488 lines
16 KiB
HTML
488 lines
16 KiB
HTML
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
|
|
<html>
|
|
<head>
|
|
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
|
|
<meta name="Copyright" content="Copyright (c) 2006 by IBM Corporation">
|
|
<title>shmget()-Get ID of Shared Memory Segment with Key</title>
|
|
<!-- Begin Header Records ========================================== -->
|
|
<!-- Direct1 SCRIPT J converted by B2H R4.1 (346) (CMS) by V2KEA304 -->
|
|
<!-- at RCHVMW2 on 17 Feb 1999 at 11:05:09 -->
|
|
<!-- All rights reserved. Licensed Materials Property of IBM -->
|
|
<!-- US Government Users Restricted Rights -->
|
|
<!-- Use, duplication or disclosure restricted by -->
|
|
<!-- GSA ADP Schedule Contract with IBM Corp. -->
|
|
<!-- Change History: -->
|
|
<!-- YYMMDD USERID Change description -->
|
|
<!-- Edited by Kersten Feb 02 -->
|
|
<!-- This file has undergone html cleanup on 05/01/02 by JET -->
|
|
<link rel="stylesheet" type="text/css" href="../rzahg/ic.css">
|
|
</head>
|
|
<body>
|
|
<!--End Header Records -->
|
|
<!-- Java sync-link -->
|
|
<script type="text/javascript" language="Javascript" src="../rzahg/synch.js">
|
|
</script>
|
|
|
|
<a name="Top_Of_Page"></a>
|
|
|
|
<h2>shmget()--Get ID of Shared Memory Segment with Key</h2>
|
|
|
|
<div class="box" style="width: 70%;"><br>
|
|
Syntax<br>
|
|
<pre>
|
|
#include <sys/shm.h>
|
|
#include <sys/stat.h>
|
|
|
|
int shmget(key_t <em>key</em>, size_t <em>size</em>, int <em>shmflg</em>);
|
|
</pre>
|
|
|
|
<br>
|
|
Service Program Name: QP0ZUSHR<br>
|
|
<!-- iddvc RMBR -->
|
|
<br>
|
|
Default Public Authority: *USE<br>
|
|
<!-- iddvc RMBR -->
|
|
<br>
|
|
Threadsafe: Yes<br>
|
|
<!-- iddvc RMBR -->
|
|
<br>
|
|
</div>
|
|
|
|
<p>The <strong>shmget()</strong> function either creates a new shared memory
|
|
segment or returns the shared memory identifier associated with the <em>key</em>
|
|
parameter for an existing shared memory segment. A new shared memory segment is
|
|
created if one of the following is true:
|
|
|
|
<ul>
|
|
<li>The <em>key</em> parameter is equal to IPC_PRIVATE.<br>
|
|
<br>
|
|
</li>
|
|
|
|
<li>The <em>key</em> parameter does not already have a shared memory identifier
|
|
associated with it and the IPC_CREAT flag is specified in the <em>shmflg</em> parameter.<br>
|
|
</li>
|
|
</ul>
|
|
|
|
<p>The system maintains status information about a shared memory segment which can be
|
|
retrieved with the <a href="ipcshmct.htm">shmctl()</a> function. When a new
|
|
shared memory segment is created, the system initializes the members of the
|
|
<samp>shmid_ds</samp> structure as follows:
|
|
|
|
<ul>
|
|
<li><samp>shm_perm.cuid</samp> and <samp>shm_perm.uid</samp> are set
|
|
equal to the effective user ID of the calling thread.<br>
|
|
<br>
|
|
</li>
|
|
|
|
<li><samp>shm_perm.cgid</samp> and <samp>shm_perm.gid</samp> are set
|
|
equal to the effective group ID of the calling thread.<br>
|
|
<br>
|
|
</li>
|
|
|
|
<li>The low-order 9 bits of <samp>shm_perm.mode</samp> are set equal to the
|
|
low-order 9 bits of the <em>shmflg</em> parameter.<br>
|
|
<br>
|
|
</li>
|
|
|
|
<li><samp>shm_segsz</samp> is set to the value specified in the <em>
|
|
size</em> parameter.<br>
|
|
<br>
|
|
</li>
|
|
|
|
<li><samp>shm_ctime</samp> is set to the current time.<br>
|
|
<br>
|
|
</li>
|
|
|
|
<li><samp>shm_lpid</samp>, <samp>shm_nattch</samp>, <samp>shm_atime</samp>, and
|
|
<samp>shm_dtime</samp> are set to zero.<br>
|
|
<br>
|
|
</li>
|
|
|
|
</ul>
|
|
|
|
<p>There are two types of shared memory segments: teraspace shared memory segments and
|
|
nonteraspace shared memory segments. A teraspace shared memory segment is accessed by
|
|
adding the shared memory segment to a job's teraspace.
|
|
A nonteraspace shared memory segment creates shared memory using i5/OS
|
|
space objects.</p>
|
|
|
|
<p>Shared memory segments larger than 16 773 120 bytes (16 MB minus
|
|
4096 bytes) should be created as teraspace shared memory segments.
|
|
The maximum size of a teraspace shared memory segment is
|
|
4 294 967 295 bytes (4GB - 1). The maximum size of a resizeable
|
|
teraspace shared memory segment is 268 435 456 bytes (256 MB).</p>
|
|
|
|
<p>The maximum size of a nonteraspace shared memory segments is
|
|
16 776 960 bytes (16 MB - 256 bytes). When the
|
|
operating system accesses a nonteraspace shared memory segment that has a size
|
|
in the range 16 773 120 bytes (16 MB minus 4096 bytes) to
|
|
16 776 960 bytes (16 MB minus 256 bytes), a performance degradation
|
|
will be observed.</p>
|
|
|
|
<p>The size of the shared memory segment can be changed after it is created using the
|
|
<a href="ipcshmct.htm">shmctl()</a> function. The size can only be changed if it is
|
|
nonteraspace shared memory segment or if it is a teraspace shared memory segment and
|
|
SHM_RESIZE_NP is specified in the <em>shmflg</em> parameter.</p>
|
|
|
|
<br>
|
|
<h3>Parameters</h3>
|
|
|
|
<dl>
|
|
<dt><strong>key</strong></dt>
|
|
|
|
<dd>(Input) The key associated with the shared memory segment. A key of
|
|
IPC_PRIVATE (0x00000000) guarantees that a unique shared memory segment
|
|
is created. A key can also be specified by the caller or generated by the
|
|
<a href="p0zftok.htm">ftok()</a> function.<br>
|
|
<br>
|
|
</dd>
|
|
|
|
<dt><strong>size</strong></dt>
|
|
|
|
<dd>(Input) The size of the shared memory segment being created. If an existing shared
|
|
memory segment is being accessed, <em>size</em> may be zero.<br>
|
|
<br>
|
|
</dd>
|
|
|
|
<dt><strong>shmflg</strong></dt>
|
|
|
|
<dd>(Input) Operation and permission flags. The value of the <em>shmflg</em> parameter
|
|
is either zero or is obtained by performing an OR operation on one or more of the
|
|
following constants:<p>
|
|
|
|
<dl>
|
|
<dt><strong>S_IRUSR (0x00000100)</strong></dt>
|
|
<dd>Allow the owner of the shared memory segment to attach to it in read mode.<p></dd>
|
|
|
|
<dt><strong>S_IWUSR (0x00000080)</strong></dt>
|
|
<dd>Allow the owner of the shared memory segment to attach to it in write mode.<p></dd>
|
|
|
|
<dt><strong>S_IRGRP (0x00000020)</strong></dt>
|
|
<dd>Allow the group of the shared memory segment to attach to it in read mode.<p></dd>
|
|
|
|
<dt><strong>S_IWGRP (0x00000010)</strong></dt>
|
|
<dd>Allow the group of the shared memory segment to attach to it in write mode.<p></dd>
|
|
|
|
<dt><strong>S_IROTH (0x00000004)</strong></dt>
|
|
<dd>Allow others to attach to the shared memory segment in read mode.<p></dd>
|
|
|
|
<dt><strong>S_IWOTH (0x00000002)</strong></dt>
|
|
<dd>Allow others to attach to the shared memory segment in write mode.<p></dd>
|
|
|
|
<dt><strong>IPC_CREAT (0x00000200)</strong></dt>
|
|
<dd>Create the shared memory segment if it does not exist.<p></dd>
|
|
|
|
<dt><strong>IPC_EXCL (0x00000400)</strong></dt>
|
|
<dd>Return an error if the IPC_CREAT flag is set and the shared memory segment already exists.<p></dd>
|
|
|
|
<dt><strong>SHM_TS_NP (0x00010000)</strong></dt>
|
|
<dd>If creating a new shared memory segment, make the shared memory segment a
|
|
teraspace shared memory segment. When a job attaches to the shared memory
|
|
segment, the shared memory segment will be added to the job's teraspace.
|
|
Some compilers permit the user to indicate that the teraspace versions of
|
|
storage functions should be used. For example, if a C module is compiled using
|
|
CRTCMOD TERASPACE(*YES *TSIFC), this flag will be set automatically.
|
|
|
|
<p>If accessing an existing shared memory segment, only specify this constant
|
|
if it was specified when the shared memory segment was created.<p>
|
|
</dd>
|
|
|
|
<dt><strong>SHM_RESIZE_NP (0x00040000)</strong></dt>
|
|
<dd>If creating a new teraspace shared memory segment, allow the size of the
|
|
shared memory segment to be changed with the <a href="ipcshmct.htm">shmctl()</a>
|
|
function. The maximum size of this teraspace shared memory segment is
|
|
268 435 456 bytes (256 MB). This
|
|
flag is ignored for nonteraspace shared memory segments. A nonteraspace shared
|
|
memory segment may always be resized up to 16 773 120 bytes (16 MB -
|
|
4096 bytes).<p></dd>
|
|
|
|
<dt><strong>SHM_MAP_FIXED_NP (0x00100000)</strong></dt>
|
|
<dd>If creating a new teraspace shared memory segment, make all jobs that successfully
|
|
attach to the shared memory segment attach to the shared memory segment at the same
|
|
address. The shared memory segment may not be attached in read-only mode. This
|
|
flag is ignored for nonteraspace shared memory segments.
|
|
|
|
<p>If accessing an existing shared memory segment, only specify this constant
|
|
if it was specified when the shared memory segment was created.<p>
|
|
</dd>
|
|
|
|
</dl>
|
|
</dd>
|
|
</dl>
|
|
<br>
|
|
<br>
|
|
<h3>Authorities</h3>
|
|
|
|
<p><strong><a name="TBLAHGET1">Authorization Required for
|
|
shmget()</a></strong></p>
|
|
|
|
<table border cellpadding="5">
|
|
<!-- cols="60 20 20" -->
|
|
<tr>
|
|
<th align="left" valign="bottom">Object Referred to</th>
|
|
<th align="left" valign="bottom">Authority Required</th>
|
|
<th align="left" valign="bottom">errno</th>
|
|
</tr>
|
|
|
|
<tr>
|
|
<td align="left" valign="top">Shared memory segment to be created</td>
|
|
<td align="left" valign="top">None</td>
|
|
<td align="left" valign="top">None</td>
|
|
</tr>
|
|
|
|
<tr>
|
|
<td align="left" valign="top">Existing shared memory segment to be
|
|
accessed</td>
|
|
<td align="left" valign="top">See Note</td>
|
|
<td align="left" valign="top">EACCES</td>
|
|
</tr>
|
|
</table>
|
|
|
|
<p><strong>Note:</strong> If the thread is accessing a shared memory segment
|
|
that already exists, the mode specified in the last 9 bits of the <em>shmflg</em>
|
|
parameter must be a subset of the mode of the existing shared memory segment.</p>
|
|
|
|
<br>
|
|
<h3>Return Value</h3>
|
|
|
|
<table cellpadding="5">
|
|
<!-- cols="10 90" -->
|
|
<tr>
|
|
<td align="left" valign="top"><em>value</em></td>
|
|
<td align="left" valign="top"><strong>shmget()</strong>was successful. The
|
|
value returned is the shared memory identifier associated with the <em>key</em> parameter.</td>
|
|
</tr>
|
|
|
|
<tr>
|
|
<td align="left" valign="top"><em>-1</em></td>
|
|
<td align="left" valign="top"><strong>shmget()</strong> was not successful. The
|
|
<em>errno</em> variable is set to indicate the error.</td>
|
|
</tr>
|
|
</table>
|
|
|
|
<br>
|
|
<br>
|
|
<h3>Error Conditions</h3>
|
|
|
|
<p>If <strong>shmget()</strong> is not successful, <em>errno</em> usually
|
|
indicates one of the following errors. Under some conditions, <em>errno</em>
|
|
could indicate an error other than those listed here.</p>
|
|
|
|
<dl>
|
|
<dt><em>[EACCES]</em></dt>
|
|
|
|
<dd>
|
|
<p>Permission denied.</p>
|
|
|
|
<p>An attempt was made to access an object in a way forbidden by its object
|
|
access permissions.</p>
|
|
|
|
<p>The thread does not have access to the specified file, directory, component,
|
|
or path.</p>
|
|
|
|
<p>A shared memory identifier exists for the parameter <em>key</em>, but
|
|
permissions specified in the low-order 9 bits of <em>semflg</em>
|
|
are not a subset of the current permissions.</p>
|
|
|
|
<p>Shared memory operations are not permitted because the QSHRMEMCTL system
|
|
value is set to 0.</p>
|
|
</dd>
|
|
|
|
<dt><em>[EDAMAGE]</em></dt>
|
|
|
|
<dd>
|
|
<p>A damaged object was encountered.</p>
|
|
|
|
<p>The shared memory segment has been damaged by a previous shared memory operation.</p>
|
|
</dd>
|
|
|
|
<dt><em>[EEXIST]</em></dt>
|
|
|
|
<dd>
|
|
<p>File exists.</p>
|
|
|
|
<p>The file specified already exists and the specified operation requires that
|
|
it not exist.</p>
|
|
|
|
<p>The named file, directory, or path already exists.</p>
|
|
|
|
<p>A shared memory identifier exists for the <em>key</em> parameter and both
|
|
the IPC_CREAT and IPC_EXCL flags are set in the <em>shmflg</em> parameter.</p>
|
|
</dd>
|
|
|
|
<dt><em>[EINVAL]</em></dt>
|
|
|
|
<dd>
|
|
<p>The value specified for the argument is not correct.</p>
|
|
|
|
<p>A function was passed incorrect argument values, or an operation was
|
|
attempted on an object and the operation specified is not supported for that
|
|
type of object.</p>
|
|
|
|
<p>An argument value is not valid, out of range, or NULL.</p>
|
|
|
|
<p>One of the following has occurred:</p>
|
|
|
|
<ul>
|
|
<li>The value of the <em>size</em> parameter is less than the system-imposed
|
|
minimum or greater than the system-imposed maximum.</li>
|
|
|
|
<li>A shared memory identifier exists for the <em>key</em> parameter and the
|
|
size of the segment associated with it is less than <em>size</em> and <em>
|
|
size</em> is not zero.</li>
|
|
|
|
<li>A shared memory identifier exists for the <em>key</em> parameter and the
|
|
SHM_MAP_FIXED_NP or SHM_TS_NP attributes of the shared memory segment do not
|
|
match the flags set in the <em>shmflg</em> parameter.</li>
|
|
</ul><br>
|
|
</dd>
|
|
|
|
<dt><em>[ENOENT]</em></dt>
|
|
|
|
<dd>
|
|
<p>No such path or directory.</p>
|
|
|
|
<p>The directory or a component of the path name specified does not exist.</p>
|
|
|
|
<p>A named file or directory does not exist or is an empty string.</p>
|
|
|
|
<p>A shared memory identifier does not exist for the <em>key</em> parameter
|
|
and the IPC_CREAT flag is not set in the <em>shmflg</em> parameter.</p>
|
|
</dd>
|
|
|
|
<dt><em>[ENOMEM]</em></dt>
|
|
|
|
<dd>
|
|
<p>Storage allocation request failed.</p>
|
|
|
|
<p>A function needed to allocate storage, but no storage is available.</p>
|
|
|
|
<p>A new shared memory segment is being created and the amount of available
|
|
physical memory is not sufficient to fulfill the request.</p>
|
|
</dd>
|
|
|
|
<dt><em>[ENOSPC]</em></dt>
|
|
|
|
<dd>
|
|
<p>No space available.</p>
|
|
|
|
<p>The requested operations required additional space on the device and there
|
|
is no space left. This could also be caused by exceeding the user profile
|
|
storage limit when creating or transferring ownership of an object.</p>
|
|
|
|
<p>Insufficient space remains to hold the intended file, directory, or
|
|
link.</p>
|
|
|
|
<p>A shared memory identifier cannot be created because the system limit on
|
|
the maximum number of allowed shared memory identifiers would be
|
|
exceeded.</p>
|
|
</dd>
|
|
|
|
<dt><em>[EUNKNOWN]</em></dt>
|
|
|
|
<dd>
|
|
<p>Unknown system state.</p>
|
|
|
|
<p>The operation failed because of an unknown system state. See any messages in
|
|
the job log and correct any errors that are indicated, then retry the
|
|
operation.</p>
|
|
</dd>
|
|
</dl>
|
|
|
|
<br>
|
|
<h3>Error Messages</h3>
|
|
|
|
<p>None.</p>
|
|
|
|
<br>
|
|
<h3><a name="USAGE_NOTES">Usage Notes</a></h3>
|
|
|
|
<ol>
|
|
<li>The best way to generate a unique key is to use the <a href="p0zftok.htm">ftok()</a>
|
|
function.<br>
|
|
<br>
|
|
</li>
|
|
|
|
<li>When the operating system accesses a nonteraspace shared memory segment that has a size
|
|
in the range 16 773 120 bytes (16 MB minus 4096 bytes) to
|
|
16 776 960 bytes (16 MB minus 256 bytes), a performance degradation
|
|
will be observed. Use a teraspace shared memory segment if the size of the segment
|
|
is larger than 16 773 120 bytes.<br>
|
|
<br>
|
|
</li>
|
|
|
|
<li>Use the <a href="ipcshmat.htm">shmat()</a> function to get addressability
|
|
to the shared memory segment after the shared memory identifier is obtained.<br>
|
|
<br>
|
|
</li>
|
|
|
|
<li>The storage for a shared memory segment is not allocated until it is
|
|
attached to a job. A job will not be able to attach to a shared memory
|
|
segment that is larger than the amount of storage available on the system.<br>
|
|
<br>
|
|
</li>
|
|
|
|
<li>Jobs cannot attach a nonteraspace shared memory segment in read-only
|
|
or write-only mode. Consequently, permissions that specify read-only or
|
|
write-only will always result in <strong>shmat()</strong> returning with a
|
|
return value of -1 and <em>errno</em> set to EOPNOTSUPP. Jobs
|
|
are permitted to attach a teraspace shared memory segment in read-only
|
|
mode.<br>
|
|
<br>
|
|
</li>
|
|
|
|
</ol>
|
|
|
|
<br>
|
|
<h3>Related Information</h3>
|
|
|
|
<ul>
|
|
<li>The <<strong>sys/shm.h</strong>> file (see <a href="unix13.htm">
|
|
Header Files for UNIX-Type Functions</a>)<br>
|
|
<br>
|
|
</li>
|
|
|
|
<li><a href="p0zftok.htm">ftok()</a>--Generate IPC Key from File Name<br>
|
|
<br>
|
|
</li>
|
|
|
|
<li><a href="ipcshmat.htm">shmat()</a>--Attach Shared Memory Segment to Current
|
|
Process<br>
|
|
<br>
|
|
</li>
|
|
|
|
<li><a href="ipcshmct.htm">shmctl()</a>--Perform Shared Memory Control
|
|
Operations<br>
|
|
<br>
|
|
</li>
|
|
|
|
<li><a href="ipcshmdt.htm">shmdt()</a>--Detach Shared Memory Segment from
|
|
Calling Process</li>
|
|
</ul>
|
|
|
|
<br>
|
|
<h3>Example</h3>
|
|
|
|
<p>See <a href="../apiref/aboutapis.htm#codedisclaimer">Code disclaimer information</a>
|
|
for information pertaining to code examples.</p>
|
|
|
|
<p>For an example of using this function, see <a href="../apiref/apiexusmem.htm">
|
|
Using Semaphores and Shared Memory</a> in Examples: APIs.</p>
|
|
|
|
<br>
|
|
<hr>
|
|
API introduced: V3R6
|
|
|
|
<hr>
|
|
<center>
|
|
<table cellpadding="2" cellspacing="2">
|
|
<tr align="center">
|
|
<td valign="middle" align="center"><a href="#Top_Of_Page">Top</a> | <a href=
|
|
"unix.htm">UNIX-Type APIs</a> | <a href="aplist.htm">APIs by category</a></td>
|
|
</tr>
|
|
</table>
|
|
</center>
|
|
</body>
|
|
</html>
|
|
|