ipc.texi

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@setfilename ipc.info
@settitle Inter Process Communication.
@setchapternewpage odd
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@ifinfo
This file documents the System V style inter process communication
primitives available under linux.

Copyright @copyright{} 1992  krishna balasubramanian
    
Permission is granted to use this material and the accompanying
programs within the terms of the GNU GPL.
@end ifinfo
    
@titlepage
@sp 10
@center @titlefont{System V Inter Process Communication}
@sp 2
@center krishna balasubramanian,
    
@comment  The following two commands start the copyright page.
@page
@vskip 0pt plus 1filll
Copyright @copyright{} 1992  krishna balasubramanian
    
Permission is granted to use this material and the accompanying
programs within the terms of the GNU GPL.
@end titlepage

@dircategory Miscellaneous
@direntry
* ipc: (ipc).           System V style inter process communication
@end direntry

@node Top, Overview, Notes, (dir)
@chapter 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).@refill

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.
@end menu

@node Overview, example, Top, Top
@section Overview

@noindent System V IPC consists of three mechanisms:

@itemize @bullet
@item 
Messages : exchange messages with any process or server.
@item 
Semaphores : allow unrelated processes to synchronize execution.
@item 
Shared memory : allow unrelated processes to share memory.
@end itemize

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

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

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

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.@refill

A resource is identified by a numeric @var{id}. Typically a creator
defines a @var{key} that may be used to access the resource. The user
process may then use this @var{key} in the @dfn{get} system call to obtain
the @var{id} for the corresponding resource. This @var{id} is then used for
all further access. A library call @dfn{ftok} is provided to translate
pathnames or strings to numeric keys.@refill

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 (@xref{msglimits}, @xref{semlimits},
@xref{shmlimits}).@refill

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



@node example, perms, Overview, Overview
@section example

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

In a typical access sequence the creator allocates a new instance
of the resource with the @code{get} system call using the IPC_CREAT
flag.@refill

@noindent creator process:@*

@example
#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 */
@end example

@noindent
Users then gain access to the resource using the same key.@*
@noindent
Client process:
@example
#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 */
@end example

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


@node perms, syscalls, example, Overview
@section Permissions

Each resource has an associated @code{ipc_perm} struct which defines the
creator, owner and access perms for the resource.@refill

@example
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 */
@end example

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.@refill

The lowest nine bits of the flags parameter supplied by the user to the
system call are compared with the values stored in @code{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.@refill

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.@refill

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

The @code{cuid}, @code{cgid}, @code{key} and @code{seq} fields
cannot be changed by the user.@refill



@node syscalls, Messages, perms, Overview
@section IPC system calls

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

Each type of mechanism provides a @dfn{get}, @dfn{ctl} and one or more
@dfn{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).@refill
	


@subsection The @dfn{get} system calls

The @code{get} call typically takes a @var{key} and returns a numeric
@var{id} that is used for further access.
The @var{id} is an index into the resource table. A sequence
number is maintained and incremented when a resource is
destroyed so that access using an obsolete @var{id} is likely to fail.@refill

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:@refill
@example
msgflg = IPC_CREAT | IPC_EXCL | 0666;
id = msgget (key, msgflg);
@end example
@itemize @bullet
@item
@code{key} : IPC_PRIVATE => new instance of resource is initialized.
@item
@code{flags} :
@itemize @asis
@item
IPC_CREAT : resource created for @var{key} if it does not exist.
@item
IPC_CREAT | IPC_EXCL : fail if resource exists for @var{key}.
@end itemize
@item
returns : an identifier used for all further access to the resource.
@end itemize

Note that IPC_PRIVATE is not a flag but a special @code{key}
that ensures (when the call is successful) that a new resource is
created.@refill

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

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.@refill

@noindent
See Also : @xref{msgget}, @xref{semget}, @xref{shmget}.@refill



@subsection The @dfn{ctl} system calls

Provides or alters the information stored in the structure that describes
the resource indexed by @var{id}.@refill

@example
#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);
        ....
@end example

@noindent
Commands supported by all @code{ctl} calls:@*
@itemize @bullet
@item
IPC_STAT : read info on resource  specified by id into user allocated
buffer. The user must have read access to the resource.@refill
@item
IPC_SET : write info from buffer into resource data structure. The
user must be owner creator or super-user.@refill
@item
IPC_RMID : remove resource. The user must be the owner, creator or
super-user.@refill
@end itemize

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.@refill

The @code{semctl} call provides a number of command options that allow
the user to determine or set the values of the semaphores in an array.@refill

@noindent
See Also: @xref{msgctl}, @xref{semctl}, @xref{shmctl}.@refill


@subsection The @dfn{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.@refill

@noindent
@code{flags} : IPC_NOWAIT  => return with error if a wait is required.

@noindent
See Also: @xref{msgsnd},@xref{msgrcv},@xref{semop},@xref{shmat},
@xref{shmdt}.@refill



@node Messages, msgget, syscalls, Top
@section Messages

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

@example
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 */
@end example

To send or receive a message the user allocates a structure that looks
like a @code{msgbuf} but with an array @code{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.@refill

@example
struct msgbuf
    long mtype;      type of message (@xref{msgrcv}).
    char mtext[1];   message text .. why is this not a ptr?
@end example

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

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


@menu
* msgget::
* msgsnd::
* msgrcv::
* msgctl::
* msglimits:: Implementation defined limits.
@end menu


@node msgget, msgsnd, Messages, Messages
@subsection msgget

@noindent
A message queue is allocated by a msgget system call :

@example
msqid = msgget (key_t key, int msgflg);
@end example

@itemize @bullet
@item
@code{key}: an integer usually got from @code{ftok()} or IPC_PRIVATE.@refill
@item
@code{msgflg}:
@itemize @asis
@item
IPC_CREAT : used to create a new resource if it does not already exist.
@item
IPC_EXCL | IPC_CREAT : used to ensure failure of the call if the
resource already exists.@refill
@item
rwxrwxrwx : access permissions.
@end itemize
@item
returns: msqid (an integer used for all further access) on success.
-1 on failure.@refill
@end itemize

A message queue is allocated if there is no resource corresponding
to the given key. The access permissions specified are then copied
into the @code{msg_perm} struct and the fields in @code{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
@var{key} already exists, the access permissions are verified.@refill

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




@node msgsnd, msgrcv, msgget, Messages
@subsection msgsnd