<?xml version="1.0" encoding="ISO8859-1"?>
<fpdoc-descriptions>
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   This file is part of the FPC documentation. 
   Copyright (C) 1997, by Michael Van Canneyt 
 
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 -->
<package name="rtl">
<module name="ipc">
<short>Unix Inter Process Communication functionality.</short>
<!-- \FPCexampledir{ipcex} -->
<descr>
<p>
This document describes the IPC unit for Free Pascal. It was written for
linux by Michael Van Canneyt. It gives all the functionality of system V 
Inter-Process Communication: shared memory, semaphores and messages.
It works only on the linux operating system.
</p>
<p>
Many constants here are provided for completeness only, and should under
normal circumstances not be used by the programmer.
</p>
</descr>

<element name="IPCerror">
<short>Last IPC error code</short>
<descr>
The <var>IPCerror</var> variable is used to report errors, by all calls.
</descr>
</element>

<element name="IPC_CREAT">
<short>Create if key is nonexistent</short>
</element>

<element name="IPC_EXCL">
<short>fail if key exists</short>
</element>

<element name="IPC_NOWAIT">
<short>return error on wait</short>
</element>

<element name="IPC_RMID">
<short>Remove resource</short>
</element>
<element name="IPC_SET">
<short>set ipc_perm options</short>
</element>
<element name="IPC_STAT">
<short>get ipc_perm options</short>
</element>
<element name="IPC_INFO">
<short>For ipcs call</short>
</element>

<element name="MSG_NOERROR">
<short>Internal Message control code. Do not use</short>
</element>
<element name="MSG_EXCEPT">
<short>Internal Message control code. Do not use</short>
</element>
<element name="MSGMNI">
<short>Internal Message control code. Do not use</short>
</element>
<element name="MSGMAX">
<short>Internal Message control code. Do not use</short>
</element>
<element name="MSGMNB">
<short>Internal Message control code. Do not use</short>
</element>

<element name="SEM_UNDO">
<short>Constant for use in <link id="semop"/></short>
</element>
<element name="GETPID">
<short>Constant for use in <link id="semop"/></short>
</element>
<element name="GETVAL">
<short>Constant for use in <link id="semop"/></short>
</element>
<element name="GETALL">
<short>Constant for use in <link id="semop"/></short>
</element>
<element name="GETNCNT">
<short>Constant for use in <link id="semop"/></short>
</element>
<element name="GETZCNT">
<short>Constant for use in <link id="semop"/></short>
</element>
<element name="SETVAL">
<short>Constant for use in <link id="semop"/></short>
</element>
<element name="SETALL">
<short>Constant for use in <link id="semop"/></short>
</element>

<element name="SEMMNI">
<short>Internal semaphore system constant. Do not use.</short>
</element>
<element name="SEMMSL">
<short>Internal semaphore system constant. Do not use.</short>
</element>
<element name="SEMMNS">
<short>Internal semaphore system constant. Do not use.</short>
</element>
<element name="SEMOPM">
<short>Internal semaphore system constant. Do not use.</short>
</element>
<element name="SEMVMX">
<short>Internal semaphore system constant. Do not use.</short>
</element>

<element name="SHM_R">
<short>This constant is used in the <link id="shmctl"/> call.</short>
</element>
<element name="SHM_W">
<short>This constant is used in the <link id="shmctl"/> call.</short>
</element>
<element name="SHM_RDONLY">
<short>This constant is used in the <link id="shmctl"/> call.</short>
</element>
<element name="SHM_RND">
<short>This constant is used in the <link id="shmctl"/> call.</short>
</element>
<element name="SHM_REMAP">
<short>This constant is used in the <link id="shmctl"/> call.</short>
</element>
<element name="SHM_LOCK">
<short>This constant is used in the <link id="shmctl"/> call.</short>
</element>
<element name="SHM_UNLOCK">
<short>This constant is used in the <link id="shmctl"/> call.</short>
</element>

<element name="BaseUnix">
<short>Some base types</short>
</element>

<element name="key_t">
<short>Alias for <link id="TKey"/> type</short>
</element>

<element name="TIPC_Perm.mode">
<short>Creation Mode</short>
</element>

<element name="TSHMinfo.shmmni">
<short>?</short>
</element>

<element name="msglen_t">
<short>Message length type</short>
</element>

<element name="msgqnum_t">
<short>Message queue number type</short>
</element>

<element name="PMSG">
<short>Pointer to <link id="TMSG"/> record</short>
</element>

<element name="PMSQid_ds">
<short>Pointer to <link id="TMSQid_ds"/></short>
</element>

<element name="PMSGbuf">
<short>Pointer to <link id="TMsgBuf"/> rcord</short>
</element>

<element name="SEM_GETPID">
<short>Semaphore operation: Get process ID of last operation.</short>
</element>

<element name="SEM_GETVAL">
<short>Semaphore operation: Get current value of semaphore</short>
</element>

<element name="SEM_GETALL">
<short>Semaphore operation: Get all semaphore values</short>
</element>

<element name="SEM_GETNCNT">
<short>Semaphore operation: Get number of processes waiting for resource.</short>
</element>

<element name="SEM_GETZCNT">
<short>Semaphore operation: Get number of processes waiting for semaphores to reach zero </short>
</element>

<element name="SEM_SETVAL">
<short>Semaphore operation: Set semaphore value</short>
</element>

<element name="SEM_SETALL">
<short>Semaphore operation: Set all semaphore values</short>
</element>


<element name="SEM_SEMMNI">
<short>Semaphore operation: ?</short>
</element>


<element name="SEM_SEMMSL">
<short>Semaphore operation: ?</short>
</element>


<element name="SEM_SEMMNS">
<short>Semaphore operation: ?</short>
</element>

<element name="SEM_SEMOPM">
<short>Semaphore operation: ?</short>
</element>


<element name="SEM_SEMVMX">
<short>Semaphore operation: ?</short>
</element>


<element name="TSEMid_ds.sem_nsems">
<short>Number of semaphores</short>
</element>

<element name="PULong">
<short>Pointer to cardinal type.</short>
</element>
<element name="PWord">
<short>Pointer to word type.</short>
</element>

<element name="TKey">
<short>Type returned by the <link id="ftok"/> key generating
function.</short>
</element>

<element name="TIPC_Perm">
<short>Record used in all IPC systems to specify the permissions.</short>
<descr>
<var>TIPC_Perm</var> is used in all IPC systems to specify the permissions.
It should never be used directly.
</descr>
</element>
<element name="TIPC_Perm.Key">
<short>key used to create resource</short>
</element>
<element name="TIPC_Perm.uid">
<short>Owner user ID</short>
</element>
<element name="TIPC_Perm.gid">
<short>Owner group id</short>
</element>
<element name="TIPC_Perm.cuid">
<short>Creator user ID</short>
</element>
<element name="TIPC_Perm.cgid">
<short>Creator group ID</short>
</element>
<element name="TIPC_Perm.seq">
<short>?</short>
</element>

<element name="PIPC_Perm">
<short>Pointer to <link id="TIPC_Perm"/> record.</short>
</element>

<element name="TSHMid_ds">
<short>Record used in the <link id="shmctl"/> call to set or retrieve settings for shared memory.</short>
</element>
<element name="PSHMid_DS">
<short>Pointer to <link id="TSHMid_ds"/> record.</short>
</element>
<element name="TSHMid_ds.shm_perm">
<short>Permissions</short>
</element>
<element name="TSHMid_ds.shm_segsz">
<short>Segment size</short>
</element>
<element name="TSHMid_ds.shm_atime">
<short>Last access time</short>
</element>
<element name="TSHMid_ds.shm_dtime">
<short>Last detach time</short>
</element>
<element name="TSHMid_ds.shm_ctime">
<short>Create time</short>
</element>
<element name="TSHMid_ds.shm_cpid">
<short>Creator PID</short>
</element>
<element name="TSHMid_ds.shm_lpid">
<short>Last operation PID</short>
</element>
<element name="TSHMid_ds.shm_nattch">
<short>Number of attachments</short>
</element>
<element name="TSHMid_ds.shm_npages">
<short>Number of pages</short>
</element>
<element name="TSHMid_ds.shm_pages">
<short>?</short>
</element>
<element name="TSHMid_ds.attaches">
<short>?</short>
</element>

<element name="TSHMinfo">
<short>Record used by the shared memory system, Do not use directly.</short>
</element>
<element name="TSHMinfo.shmmax" skip="1"/>
<element name="TSHMinfo.shmmin" skip="1"/>
<element name="TSHMinfo.smmni" skip="1"/>
<element name="TSHMinfo.shmseg" skip="1"/>
<element name="TSHMinfo.shmall" skip="1"/>
<element name="PSHMinfo" skip="1"/>

<element name="TMSG">
<short>Record used in the handling of message queues. Do not use directly.</short>
</element>
<element name="TMSG.msg_next" skip="1"/>
<element name="TMSG.msg_type" skip="1"/>
<element name="TMSG.msg_spot" skip="1"/>
<element name="TMSG.msg_stime" skip="1"/>
<element name="TMSG.msg_ts" skip="1"/>

<element name="TMSQid_ds">
<short>Record returned by the <link id="msgctl"/> call, contains all data about a message queue.</short>
<descr>
This record should never be used directly, it is an internal kernel record.
It's fields may change at any time.
</descr>
</element>

<element name="TMSQid_ds.msg_perm">
<short>Queue permissions</short>
</element>
<element name="TMSQid_ds.msg_first">
<short>Pointer to first message</short>
</element>
<element name="TMSQid_ds.msg_last">
<short>Pointer to last message</short>
</element>
<element name="TMSQid_ds.msg_stime">
<short>Last send time</short>
</element>
<element name="TMSQid_ds.msg_rtime">
<short>Last receive time</short>
</element>
<element name="TMSQid_ds.msg_ctime">
<short>Last control time</short>
</element>
<element name="TMSQid_ds.wwait">
<short></short>
</element>
<element name="TMSQid_ds.rwait">
<short></short>
</element>
<element name="TMSQid_ds.msg_cbytes">
<short></short>
</element>
<element name="TMSQid_ds.msg_qnum">
<short></short>
</element>
<element name="TMSQid_ds.msg_qbytes">
<short></short>
</element>
<element name="TMSQid_ds.msg_lspid">
<short></short>
</element>
<element name="TMSQid_ds.msg_lrpid">
<short></short>
</element>

<element name="TMSGbuf">
<short>Generic message data record</short>
<descr>
The <var>TMSGbuf</var> record is a record containing the data of a record. you
should never use this record directly, instead you should make your own
record that follows the structure of the <var>TMSGbuf</var> record, but that has
a size that is big enough to accomodate your messages. The <var>mtype</var> field
should always be present, and should always be filled.
</descr>
</element>
<element name="TMSGbuf.mtype">
<short>Message type</short>
</element>
<element name="TMSGbuf.mtext">
<short>Message data</short>
</element>

<element name="TMSGinfo" skip="1">
<short>Internal message system record. Do not use directly.</short>
</element>
<element name="TMSGinfo.msgpool" skip="1"/>
<element name="TMSGinfo.msgmap" skip="1"/>
<element name="TMSGinfo.msgmax" skip="1"/>
<element name="TMSGinfo.msgmax" skip="1"/>
<element name="TMSGinfo.msgmnb" skip="1"/>
<element name="TMSGinfo.msgmni" skip="1"/>
<element name="TMSGinfo.msgssz" skip="1"/>
<element name="TMSGinfo.msgtql" skip="1"/>
<element name="TMSGinfo.msgseg" skip="1"/>

<element name="PMSGinfo" skip="1">
<short>Pointer to <link id="TMSGinfo"/> record</short>
</element>

<element name="TSEMid_ds">
<short>Structure returned by the <link id="semctl"/> call, contains all data of a semahore</short>
</element>

<element name="PSEMid_ds">
<short>Pointer to <link id="TSEMid_ds"/> record.</short>
</element>

<element name="TSEMid_ds.sem_perm">
<short>IPC permissions</short>
</element>

<element name="TSEMid_ds.sem_otime">
<short>Last operation time</short>
</element>

<element name="TSEMid_ds.sem_ctime">
<short>Create time</short>
</element>

<element name="TSEMid_ds.sem_base">
<short>Internal data. Do not use</short>
</element>

<element name="TSEMid_ds.sem_pending">
<short>Internal data. Do not use</short>
</element>
<element name="TSEMid_ds.sem_pending_last">
<short>Internal data. Do not use</short>
</element>
<element name="TSEMid_ds.undo">
<short>Internal data. Do not use</short>
</element>
<element name="TSEMid_ds.nsems">
<short>Internal data. Do not use</short>
</element>

<element name="TSEMbuf">
<short>Record used in <link id="semop"/> call.</short>
<descr>
The <var>TSEMbuf</var> record is used in the <link id="semop"/> call, and is
used to specify which operations you want to do.
</descr>
</element>

<element name="PSEMbuf">
<short>Pointer to <link id="TSembuf"/> record.</short>
</element>
<element name="TSEMbuf.sem_num">
<short>Number of the semaphore to perform operation on.</short>
</element>
<element name="TSEMbuf.sem_op">
<short>Operation to perform on semaphore</short>
</element>
<element name="TSEMbuf.sem_flg">
<short>Flags for operation.</short>
</element>

<element name="TSEMinfo" skip="1">
<short>Internal semaphore system record. Do not use.</short>
</element>
<element name="TSEMinfo.semmap" skip="1"/>
<element name="TSEMinfo.semmni" skip="1"/>
<element name="TSEMinfo.semmns" skip="1"/>
<element name="TSEMinfo.semmnu" skip="1"/>
<element name="TSEMinfo.semmsl" skip="1"/>
<element name="TSEMinfo.semopm" skip="1"/>
<element name="TSEMinfo.semume" skip="1"/>
<element name="TSEMinfo.semusz" skip="1"/>
<element name="TSEMinfo.semvmx" skip="1"/>
<element name="TSEMinfo.semaem" skip="1"/>

<element name="PSEMinfo">
<short>Pointer to <link id="TSEMinfo"/> record.</short>
</element>

<element name="TSEMun">
<short>Record used in <link id="semctl"/> call.</short>
</element>
<element name="PSEMun">
<short>Pointer to <link id="TSEMun"/> record</short>
</element>

<element name="ftok">
<short>Create token from filename</short>
<descr>
<p>
<var>ftok</var> returns a key that can be used in a <link id="semget"/>
<link id="shmget"/>
or <link id="msgget"/> call to access a new or existing IPC resource.
</p>
<p>
<var>Path</var> is the name of a file in the file system, <var>ID</var> is a
character of your choice. The ftok call does the same as it's C couterpart,
so a pascal program and a C program will access the same resource if
they use the same <var>Path</var> and <var>ID</var>
</p>
<p>
For an example, see <link id="msgctl"/>, <link id="semctl"/> or <link id="shmctl"/>.
</p>
</descr>
<errors>
<var>ftok</var> returns -1 if the file in <var>Path</var> doesn't exist.
</errors>
<seealso>
<link id="semget"/>
<link id="shmget"/>
<link id="msgget"/>
</seealso>
</element>


<element name="msgget">
<short>Return message queue ID, possibly creating the queue</short>
<descr>
<p>
<var>msgget</var> returns the ID of the message queue described by <var>key</var>.
Depending on the flags in <var>msgflg</var>, a new queue is created.
</p>
<p>
<var>msgflg</var> can have one or more of the following values (combined by ORs):
</p>
<dl>
<dt>IPC_CREAT</dt><dd> The queue is created if it doesn't already exist.</dd>
<dt>IPC_EXCL</dt><dd> If used in combination with <var>IPC_CREAT</var>, causes the
call to fail if the queue already exists. It cannot be used by itself.
</dd>
</dl>
<p>
Optionally, the flags can be <var>OR</var>ed with a permission mode, which is the
same mode that can be used in the file system.
</p>
<p>
For an example, see <link id="msgctl"/>.
</p>
</descr>
<errors>
On error, -1 is returned, and <var>IPCError</var> is set.
</errors>
<seealso>
<link id="ftok"/>
<link id="msgsnd"/>
<link id="msgrcv"/>
<link id="msgctl"/>
</seealso>
</element>


<element name="msgsnd">
<short>Send a message to the messague queue</short>
<descr>
<p>
<var>msgsend</var> sends a message to a message queue with ID <var>msqid</var>.
<var>msgp</var> is a pointer to a message buffer, that should be based on the
<var>TMsgBuf</var> type. <var>msgsiz</var> is the size of the message (NOT of the
message buffer record !)
</p>
<p>
The <var>msgflg</var> can have a combination of the following values (ORed
together):
</p>
<dl>
<dt>0</dt><dd>No special meaning. The message will be written to the queue.
If the queue is full, then the process is blocked.</dd>
<dt>IPC_NOWAIT</dt><dd> If the queue is full, then no message is written,
and the call returns immediatly.
</dd>
</dl>
<p>
The function returns <var>True</var> if the message was sent successfully, 
<var>False</var> otherwise.
</p>
<p>
For an example, see <link id="msgctl"/>.
</p>
</descr>
<errors>
In case of error, the call returns <var>False</var>, and <var>IPCerror</var> is set.
</errors>
<seealso>
<link id="msgget"/>
<link id="msgrcv"/>
<link id="msgctl"/>
</seealso>
</element>


<element name="msgrcv">
<short>Retrieve a message from the queue</short>
<descr>
<p>
<var>msgrcv</var> retrieves a message of type <var>msgtyp</var> from the message 
queue with ID <var>msqid</var>. <var>msgtyp</var> corresponds to the <var>mtype</var> 
field of the <var>TMSGbuf</var> record. The message is stored in the <var>MSGbuf</var>
structure pointed to by <var>msgp</var>.
</p>
<p>
The <var>msgflg</var> parameter can be used to control the behaviour of the
<var>msgrcv</var> call. It consists of an ORed combination of the following
flags:
</p>
<dl>
<dt>0</dt><dd> No special meaning.</dd>
<dt>IPC_NOWAIT</dt><dd>if no messages are available, then the call returns
immediatly, with the <var>ENOMSG</var> error.</dd>
<dt>MSG_NOERROR</dt><dd> If the message size is wrong (too large), 
no error is generated, instead the message is truncated. 
Normally, in such cases, the call returns an error (E2BIG)
</dd>
</dl>
<p>
The function returns <var>True</var> if the message was received correctly,
<var>False</var> otherwise.
</p>
<p>
For an example, see <link id="msgctl"/>.
</p>
</descr>
<errors>
In case of error, <var>False</var> is returned, and <var>IPCerror</var> is set.
</errors>
<seealso>
<link id="msgget"/>
<link id="msgsnd"/>
<link id="msgctl"/>
</seealso>
</element>


<element name="msgctl">
<short>Perform various operations on a message queue</short>
<descr>
<p>
<var>msgctl</var> performs various operations on the message queue with id
<var>ID</var>. Which operation is performed, depends on the <var>cmd</var> 
parameter, which can have one of the following values:
</p>
<dl>
<dt>IPC_STAT</dt><dd> In this case, the <var>msgctl</var> call fills the
<var>TMSQid_ds</var> structure with information about the message queue.
</dd>
<dt>IPC_SET</dt><dd> in this case, the <var>msgctl</var> call sets the permissions
of the queue as specified in the <var>ipc_perm</var> record inside <var>buf</var>.
</dd><dt>IPC_RMID</dt><dd> If this is specified, the message queue will be removed 
from the system.
</dd>
</dl>
<p>
<var>buf</var> contains the data that are needed by the call. It can be 
<var>Nil</var> in case the message queue should be removed.
</p>
<p>
The function returns <var>True</var> if successfull, <var>False</var> otherwise.
</p>
</descr>
<errors>
On error, <var>False</var> is returned, and <var>IPCerror</var> is set accordingly.
</errors>
<seealso>
<link id="msgget"/>
<link id="msgsnd"/>
<link id="msgrcv"/>
</seealso>
<example file="ipcex/msgtool"/>
</element>


<element name="semget">
<short>Return the ID of a semaphore set, possibly creating the set</short>
<descr>
<p>
<var>msgget</var> returns the ID of the semaphore set described by <var>key</var>.
Depending on the flags in <var>semflg</var>, a new queue is created.
</p>
<p>
<var>semflg</var> can have one or more of the following values (combined by ORs):
</p>
<dl>
<dt>IPC_CREAT</dt><dd> The queue is created if it doesn't already exist.</dd>
<dt>IPC_EXCL</dt><dd> If used in combination with <var>IPC_CREAT</var>, causes the
call to fail if the set already exists. It cannot be used by itself.</dd>
</dl>
<p>
Optionally, the flags can be <var>OR</var>ed with a permission mode, which is the
same mode that can be used in the file system.
</p>
<p>
if a new set of semaphores is created, then there will be <var>nsems</var>
semaphores in it.
</p>
</descr>
<errors>
On error, -1 is returned, and <var>IPCError</var> is set.
</errors>
<seealso>
<link id="ftok"/>
<link id="semop"/>
<link id="semctl"/>
</seealso>
</element>

<element name="semop">
<short>Perform semaphore operation.</short>
<descr>
<p>
<var>semop</var> performs a set of operations on a message queue.
<var>sops</var> points to an array of type <var>TSEMbuf</var>. The array should
contain <var>nsops</var> elements.
</p>
<p>
The fields of the <link id="TSEMbuf"/> structure 
</p>
<code>
  TSEMbuf = record
    sem_num : word;
    sem_op  : integer;
    sem_flg : integer;
</code>
<p>
should be filled as follows:
</p>
<dl>
<dt>sem_num</dt><dd> The number of the semaphore in the set on which the
operation must be performed.</dd>
<dt>sem_op</dt><dd>
The operation to be performed. The operation depends on the
sign of <var>sem_op</var>:
A positive  number is simply added to the current value of the
semaphore. If 0 (zero) is specified, then the process is suspended until the 
specified semaphore reaches zero. If a negative number is specified, it is substracted from the
current value of the semaphore. If the value would become negative
then the process is suspended until the value becomes big enough, unless
<var>IPC_NOWAIT</var> is specified in the <var>sem_flg</var>.
</dd>
<dt>sem_flg</dt>
<dd> Optional flags: if <var>IPC_NOWAIT</var> is specified, then the
calling process will never be suspended.
</dd>
</dl>
<p>
The function returns <var>True</var> if the operations were successful,
<var>False</var> otherwise.
</p>
</descr>
<errors>
In case of error, <var>False</var> is returned, and <var>IPCerror</var> is set.
</errors>
<seealso>
<link id="semget"/>
<link id="semctl"/>
</seealso>
</element>

<element name="semctl">
<short>Perform various control operations on a semaphore set</short>
<descr>
<p>
<var>semctl</var> performs various operations on the semaphore <var>semnum</var> w
ith semaphore set id <var>ID</var>. 
</p>
<p>
The <var>arg</var> parameter supplies the data needed for each call. This is
a variant record that should be filled differently, according to the
command:
</p>
<code>
Type
  TSEMun = record
   case longint of
      0 : ( val : longint );
      1 : ( buf : PSEMid_ds );
      2 : ( arr : PWord );
      3 : ( padbuf : PSeminfo );
      4 : ( padpad : pointer );
   end;
</code>
<p>
Which operation is performed, depends on the <var>cmd</var> 
parameter, which can have one of the following values:
</p>
<dl>
<dt>IPC_STAT</dt>
<dd> In this case, the arg record should have it's <var>buf</var>
field set to the address of a <var>TSEMid_ds</var> record. 
The <var>semctl</var> call fills this <var>TSEMid_ds</var> structure with information 
about the semaphore set. 
</dd>
<dt>IPC_SET</dt>
<dd> In this case, the <var>arg</var> record should have it's <var>buf</var>
field set to the address of a <var>TSEMid_ds</var> record.
The <var>semctl</var> call sets the permissions of the queue as specified in 
the <var>ipc_perm</var> record.
</dd>
<dt>IPC_RMID</dt>
<dd> If this is specified, the semaphore set is removed from 
from the system.
</dd>
<dt>GETALL</dt>
<dd> In this case, the <var>arr</var> field of <var>arg</var> should point
to a memory area where the values of the semaphores will be stored.
The size of this memory area is \var{SizeOf(Word)* Number of semaphores
in the set}.
This call will then fill the memory array with all the values of the
semaphores.
</dd>
<dt>GETNCNT</dt>
<dd> This will fill the <var>val</var> field of the <var>arg</var> union
with the number of processes waiting for resources.
</dd>
<dt>GETPID</dt>
<dd> <var>semctl</var> returns the process ID of the process that
performed the last <link id="semop"/> call.
</dd>
<dt>GETVAL</dt>
<dd> <var>semctl</var> returns the value of the semaphore with number
<var>semnum</var>.
</dd>
<dt>GETZCNT</dt>
<dd> <var>semctl</var> returns the number of processes waiting for 
semaphores that reach value zero.
</dd>
<dt>SETALL</dt>
<dd> In this case, the <var>arr</var> field of <var>arg</var> should point
to a memory area where the values of the semaphores will be retrieved from.
The size of this memory area is \var{SizeOf(Word)* Number of semaphores
in the set}.
This call will then set the values of the semaphores from the memory array.
</dd>
<dt>SETVAL</dt>
<dd> This will set the value of semaphore <var>semnum</var> to the value
in the <var>val</var> field of the <var>arg</var> parameter.
</dd>
</dl>
<p>
The function returns -1 on error.
</p>
</descr>
<errors>
The function returns -1 on error, and <var>IPCerror</var> is set accordingly.
</errors>
<seealso>
<link id="semget"/>
<link id="semop"/>
</seealso>
<example file="ipcex/semtool"/>
</element>


<element name="shmget">
<short>Return the ID of a shared memory block, possibly creating it</short>
<descr>
<p>
<var>shmget</var> returns the ID of a shared memory block, described by <var>key</var>.
Depending on the flags in <var>flag</var>, a new memory block is created.
</p>
<p>
<var>flag</var> can have one or more of the following values (combined by ORs):
</p>
<dl>
<dt>IPC_CREAT</dt>
<dd> The queue is created if it doesn't already exist.
</dd>
<dt>IPC_EXCL</dt>
<dd> If used in combination with <var>IPC_CREAT</var>, causes the
call to fail if the queue already exists. It cannot be used by itself.
</dd>
</dl>
<p>
Optionally, the flags can be <var>OR</var>ed with a permission mode, which is the
same mode that can be used in the file system.
</p>
<p>
if a new memory block is created, then it will have size <var>Size</var>
bytes in it.
</p>
</descr>
<errors>
On error, -1 is returned, and <var>IPCError</var> is set.
</errors>
<seealso>
</seealso>
</element>

<element name="shmat">
<short>Attach a shared memory block.</short>
<descr>
<p>
<var>shmat</var> attaches a shared memory block with identified <var>shmid</var> 
to the current process. The function returns a pointer to the shared memory
block.
</p>
<p>
If <var>shmaddr</var> is <var>Nil</var>, then the system chooses a free unmapped
memory region, as high up in memory space as possible.
</p>
<p>
If <var>shmaddr</var> is non-nil, and <var>SHM_RND</var> is in <var>shmflg</var>, then 
the returned address is <var>shmaddr</var>, rounded down to <var>SHMLBA</var>.
If <var>SHM_RND</var> is not specified, then <var>shmaddr</var> must be a
page-aligned address.
</p>
<p>
The parameter <var>shmflg</var> can be used to control the behaviour of the
<var>shmat</var> call. It consists of a ORed combination of the following
constants:
</p>
<dl>
<dt>SHM_RND</dt> <dd> The suggested address in <var>shmaddr</var> is rounded down to
<var>SHMLBA</var>.
</dd>
<dt>SHM_RDONLY</dt> <dd> the shared memory is attached for read access only.
Otherwise the memory is attached for read-write. The process then needs
read-write permissions to access the shared memory.
</dd>
</dl>
<p>
For an example, see <link id="shmctl"/>.
</p>
</descr>
<errors>
If an error occurs, -1 is returned, and <var>IPCerror</var> is set.
</errors>
<seealso>
<link id="shmget"/>
<link id="shmdt"/>
<link id="shmctl"/>
</seealso>
</element>


<element name="shmdt">
<short>Detach shared memory block.</short>
<descr>
<p>
<var>shmdt</var> detaches the shared memory at address <var>shmaddr</var>. This shared
memory block is unavailable to the current process, until it is attached
again by a call to <link id="shmat"/>.
</p>
<p>
The function returns <var>True</var> if the memory block was detached
successfully, <var>False</var> otherwise.
</p>
</descr>
<errors>
On error, False is returned, and IPCerror is set.
</errors>
<seealso>
<link id="shmget"/>
<link id="shmat"/>
<link id="shmctl"/>
</seealso>
</element>

<element name="shmctl">
<short>Perform control operations on a shared memory block.</short>
<descr>
<p>
<var>shmctl</var> performs various operations on the shared memory block
identified by identifier <var>shmid</var>.
</p>
<p>
The <var>buf</var> parameter points to a <var>TSHMid_ds</var> record. The 
<var>cmd</var> parameter is used to pass which operation is to be performed.
It can have one of the following values :
</p>
<dl>
<dt>IPC_STAT</dt><dd> <var>shmctl</var> fills the <var>TSHMid_ds</var> record that 
<var>buf</var> points to with the available information about the shared memory
block.</dd>
<dt>IPC_SET</dt><dd> applies the values in the <var>ipc_perm</var> record that
<var>buf</var> points to, to the shared memory block.</dd>
<dt>IPC_RMID</dt><dd> the shared memory block is destroyed (after all processes
to which the block is attached, have detached from it).</dd>
</dl>
<p>
If successful, the function returns <var>True</var>, <var>False</var> otherwise.
</p>
</descr>
<errors>
If an error occurs, the function returns <var>False</var>, and <var>IPCerror</var>
is set.
</errors>
<seealso>
<link id="shmget"/>
<link id="shmat"/>
<link id="shmdt"/>
</seealso>
<example file="ipcex/shmtool"/>
</element>

</module>
</package>
</fpdoc-descriptions>
