next: perms,  Prev: Overview,  Up: Overview

example
=======

   Here is a code fragment with pointers on how to use shared memory.
The same methods are applicable to other resources.

   In a typical access sequence the creator allocates a new instance of
the resource with the `get' system call using the IPC_CREAT flag.

creator process:
     #include <sys/shm.h>
     int id;
     key_t key;
     char proc_id = 'C';
     int size = 0x5000;	/* 20 K */
     int flags = 0664 | IPC_CREAT;		/* read-only for others */
     
     key = ftok ("~creator/ipckey", proc_id);
     id = shmget (key, size, flags);
     exit (0);	/* quit leaving resource allocated */

Users then gain access to the resource using the same key.
Client process:
     #include <sys/shm.h>
     char *shmaddr;
     int id;
     key_t key;
     char proc_id = 'C';
     
     key = ftok ("~creator/ipckey", proc_id);
     
     id = shmget (key, 0, 004);		/* default size   */
     if (id == -1)
           perror ("shmget ...");
     
     shmaddr = shmat (id, 0, SHM_RDONLY); /* attach segment for reading */
     if (shmaddr == (char *) -1)
           perror ("shmat ...");
     
     local_var = *(shmaddr + 3); 	/* read segment etc. */
     
     shmdt (shmaddr);		/* detach segment */

When the resource is no longer needed the creator should remove it.
Creator/owner process 2:
     key = ftok ("~creator/ipckey", proc_id)
     id = shmget (key, 0, 0);
     shmctl (id, IPC_RMID, NULL);


File: ipc.info,  Node: perms,  Next: syscalls,  Prev: example,  Up: Overview

Permissions
===========

   Each resource has an associated `ipc_perm' struct which defines the
creator, owner and access perms for the resource.

     struct ipc_perm
             key_t key;    /* set by creator */
             ushort uid;   /* owner euid and egid */
             ushort gid;
             ushort cuid;  /* creator euid and egid */
             ushort cgid;
             ushort mode;  /* access modes in lower 9 bits */
             ushort seq;   /* sequence number */

   The creating process is the default owner. The owner can be
reassigned by the creator and has creator perms. Only the owner,
creator or super-user can delete the resource.

   The lowest nine bits of the flags parameter supplied by the user to
the system call are compared with the values stored in `ipc_perms.mode'
to determine if the requested access is allowed. In the case that the
system call creates the resource, these bits are initialized from the
user supplied value.

   As for files, access permissions are specified as read, write and
exec for user, group or other (though the exec perms are unused). For
example 0624 grants read-write to owner, write-only to group and
read-only access to others.

   For shared memory, note that read-write access for segments is
determined by a separate flag which is not stored in the `mode' field.
Shared memory segments attached with write access can be read.

   The `cuid', `cgid', `key' and `seq' fields cannot be changed by the
user.


File: ipc.info,  Node: syscalls,  Next: Messages,  Prev: perms,  Up: Overview

IPC system calls
================

   This section provides an overview of the IPC system calls. See the
specific sections on each type of resource for details.

   Each type of mechanism provides a "get", "ctl" and one or more "op"
system calls that allow the user to create or procure the resource
(get), define its behaviour or destroy it (ctl) and manipulate the
resources (op).

The "get" system calls
----------------------

   The `get' call typically takes a KEY and returns a numeric ID that
is used for further access. The ID is an index into the resource table.
A sequence number is maintained and incremented when a resource is
destroyed so that acceses using an obselete ID is likely to fail.

   The user also specifies the permissions and other behaviour
charecteristics for the current access. The flags are or-ed with the
permissions when invoking system calls as in:
     msgflg = IPC_CREAT | IPC_EXCL | 0666;
     id = msgget (key, msgflg);

   * `key' : IPC_PRIVATE => new instance of resource is initialized.

   * `flags' :
          IPC_CREAT : resource created for KEY if it does not exist.

          IPC_CREAT | IPC_EXCL : fail if resource exists for KEY.

   * returns : an identifier used for all further access to the
     resource.

   Note that IPC_PRIVATE is not a flag but a special `key' that ensures
(when the call is successful) that a new resource is created.

   Use of IPC_PRIVATE does not make the resource inaccessible to other
users. For this you must set the access permissions appropriately.

   There is currently no way for a process to ensure exclusive access
to a resource. IPC_CREAT | IPC_EXCL only ensures (on success) that a new
resource was initialized. It does not imply exclusive access.

See Also : *Note msgget::, *Note semget::, *Note shmget::.

The "ctl" system calls
----------------------

   Provides or alters the information stored in the structure that
describes the resource indexed by ID.

     #include <sys/msg.h>
     struct msqid_ds buf;
     err = msgctl (id, IPC_STAT, &buf);
     if (err)
             !$#%*
     else
             printf ("creator uid = %d\n", buf.msg_perm.cuid);
             ....

Commands supported by all `ctl' calls:
   * IPC_STAT : read info on resource  specified by id into user
     allocated buffer. The user must have read access to the resource.

   * IPC_SET : write info from buffer into resource data structure. The
     user must be owner creator or super-user.

   * IPC_RMID : remove resource. The user must be the owner, creator or
     super-user.

The IPC_RMID command results in immediate removal of a message queue or
semaphore array. Shared memory segments however, are only destroyed
upon the last detach after IPC_RMID is executed.

   The `semctl' call provides a number of command options that allow
the user to determine or set the values of the semaphores in an array.

See Also: *Note msgctl::, *Note semctl::, *Note shmctl::.

The "op" system calls
---------------------

   Used to send or receive messages, read or alter semaphore values,
attach or detach shared memory segments. The IPC_NOWAIT flag will cause
the operation to fail with error EAGAIN if the process has to wait on
the call.

`flags' : IPC_NOWAIT  => return with error if a wait is required.

See Also: *Note msgsnd::,*Note msgrcv::,*Note semop::,*Note shmat::,
*Note shmdt::.


File: ipc.info,  Node: Messages,  Next: msgget,  Prev: syscalls,  Up: top

Messages
========

   A message resource is described by a struct `msqid_ds' which is
allocated and initialized when the resource is created. Some fields in
`msqid_ds' can then be altered (if desired) by invoking `msgctl'. The
memory used by the resource is released when it is destroyed by a
`msgctl' call.

     struct msqid_ds
         struct ipc_perm msg_perm;
         struct msg *msg_first;  /* first message on queue (internal) */
         struct msg *msg_last;   /* last message in queue (internal) */
         time_t msg_stime;       /* last msgsnd time */
         time_t msg_rtime;       /* last msgrcv time */
         time_t msg_ctime;       /* last change time */
         struct wait_queue *wwait; /* writers waiting (internal) */
         struct wait_queue *rwait; /* readers waiting (internal) */
         ushort msg_cbytes;      /* number of bytes used on queue */
         ushort msg_qnum;        /* number of messages in queue */
         ushort msg_qbytes;      /* max number of bytes on queue */
         ushort msg_lspid;       /* pid of last msgsnd */
         ushort msg_lrpid;       /* pid of last msgrcv */

   To send or receive a message the user allocates a structure that
looks like a `msgbuf' but with an array `mtext' of the required size.
Messages have a type (positive integer) associated with them so that
(for example) a listener can choose to receive only messages of a given
type.

     struct msgbuf
         long mtype;      type of message (*Note msgrcv::).
         char mtext[1];   message text .. why is this not a ptr?

   The user must have write permissions to send and read permissions to
receive messages on a queue.

   When `msgsnd' is invoked, the user's message is copied into an
internal struct `msg' and added to the queue. A `msgrcv' will then read
this message and free the associated struct `msg'.

* Menu:

* msgget::
* msgsnd::
* msgrcv::
* msgctl::
* msglimits:: Implementation defined limits.


File: ipc.info,  Node: msgget,  Next: msgsnd,  Prev: Messages,  Up: Messages

msgget
------

A message queue is allocated by a msgget system call :

     msqid = msgget (key_t key, int msgflg);

   * `key': an integer usually got from `ftok()' or IPC_PRIVATE.

   * `msgflg':
          IPC_CREAT : used to create a new resource if it does not
          already exist.

          IPC_EXCL | IPC_CREAT : used to ensure failure of the call if
          the resource already exists.

          rwxrwxrwx : access permissions.

   * returns: msqid (an integer used for all further access) on success.
     -1 on failure.

   A message queue is allocated if there is no resource corresponding
to the given key. The access permissions specified are then copied into
the `msg_perm' struct and the fields in `msqid_ds' initialized. The
user must use the IPC_CREAT flag or key = IPC_PRIVATE, if a new
instance is to be allocated. If a resource corresponding to KEY already
exists, the access permissions are verified.

Errors:
EACCES : (procure) Do not have permission for requested access.
EEXIST : (allocate) IPC_CREAT | IPC_EXCL specified and resource exists.
EIDRM  : (procure) The resource was removed.
ENOSPC : All id's are taken (max of MSGMNI id's system-wide).
ENOENT : Resource does not exist and IPC_CREAT not specified.
ENOMEM : A new `msqid_ds' was to be created but ... nomem.


File: ipc.info,  Node: msgsnd,  Next: msgrcv,  Prev: msgget,  Up: Messages

msgsnd
------

     int msgsnd (int msqid, struct msgbuf *msgp, int msgsz, int msgflg);

   * `msqid' : id obtained by a call to msgget.

   * `msgsz' : size of msg text (`mtext') in bytes.

   * `msgp' : message to be sent. (msgp->mtype must be positive).

   * `msgflg' : IPC_NOWAIT.

   * returns : msgsz on success. -1 on error.

   The message text and type are stored in the internal `msg'
structure. `msg_cbytes', `msg_qnum', `msg_lspid', and `msg_stime'
fields are updated. Readers waiting on the queue are awakened.

Errors:
EACCES : Do not have write permission on queue.
EAGAIN : IPC_NOWAIT specified and queue is full.
EFAULT : msgp not accessible.
EIDRM  : The message queue was removed.
EINTR  : Full queue ... would have slept but ... was interrupted.
EINVAL : mtype < 1, msgsz > MSGMAX, msgsz < 0, msqid < 0 or unused.
ENOMEM : Could not allocate space for header and text.

File: ipc.info,  Node: msgrcv,  Next: msgctl,  Prev: msgsnd,  Up: Messages

msgrcv
------

     int msgrcv (int msqid, struct msgbuf *msgp, int msgsz, long msgtyp,
     			int msgflg);

   * msqid  : id obtained by a call to msgget.

   * msgsz  : maximum size of message to receive.

   * msgp   : allocated by user to store the message in.

   * msgtyp :
          0 => get first message on queue.

          > 0 => get first message of matching type.

          < 0 => get message with least type  which is <= abs(msgtyp).

   * msgflg :
          IPC_NOWAIT : Return immediately if message not found.

          MSG_NOERROR : The message is truncated if it is larger than
          msgsz.

          MSG_EXCEPT : Used with msgtyp > 0 to receive any msg except
          of specified type.

   * returns : size of message if found. -1 on error.

   The first message that meets the `msgtyp' specification is
identified. For msgtyp < 0, the entire queue is searched for the
message with the smallest type.

   If its length is smaller than msgsz or if the user specified the
MSG_NOERROR flag, its text and type are copied to msgp->mtext and
msgp->mtype, and it is taken off the queue.

   The `msg_cbytes', `msg_qnum', `msg_lrpid', and `msg_rtime' fields
are updated. Writers waiting on the queue are awakened.

Errors:
E2BIG  : msg bigger than msgsz and MSG_NOERROR not specified.
EACCES : Do not have permission for reading the queue.
EFAULT : msgp not accessible.
EIDRM  : msg queue was removed.
EINTR  : msg not found ... would have slept but ... was interrupted.
EINVAL : msgsz > msgmax or msgsz < 0, msqid < 0 or unused.
ENOMSG : msg of requested type not found and IPC_NOWAIT specified.


File: ipc.info,  Node: msgctl,  Next: msglimits,  Prev: msgrcv,  Up: Messages

msgctl
------

     int msgctl (int msqid, int cmd, struct msqid_ds *buf);

   * msqid  : id obtained by a call to msgget.

   * buf    : allocated by user for reading/writing info.

   * cmd    : IPC_STAT, IPC_SET, IPC_RMID (*Note syscalls::).

   IPC_STAT results in the copy of the queue data structure into the
user supplied buffer.

   In the case of IPC_SET, the queue size (`msg_qbytes') and the `uid',
`gid', `mode' (low 9 bits) fields of the `msg_perm' struct are set from
the user supplied values. `msg_ctime' is updated.

   Note that only the super user may increase the limit on the size of a
message queue beyond MSGMNB.

   When the queue is destroyed (IPC_RMID), the sequence number is
incremented and all waiting readers and writers are awakened. These
processes will then return with `errno' set to EIDRM.

Errors:

EPERM  : Insufficient privilege to increase the size of the queue
(IPC_SET) or remove it (IPC_RMID).
EACCES : Do not have permission for reading the queue (IPC_STAT).
EFAULT : buf not accessible (IPC_STAT, IPC_SET).
EIDRM  : msg queue was removed.
EINVAL : invalid cmd, msqid < 0 or unused.


File: ipc.info,  Node: msglimits,  Next: Semaphores,  Prev: msgctl,  Up: Messages

Limis on Message Resources
--------------------------

Sizeof various structures:
     msqid_ds        52   /* 1 per message  queue .. dynamic */

     msg             16   /* 1 for each message in system .. dynamic */

     msgbuf           8   /* allocated by user */

Limits
   * MSGMNI : number of message queue identifiers ... policy.

   * MSGMAX : max size of message. Header and message space allocated
     on one page. MSGMAX = (PAGE_SIZE - sizeof(struct msg)).
     Implementation maximum MSGMAX = 4080.

   * MSGMNB : default max size of a message queue ... policy. The
     super-user can increase the size of a queue beyond MSGMNB by a
     `msgctl' call.

Unused or unimplemented:
MSGTQL  max number of message headers system-wide.
MSGPOOL total size in bytes of msg pool.


File: ipc.info,  Node: Semaphores,  Next: semget,  Prev: msglimits,  Up: top

Semaphores
==========

   Each semaphore has a value >= 0. An id provides access to an array
of `nsems' semaphores. Operations such as read, increment or decrement
semaphores in a set are performed by the `semop' call which processes
`nsops' operations at a time. Each operation is specified in a struct
`sembuf' described below. The operations are applied only if all of
them succeed.

   If you do not have a need for such arrays, you are probably better
off using the `test_bit', `set_bit' and  `clear_bit' bit-operations
defined in <asm/bitops.h>.

   Semaphore operations may also be qualified by a SEM_UNDO flag which
results in the operation being undone when the process exits.

   If a decrement cannot go through, a process will be put to sleep on
a queue waiting for the `semval' to increase unless it specifies
IPC_NOWAIT. A read operation can similarly result in a sleep on a queue
waiting for `semval' to become 0. (Actually there are two queues per
semaphore array).

A semaphore array is described by:
     struct semid_ds
       struct ipc_perm sem_perm;
       time_t          sem_otime;      /* last semop time */
       time_t          sem_ctime;      /* last change time */
       struct wait_queue *eventn;	  /* wait for a semval to increase */
       struct wait_queue *eventz;      /* wait for a semval to become 0 */
       struct sem_undo  *undo;         /* undo entries */
       ushort          sem_nsems;      /* no. of semaphores in array */

Each semaphore is described internally by :
     struct sem
       short   sempid;         /* pid of last semop() */
       ushort  semval;         /* current value */
       ushort  semncnt;        /* num procs awaiting increase in semval */
       ushort  semzcnt;        /* num procs awaiting semval = 0 */

* Menu:

* semget::
* semop::
* semctl::
* semlimits:: Limits imposed by this implementation.


File: ipc.info,  Node: semget,  Next: semop,  Prev: Semaphores,  Up: Semaphores

semget
------

A semaphore array is allocated by a semget system call:

     semid = semget (key_t key, int nsems, int semflg);

   * `key' : an integer usually got from `ftok' or IPC_PRIVATE

   * `nsems' :
          # of semaphores in array (0 <= nsems <= SEMMSL <= SEMMNS)

          0 => dont care can be used when not creating the resource. If
          successful you always get access to the entire array anyway.

   * semflg :
          IPC_CREAT used to create a new resource

          IPC_EXCL used with IPC_CREAT to ensure failure if the
          resource exists.

          rwxrwxrwx  access permissions.

   * returns : semid on success. -1 on failure.

   An array of nsems semaphores is allocated if there is no resource
corresponding to the given key. The access permissions specified are
then copied into the `sem_perm' struct for the array along with the
user-id etc. The user must use the IPC_CREAT flag or key = IPC_PRIVATE
if a new resource is to be created.

Errors:
EINVAL : nsems not in above range (allocate).
nsems greater than number in array (procure).
EEXIST : (allocate) IPC_CREAT | IPC_EXCL specified and resource exists.
EIDRM  : (procure) The resource was removed.
ENOMEM : could not allocate space for semaphore array.
ENOSPC : No arrays available (SEMMNI), too few semaphores available
(SEMMNS).
ENOENT : Resource does not exist and IPC_CREAT not specified.
EACCES : (procure) do not have permission for specified access.


File: ipc.info,  Node: semop,  Next: semctl,  Prev: semget,  Up: Semaphores

semop
-----

Operations on semaphore arrays are performed by calling semop :

     int semop (int semid, struct sembuf *sops, unsigned nsops);

   * semid : id obtained by a call to semget.

   * sops : array of semaphore operations.

   * nsops : number of operations in array (0 < nsops < SEMOPM).

   * returns : semval for last operation. -1 on failure.

Operations are described by a structure sembuf:
     struct sembuf
         ushort  sem_num;        /* semaphore index in array */
         short   sem_op;         /* semaphore operation */
         short   sem_flg;        /* operation flags */

   The value `sem_op' is to be added (signed) to the current value
semval of the semaphore with index sem_num (0 .. nsems -1) in the set.
Flags recognized in sem_flg are IPC_NOWAIT and SEM_UNDO.

Two kinds of operations can result in wait:
  1. If sem_op is 0 (read operation) and semval is non-zero, the process
     sleeps on a queue waiting for semval to become zero or returns with
     error EAGAIN if (IPC_NOWAIT | sem_flg) is true.

  2. If (sem_op < 0) and (semval + sem_op < 0), the process either
     sleeps on a queue waiting for semval to increase or returns with
     error EAGAIN if (sem_flg & IPC_NOWAIT) is true.

   The array sops is first read in and preliminary checks performed on
the arguments. The operations are parsed to determine if any of them
needs write permissions or requests an undo operation.

   The operations are then tried and the process sleeps if any operation
that does not specify IPC_NOWAIT cannot go through. If a process sleeps
it repeats these checks on waking up. If any operation that requests
IPC_NOWAIT, cannot go through at any stage, the call returns with errno
set to EAGAIN.

   Finally, operations are committed when all go through without an
intervening sleep. Processes waiting on the zero_queue or
increment_queue are awakened if any of the semval's becomes zero or is
incremented respectively.

Errors:
E2BIG  : nsops > SEMOPM.
EACCES : Do not have permission for requested (read/alter) access.
EAGAIN : An operation with IPC_NOWAIT specified could not go through.
EFAULT : The array sops is not accessible.
EFBIG  : An operation had semnum >= nsems.
EIDRM  : The resource was removed.
EINTR  : The process was interrupted on its way to a wait queue.
EINVAL : nsops is 0, semid < 0 or unused.
ENOMEM : SEM_UNDO requested. Could not allocate space for undo
structure.
ERANGE : sem_op + semval > SEMVMX for some operation.


File: ipc.info,  Node: semctl,  Next: semlimits,  Prev: semop,  Up: Semaphores

semctl
------

     int semctl (int semid, int semnum, int cmd, union semun arg);

   * semid : id obtained by a call to semget.

   * cmd :
          GETPID  return pid for the process that executed the last
          semop.

          GETVAL  return semval of semaphore with index semnum.

          GETNCNT return number of processes waiting for semval to
          increase.

          GETZCNT return number of processes waiting for semval to
          become 0

          SETVAL  set semval = arg.val.

          GETALL  read all semval's into arg.array.

          SETALL  set all semval's with values given in arg.array.

   * returns : 0 on success or as given above. -1 on failure.

   The first 4 operate on the semaphore with index semnum in the set.
The last two operate on all semaphores in the set.

   `arg' is a union :
     union semun
         int val;               value for SETVAL.
         struct semid_ds *buf;  buffer for IPC_STAT and IPC_SET.
         ushort *array;         array for GETALL and SETALL

   * IPC_SET, SETVAL, SETALL : sem_ctime is updated.

   * SETVAL, SETALL : Undo entries are cleared for altered semaphores in
     all processes. Processes sleeping on the wait queues are awakened
     if a semval becomes 0 or increases.

   * IPC_SET : sem_perm.uid, sem_perm.gid, sem_perm.mode are updated
     from user supplied values.

Errors:

EACCES : do not have permission for specified access.
EFAULT : arg is not accessible.
EIDRM  : The resource was removed.
EINVAL : semid < 0 or semnum < 0 or semnum >= nsems.
EPERM  : IPC_RMID, IPC_SET ... not creator, owner or super-user.
ERANGE : arg.array[i].semval > SEMVMX or < 0 for some i.


File: ipc.info,  Node: semlimits,  Next: Shared Memory,  Prev: semctl,  Up: Semaphores

Limits on Semaphore Resources
-----------------------------

Sizeof various structures:
     semid_ds    44   /* 1 per semaphore array .. dynamic */
     sem          8   /* 1 for each semaphore in system .. dynamic */
     sembuf       6   /* allocated by user */
     sem_undo    20   /* 1 for each undo request .. dynamic */

Limits :
   * SEMVMX  32767  semaphore maximum value (short).

   * SEMMNI  number of semaphore identifiers (or arrays) system
     wide...policy.

   * SEMMSL  maximum  number  of semaphores per id. 1 semid_ds per
     array, 1 struct sem per semaphore => SEMMSL =  (PAGE_SIZE -
     sizeof(semid_ds)) / sizeof(sem). Implementation maximum SEMMSL =
     500.

   * SEMMNS  maximum number of semaphores system wide ... policy.
     Setting SEMMNS >= SEMMSL*SEMMNI makes it irrelevent.

   * SEMOPM 	Maximum number of operations in one semop
     call...policy.

Unused or unimplemented:
SEMAEM  adjust on exit max value.
SEMMNU  number of undo structures system-wide.
SEMUME  maximum number of undo entries per process.


File: ipc.info,  Node: Shared Memory,  Next: shmget,  Prev: semlimits,  Up: top

Shared Memory
=============

   Shared memory is distinct from the sharing of read-only code pages or
the sharing of unaltered data pages that is available due to the
copy-on-write mechanism. The essential difference is that the shared
pages are dirty (in the case of Shared memory) and can be made to
appear at a convenient location in the process' address space.

A shared segment is described by :
     struct shmid_ds
         struct  ipc_perm shm_perm;
         int     shm_segsz;              /* size of segment (bytes) */
         time_t  shm_atime;              /* last attach time */
         time_t  shm_dtime;              /* last detach time */
         time_t  shm_ctime;              /* last change time */
         ulong   *shm_pages;             /* internal page table */
         ushort  shm_cpid;               /* pid, creator */
         ushort  shm_lpid;               /* pid, last operation */
         short   shm_nattch;             /* no. of current attaches */

   A shmget allocates a shmid_ds and an internal page table. A shmat
maps the segment into the process' address space with pointers into the
internal page table and the actual pages are faulted in as needed. The
memory associated with the segment must be explicitly destroyed by
calling shmctl with IPC_RMID.

* Menu:

* shmget::
* shmat::
* shmdt::
* shmctl::
* shmlimits:: Limits imposed by this implementation.


File: ipc.info,  Node: shmget,  Next: shmat,  Prev: Shared Memory,  Up: Shared Memory

shmget
------

A shared memory segment is allocated by a shmget system call:

     int shmget(key_t key, int size, int shmflg);

   * key : an integer usually got from `ftok' or IPC_PRIVATE

   * size : size of the segment in bytes (SHMMIN <= size <= SHMMAX).

   * shmflg :
          IPC_CREAT used to create a new resource

          IPC_EXCL used with IPC_CREAT to ensure failure if the
          resource exists.

          rwxrwxrwx  access permissions.

   * returns : shmid on success. -1 on failure.

   A descriptor for a shared memory segment is allocated if there isn't
one corresponding to the given key. The access permissions specified are
then copied into the `shm_perm' struct for the segment along with the
user-id etc. The user must use the IPC_CREAT flag or key = IPC_PRIVATE
to allocate a new segment.

   If the segment already exists, the access permissions are verified,
and a check is made to see that it is not marked for destruction.

   `size' is effectively rounded up to a multiple of PAGE_SIZE as shared
memory is allocated in pages.

Errors:
EINVAL : (allocate) Size not in range specified above.
(procure) Size greater than size of segment.
EEXIST : (allocate) IPC_CREAT | IPC_EXCL specified and resource exists.
EIDRM  : (procure) The resource is marked destroyed or was removed.
ENOSPC : (allocate) All id's are taken (max of SHMMNI id's system-wide).
Allocating a segment of the requested size would exceed the system wide
limit on total shared memory (SHMALL).
ENOENT : (procure) Resource does not exist and IPC_CREAT not specified.
EACCES : (procure) Do not have permission for specified access.
ENOMEM : (allocate) Could not allocate memory for shmid_ds or pg_table.


File: ipc.info,  Node: shmat,  Next: shmdt,  Prev: shmget,  Up: Shared Memory

shmat
-----

Maps a shared segment into the process' address space.

     char *virt_addr;
     virt_addr =  shmat (int shmid, char *shmaddr, int shmflg);

   * shmid : id got from call to shmget.

   * shmaddr : requested attach address.
     If shmaddr is 0 the system finds an unmapped region.
     If a non-zero value is indicated the value must be page    
     aligned or the user must specify the SHM_RND flag.

   * shmflg :
     SHM_RDONLY : request read-only attach.
     SHM_RND : attach address is rounded DOWN to a multiple of SHMLBA.

   * returns: virtual address of attached segment. -1 on failure.

   When shmaddr is 0, the attach address is determined by finding an
unmapped region in the address range 1G to 1.5G, starting at 1.5G and
coming down from there. The algorithm is very simple so you are
encouraged to avoid non-specific attaches.

Algorithm:
     Determine attach address as described above.
     Check region (shmaddr, shmaddr + size) is not mapped and allocate
         page tables (undocumented SHM_REMAP flag!).
     Map the region by setting up pointers into the internal page table.
     Add a descriptor for the attach to the task struct for the process.
     `shm_nattch', `shm_lpid', `shm_atime' are updated.

Notes:
The `brk' value is not altered. The segment is automatically detached
when the process exits. The same segment may be attached as read-only
or read-write and     more than once in the process' address space. A
shmat can succeed on a segment marked for destruction. The request for
a particular type of attach is made using the SHM_RDONLY flag. There is
no notion of a write-only attach. The requested attach     permissions
must fall within those allowed by `shm_perm.mode'.

Errors:
EACCES : Do not have permission for requested access.
EINVAL : shmid < 0 or unused, shmaddr not aligned, attach at brk failed.
EIDRM  : resource was removed.
ENOMEM : Could not allocate memory for descriptor or page tables.


File: ipc.info,  Node: shmdt,  Next: shmctl,  Prev: shmat,  Up: Shared Memory

shmdt
-----

     int shmdt (char *shmaddr);

   * shmaddr : attach address of segment (returned by shmat).

   * returns : 0 on success. -1 on failure.

   An attached segment is detached and `shm_nattch' decremented. The
occupied region in user space is unmapped. The segment is destroyed if
it is marked for destruction and `shm_nattch' is 0. `shm_lpid' and
`shm_dtime' are updated.

Errors:
EINVAL : No shared memory segment attached at shmaddr.


File: ipc.info,  Node: shmctl,  Next: shmlimits,  Prev: shmdt,  Up: Shared Memory

shmctl
------

Destroys allocated segments. Reads/Writes the control structures.

     int shmctl (int shmid, int cmd, struct shmid_ds *buf);

   * shmid : id got from call to shmget.

   * cmd : IPC_STAT, IPC_SET, IPC_RMID (*Note syscalls::).
          IPC_SET : Used to set the owner uid, gid, and shm_perms.mode
          field.

          IPC_RMID : The segment is marked destroyed. It is only
          destroyed on the last detach.

          IPC_STAT : The shmid_ds structure is copied into the user
          allocated buffer.

   * buf : used to read (IPC_STAT) or write (IPC_SET) information.

   * returns : 0 on success, -1 on failure.

   The user must execute an IPC_RMID shmctl call to free the memory
allocated by the shared segment. Otherwise all the pages faulted in
will continue to live in memory or swap.

Errors:
EACCES : Do not have permission for requested access.
EFAULT : buf is not accessible.
EINVAL : shmid < 0 or unused.
EIDRM  : identifier destroyed.
EPERM  : not creator, owner or super-user (IPC_SET, IPC_RMID).


File: ipc.info,  Node: shmlimits,  Next: Notes,  Prev: shmctl,  Up: Shared Memory

Limits on Shared Memory Resources
---------------------------------

Limits:
   * SHMMNI  max num of shared segments system wide ... 4096.

   * SHMMAX  max shared memory segment size (bytes) ... 4M

   * SHMMIN  min shared memory segment size (bytes). 1 byte (though
     PAGE_SIZE is the effective minimum size).

   * SHMALL  max shared mem system wide (in pages) ... policy.

   * SHMLBA  segment low boundary address multiple. Must be page
     aligned. SHMLBA = PAGE_SIZE.

Unused or unimplemented:
SHMSEG : maximum number of shared segments per process.


File: ipc.info,  Node: Notes,  Next: top,  Prev: shmlimits,  Up: top

Miscellaneous Notes
===================

   The system calls are mapped into one -- `sys_ipc'. This should be
transparent to the user.

Semaphore `undo' requests
-------------------------

   There is one sem_undo structure associated with a process for each
semaphore which was altered (with an undo request) by the process.
`sem_undo' structures are freed only when the process exits.

   One major cause for unhappiness with the undo mechanism is that it
does not fit in with the notion of having an atomic set of operations
on an array. The undo requests for an array and each semaphore therein
may have been accumulated over many `semop' calls. Thus use the undo
mechanism with private semaphores only.

   Should the process sleep in `exit' or should all undo operations be
applied with the IPC_NOWAIT flag in effect? Currently  those undo
operations which go through immediately are applied and those that
require a wait are ignored silently.

Shared memory, `malloc' and the `brk'.
--------------------------------------

   Note that since this section was written the implementation was
changed so that non-specific attaches are done in the region 1G - 1.5G.
However much of the following is still worth thinking about so I left
it in.

   On many systems, the shared memory is allocated in a special region
of the address space ... way up somewhere. As mentioned earlier, this
implementation attaches shared segments at the lowest possible address.
Thus if you plan to use `malloc', it is wise to malloc a large space
and then proceed to attach the shared segments. This way malloc sets
the brk sufficiently above the region it will use.

   Alternatively you can use `sbrk' to adjust the `brk' value as you
make shared memory attaches. The implementation is not very smart about
selecting attach addresses. Using the system default addresses will
result in fragmentation if detaches do not occur in the reverse
sequence as attaches.

   Taking control of the matter is probably best. The rule applied is
that attaches are allowed in unmapped regions other than in the text
space (see <a.out.h>). Also remember that attach addresses and segment
sizes are multiples of PAGE_SIZE.

   One more trap (I quote Bruno on this). If you use malloc() to get
space for your shared memory (ie. to fix the `brk'), you must ensure you
get an unmapped address range. This means you must mallocate more memory
than you had ever allocated before. Memory returned by malloc(), used,
then freed by free() and then again returned by malloc is no good.
Neither is calloced memory.

   Note that a shared memory region remains a shared memory region until
you unmap it. Attaching a segment at the `brk' and calling malloc after
that will result in an overlap of what malloc thinks is its space with
what is really a shared memory region. For example in the case of a
read-only attach, you will not be able to write to the overlapped
portion.

Fork, exec and exit
-------------------

   On a fork, the child inherits attached shared memory segments but
not the semaphore undo information.

   In the case of an exec, the attached shared segments are detached.
The sem undo information however remains intact.

   Upon exit, all attached shared memory segments are detached. The
adjust values in the undo structures are added to the relevant semvals
if the operations are permitted. Disallowed operations are ignored.

Other Features
--------------

   These features of the current implementation are likely to be
modified in the future.

   The SHM_LOCK and SHM_UNLOCK flag are available (super-user) for use
with the `shmctl' call to prevent swapping of a shared segment. The user
must fault in any pages that are required to be present after locking
is enabled.

   The IPC_INFO, MSG_STAT, MSG_INFO, SHM_STAT, SHM_INFO, SEM_STAT,
SEMINFO `ctl' calls are used by the `ipcs' program to provide
information on allocated resources. These can be modified as needed or
moved to a proc file system interface.

   Thanks to Ove Ewerlid, Bruno Haible, Ulrich Pegelow and Linus
Torvalds for ideas, tutorials, bug reports and fixes, and merriment.
And more thanks to Bruno.



Tag Table:
Node: top349
Node: Overview1049
Node: example2881
Node: perms4383
Node: syscalls5943
Node: Messages9392
Node: msgget11426
Node: msgsnd12810
Node: msgrcv13778
Node: msgctl15477
Node: msglimits16684
Node: Semaphores17558
Node: semget19517
Node: semop21060
Node: semctl23624
Node: semlimits25383
Node: Shared Memory26523
Node: shmget28008
Node: shmat29803
Node: shmdt31851
Node: shmctl32383
Node: shmlimits33514
Node: Notes34161

End Tag Table