212 lines
8.4 KiB
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212 lines
8.4 KiB
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<meta name="Copyright" content="Copyright (c) 2006 by IBM Corporation">
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<title>Identifier Based Services</title>
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<h2>Identifier Based Services</h2>
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<p>Although each IPC service provides a specific type of interprocess
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communication, the three identifier based services share many similarities.
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Each service defines a mechanism through which its communications take place.
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For message queues, that mechanism is a message queue; for semaphore sets, it
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is a semaphore set; and for shared memory, it is a shared memory segment. These
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mechanisms are identified by a unique positive integer called, respectively, a
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message queue identifier (<em>msqid</em>), a semaphore set identifier
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(<em>semid</em>), and a shared memory identifier (<em>shmid</em>).</p>
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<p><strong>Note:</strong> Throughout the Interprocess Communication APIs, the
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term thread is used extensively. This does not mean that IPC objects can be
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used only between threads within one process, but rather that authorization
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checks and waits occur for the calling thread within a process.</p>
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<p>Associated with each identifier is a data structure that contains state
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information for the IPC mechanism, as well as ownership and permissions
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information. The ownership and permissions information is defined in a
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structure in the <strong><sys/ipc.h></strong> header file as follows:</p>
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<pre>
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typedef struct ipc_perm {
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uid_t uid; /* Owner's user ID */
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gid_t gid; /* Owner's group ID */
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uid_t cuid; /* Creator's user ID */
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gid_t cgid; /* Creator's group ID */
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mode_t mode; /* Access modes */
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} ipc_perm_t;
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</pre>
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<p>This structure is similar to a file permissions structure, and is
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initialized by the thread that creates the IPC mechanism. It is then checked by
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all subsequent IPC operations to determine if the requesting thread has the
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required permissions to perform the operation.</p>
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<p>To get an identifier, a thread must either create a new IPC mechanism or
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access an existing mechanism. This is done through the <strong>
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msgget()</strong>, <strong>semget()</strong>, and <strong>shmget()</strong>
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functions. Each get operation takes as input a <em>key</em> parameter and
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returns an identifier. Each get operation also takes a <em>flag</em> parameter.
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This <em>flag</em> parameter contains the IPC permissions for the mechanism as
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well as bits that determine whether or not a new mechanism is created. The
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rules for whether a new mechanism is created or an existing one is referred to
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are as follows:</p>
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<ul>
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<li>Specifying a <em>key</em> of IPC_PRIVATE guarantees a new mechanism is
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created.<br>
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<br>
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</li>
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<li>Setting the IPC_CREAT bit in the <em>flag</em> parameter creates a new
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mechanism for the specified <em>key</em> if one does not already exist. If an
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existing mechanism is found, its identifier is returned.<br>
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<br>
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</li>
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<li>Setting both IPC_CREAT and IPC_EXCL creates a new mechanism for the
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specified <em>key</em> only if a mechanism does not already exist. If an
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existing mechanism is found, an error is returned.</li>
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</ul>
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<p>When a message queue, semaphore set, or shared memory segment is created,
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the thread that creates it determines how it can be accessed. The thread does
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this by passing mode information in the low-order 9 bits of the <em>flag</em>
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parameter on the <strong>msgget()</strong>, <strong>semget()</strong>, or
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<strong>shmget()</strong> function call. This information is used to initialize
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the <samp>mode</samp> field in the <samp>ipc_perm</samp> structure. The values of the
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bits are given below in hexadecimal notation:</p>
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<table cellpadding="5">
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<!-- cols="15 85" -->
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<tr>
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<th align="left" valign="top">Bit</th>
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<th align="left" valign="top">Meaning</th>
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</tr>
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<tr>
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<td align="left" valign="top"><em>X'0100'</em></td>
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<td align="left" valign="top">Read by user</td>
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</tr>
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<tr>
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<td align="left" valign="top"><em>X'0080'</em></td>
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<td align="left" valign="top">Write by user</td>
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</tr>
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<tr>
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<td align="left" valign="top"><em>X'0020'</em></td>
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<td align="left" valign="top">Read by group</td>
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</tr>
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<tr>
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<td align="left" valign="top"><em>X'0010'</em></td>
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<td align="left" valign="top">Write by group</td>
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</tr>
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<tr>
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<td align="left" valign="top"><em>X'0004'</em></td>
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<td align="left" valign="top">Read by others</td>
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</tr>
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<tr>
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<td align="left" valign="top"><em>X'0002'</em></td>
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<td align="left" valign="top">Write by others</td>
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</tr>
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</table>
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<p>Subsequent IPC operations do a permission test on the calling thread before
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allowing the thread to perform the requested operation. This permission test is
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done in one of three forms:</p>
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<ul>
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<li>For the <strong>msgget()</strong>, <strong>semget()</strong>, or <strong>
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shmget()</strong> calls that are accessing an existing IPC mechanism, the
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caller's <em>flag</em> parameter is checked to make sure it does not specify
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access bits that are not in the <samp>mode</samp> field of the existing IPC
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mechanism's <samp>ipc_perm</samp> structure. If the <em>flag</em> parameter does
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not contain any bits that are not in the <samp>mode</samp> field, permission is
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granted.<br>
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<br>
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</li>
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<li>For most of the other IPC APIs, the effective user ID and effective group
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ID of the thread are retrieved, and these values are compared with the data in
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the <samp>ipc_perm</samp> structure as follows:<br>
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<br>
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<ul>
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<li>If the effective user ID equals either the <samp>uid</samp> or the <samp>
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cuid</samp> field for the IPC mechanism, and if the appropriate access bit is on
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in the <samp>mode</samp> field (either <samp>Read by user</samp> or <samp>Write by
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user</samp>, depending on the operation being requested), permission is
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granted.<br>
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<br>
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</li>
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<li>If the effective group ID equals either the <samp>gid</samp> or the <samp>
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cgid</samp> field for the IPC mechanism, and if the appropriate access bit is on
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in the <samp>mode</samp> field (either <samp>Read by group</samp> or <samp>Write by
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group</samp>), permission is granted.<br>
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<br>
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</li>
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<li>If none of the above tests are true, and if the appropriate access bit is
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on for others (either <samp>Read by others</samp> or <samp>Write by others</samp>),
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permission is granted.<br>
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<br>
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</li>
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</ul>
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</li>
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<li>For the <strong>msgctl()</strong>, <strong>semctl()</strong>, or <strong>
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shmctl()</strong> APIs, some values of the <em>cmd</em> parameter require the
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caller to be the owner or creator of the IPC object, or have appropriate
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privileges. The values of <em>cmd</em> that this rule applies to depends on the
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API. This is shown in the API descriptions for <strong>msgctl()</strong>,
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<strong>semctl()</strong>, and <strong>shmctl()</strong>.</li>
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</ul>
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<br>
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<p>The Identifier Based Services are divided into the following subcategories:</p>
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<ul>
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<li><a href="unix3a1.htm">Message Queue Functions</a><br></li>
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<li><a href="unix3a2.htm">Semaphore Set Functions</a><br></li>
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<li><a href="unix3a3.htm">Shared Memory Functions</a><br></li>
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<li><a href="unix3a4.htm">IPC Key Generation Functions</a><br></li>
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</ul>
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