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143 lines
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<meta name="Copyright" content="Copyright (c) 2006 by IBM Corporation">
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<title>Pointer Based Services</title>
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<h2>Pointer Based Services</h2>
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<p>The pointer based services consist of named and unnamed semaphores.
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The named and unnamed semaphores on i5/OS<SUP>(R)</SUP> differ from the other IPC
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mechanisms in that they do not have an IPC identifier
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associated with them. Instead, pointers to the semaphore are used to operate on
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the semaphore. Before using a semaphore, a process must obtain a pointer to the
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semaphore. Unlike a semaphore set, a named or unnamed semaphore refers to
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a single semaphore only. A semaphore contains a value, a maximum value, and a
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title.</p>
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<p>There are two types of semaphores: named semaphores and unnamed semaphores. Once a
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semaphore is created and a pointer to the semaphore is obtained, the same
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operations are used to manipulate the values of both types of semaphores. Like
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the semaphores in a semaphore set, a named or unnamed semaphore has a nonzero
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value. A semaphore can be used as a resource counter or as a lock. A thread
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decrements a semaphore to obtain one or more associated resources, and
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increments the semaphore to release the resource. A semaphore also has a
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maximum value associated with it. An attempt to increment the value of a
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semaphore above its maximum value results in an error.</p>
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<p>Besides a value, named and unnamed semaphores contain a maximum value and a
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title. The maximum value sets the highest value that the semaphore value may
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obtain. The title is a null-terminated string with a maximum size of 16
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characters that are associated with the semaphore and may be used to contain
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debugging information. The titles associated with named and unnamed semaphores
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may be obtained by using the <strong>QP0ZOLIP()</strong> API.</p>
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<p>A process obtains a pointer to a named semaphore by calling the <strong>
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sem_open()</strong> or <strong>sem_open_np()</strong> functions. These
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functions find the semaphore associated with a name. The name is a character
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string, interpreted in the CCSID of the job. The name may be structured so that
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it looks like a pathname. This name, however, has no relationship to any file
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system. If the semaphore exists and the process has permission to use the
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semaphore, then the system allocates memory for the semaphore and returns a
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pointer to the caller. If the semaphore does not exist, it will be created if
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the appropriate flags are set. When a new named semaphore is created, the
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permissions of the semaphore are set using the information provided by the <em>
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mode</em> parameter. These permissions are the same as those used by the
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identifier- based IPC services. The <strong>sem_open_np()</strong> function
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permits the caller to set the maximum value and title of a semaphore when
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creating a named semaphore. When a process is finished using a named semaphore,
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it should call <strong>sem_close()</strong> to close the semaphore. The
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semaphore is also explicitly closed when a process terminates. When a named
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semaphore will no longer be needed, it can be removed from the system using
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<strong>sem_unlink()</strong>.</p>
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<p>A process obtains a pointer to an unnamed semaphore calling the <strong>
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sem_init()</strong> or <strong>sem_init_np()</strong> functions. These
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functions initialize a semaphore at the specified memory location. The <strong>
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sem_init_np()</strong> function permits the caller to set the maximum value and
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title of a unnamed semaphore when it is created. When a process is finished
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using an unnamed semaphore, it should call <strong>sem_destroy()</strong> to
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destroy the semaphore and release system resources associated with that
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semaphore.</p>
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<p>A process decrements by one the value of a semaphore using the <strong>
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sem_wait()</strong> and <strong>sem_wait_np()</strong> functions. If the value
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of the semaphore is currently zero, then the thread is blocked until the value
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of the semaphore is incremented or until the time specified on the <strong>
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sem_wait_np()</strong> has expired. The <strong>sem_trywait()</strong> call may
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be used to decrement the value of the semaphore if it is greater than zero. If
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the current value of the semaphore is zero, then <strong>sem_trywait()</strong>
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will return an error. The <strong>sem_post()</strong>and <strong>
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sem_post_np()</strong>functions are used to increment the value of a semaphore.
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After the value of the semaphore is incremented, it may be decremented
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immediately by threads that have blocked trying to decrement the semaphore.</p>
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<p>Named and unnamed semaphore waits are visible from the Work with Active Jobs
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display. A thread waiting on a named or unnamed semaphore will be in a
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semaphore wait state (SEMW).</p>
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<p>The <strong>sem_getvalue()</strong>function returns the value of the
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semaphore if the value is greater than or equal to zero. If there are threads
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waiting on the semaphore, <strong>sem_getvalue()</strong> returns a negative
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number whose absolute value is the number of threads waiting on the
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semaphore.</p>
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<p>For details on the semaphore functions, see the following:</p>
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<!-- Notify User Technologies of needed updates to be made in XML for API finder.-->
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<!--***************API BEGIN PASTE***************-->
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<ul>
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<li><A HREF="ipcsemou.htm">QlgSem_open()</A> (Open Named Semaphore (using NLS-enabled path name)) opens a named semaphore and returns a semaphore pointer that may be used on subsequent calls to sem_post(), sem_post_np(), sem_wait(), sem_wait_np(), sem_trywait(), sem_getvalue(), and sem_close().</li>
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<li><A HREF="ipcsemonu.htm">QlgSem_open_np()</A> (Open Named Semaphore with Maximum Value (using NLS-enabled path name)) opens a named semaphore and returns a semaphore pointer that may be used on subsequent calls to sem_post(), sem_post_np(), sem_wait(), sem_wait_np(), sem_trywait(), sem_getvalue(), and sem_close().</li>
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<li><A HREF="ipcsemunu.htm">QlgSem_unlink()</A> (Unlink Named Semaphore (using NLS-enabled path name)) unlinks a named semaphore.</li>
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<li><A HREF="ipcsemcl.htm">sem_close()</A> (Close Named Semaphore) closes a named semaphore that was previously opened by a thread of the current process using sem_open() or sem_open_np().</li>
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<li><A HREF="ipcsemde.htm">sem_destroy()</A> (Destroy Unnamed Semaphore) destroys an unnamed semaphore that was previously initialized using sem_init() or sem_init_np().</li>
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<li><A HREF="ipcsemge.htm">sem_getvalue()</A> (Get Semaphore Value) retrieves the value of a named or unnamed semaphore.</li>
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<li><A HREF="ipcsemi.htm">sem_init()</A> (Initialize Unnamed Semaphore) initializes an unnamed semaphore and sets its initial value.</li>
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<li><A HREF="ipcsemin.htm">sem_init_np()</A> (Initialize Unnamed Semaphore with Maximum Value) initializes an unnamed semaphore and sets its initial value.</li>
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<li><A HREF="ipcsemo.htm">sem_open()</A> (Open Named Semaphore) opens a named semaphore, returning a semaphore pointer that may be used on subsequent calls to sem_post(), sem_post_np(), sem_wait(), sem_wait_np(), sem_trywait(), sem_getvalue(), and sem_close().</li>
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<li><A HREF="ipcsemon.htm">sem_open_np()</A> (Open Named Semaphore with Maximum Value) opens a named semaphore, returning a semaphore pointer that may be used on subsequent calls to sem_post(), sem_post_np(), sem_wait(), sem_wait_np(), sem_trywait(), sem_getvalue(), and sem_close().</li>
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<li><A HREF="ipcsemp.htm">sem_post()</A> (Post to Semaphore) posts to a semaphore, incrementing its value by one.</li>
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<li><A HREF="ipcsempn.htm">sem_post_np()</A> (Post Value to Semaphore) posts to a semaphore, incrementing its value by the increment specified in the options parameter.</li>
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<li><A HREF="ipcsemtr.htm">sem_trywait()</A> (Try to Decrement Semaphore) attempts to decrement the value of the semaphore.</li>
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<li><A HREF="ipcsemun.htm">sem_unlink()</A> (Unlink Named Semaphore) unlinks a named semaphore.</li>
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<li><A HREF="ipcsemw.htm">sem_wait()</A> (Wait for Semaphore) decrements by one the value of the semaphore.</li>
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<li><A HREF="ipcsemwn.htm">sem_wait_np()</A> (Wait for Semaphore with Timeout) attempts to decrement by one the value of the semaphore.</li>
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</ul>
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