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361 lines
11 KiB
XML
361 lines
11 KiB
XML
<reference id="ref.sem">
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<title>Semaphore and Shared Memory Functions</title>
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<titleabbrev>Semaphore</titleabbrev>
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<partintro>
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<para>
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This module provides semaphore functions using System V
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semaphores. Semaphores may be used to provide exclusive access to
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resources on the current machine, or to limit the number of
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processes that may simultaneously use a resource.
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</para>
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<para>
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This module provides also shared memory functions using System V
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shared memory. Shared memory may be used to provide access to
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global variables. Different httpd-daemons and even other programs
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(such as Perl, C, ...) are able to access this data to provide a
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global data-exchange. Remember, that shared memory is NOT safe
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against simultaneous access. Use semaphores for synchronization.
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<table>
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<title>Limits of Shared Memory by the Unix OS</title>
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<tgroup cols="2">
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<tbody>
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<row>
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<entry>SHMMAX</entry>
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<entry>max size of shared memory, normally 131072 bytes</entry>
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</row>
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<row>
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<entry>SHMMIN</entry>
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<entry>minimum size of shared memory, normally 1 byte</entry>
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</row>
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<row>
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<entry>SHMMNI</entry>
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<entry>max amount of shared memory segments, normally 100</entry>
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</row>
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<row>
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<entry>SHMSEG</entry>
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<entry>max amount of shared memory per process, normally 6</entry>
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</row>
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</tbody>
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</tgroup>
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</table>
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</para>
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<note>
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<simpara>
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These functions do not work on Windows systems.
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</simpara>
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</note>
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</partintro>
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<refentry id="function.sem-get">
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<refnamediv>
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<refname>sem_get</refname>
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<refpurpose>Get a semaphore id</refpurpose>
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</refnamediv>
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<refsect1>
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<title>Description</title>
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<funcsynopsis>
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<funcprototype>
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<funcdef>int <function>sem_get</function></funcdef>
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<paramdef>int <parameter>key</parameter></paramdef>
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<paramdef>int
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<parameter><optional>max_acquire</optional></parameter>
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</paramdef>
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<paramdef>int
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<parameter><optional>perm</optional></parameter>
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</paramdef>
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</funcprototype>
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</funcsynopsis>
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<para>
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Returns: A positive semaphore identifier on success, or false on
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error.
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</para>
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<para>
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<function>Sem_get</function> returns an id that can be used to
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access the System V semaphore with the given key. The semaphore
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is created if necessary using the permission bits specified in
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perm (defaults to 0666). The number of processes that can
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acquire the semaphore simultaneously is set to max_acquire
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(defaults to 1). Actually this value is set only if the process
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finds it is the only process currently attached to the semaphore.
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</para>
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<para>
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A second call to <function>sem_get</function> for the same key
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will return a different semaphore identifier, but both
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identifiers access the same underlying semaphore.
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</para>
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<para>
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See also: <function>sem_acquire</function> and
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<function>sem_release</function>.
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</para>
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<note>
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<simpara>
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This function does not work on Windows systems
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</simpara>
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</note>
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</refsect1>
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</refentry>
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<refentry id="function.sem-acquire">
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<refnamediv>
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<refname>sem_acquire</refname>
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<refpurpose>Acquire a semaphore</refpurpose>
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</refnamediv>
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<refsect1>
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<title>Description</title>
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<funcsynopsis>
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<funcprototype>
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<funcdef>int <function>sem_acquire</function></funcdef>
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<paramdef>int <parameter>sem_identifier</parameter></paramdef>
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</funcprototype>
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</funcsynopsis>
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<para>
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Returns: true on success, false on error.
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</para>
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<para>
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<function>Sem_acquire</function> blocks (if necessary) until the
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semaphore can be acquired. A process attempting to acquire a
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semaphore which it has already acquired will block forever
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if acquiring the semaphore would cause its max_acquire value to
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be exceeded.
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</para>
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<para>
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After processing a request, any semaphores acquired by the
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process but not explicitly released will be released automatically
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and a warning will be generated.
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</para>
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<para>
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See also: <function>sem_get</function> and
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<function>sem_release</function>.
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</para>
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</refsect1>
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</refentry>
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<refentry id="function.sem-release">
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<refnamediv>
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<refname>sem_release</refname>
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<refpurpose>Release a semaphore</refpurpose>
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</refnamediv>
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<refsect1>
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<title>Description</title>
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<funcsynopsis>
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<funcprototype>
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<funcdef>int <function>sem_release</function></funcdef>
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<paramdef>int <parameter>sem_identifier</parameter></paramdef>
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</funcprototype>
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</funcsynopsis>
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<para>
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Returns: true on success, false on error.
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</para>
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<para>
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<function>Sem_release</function> releases the semaphore if it
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is currently acquired by the calling process, otherwise
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a warning is generated.
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</para>
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<para>
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After releasing the semaphore, <function>sem_acquire</function>
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may be called to re-acquire it.
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</para>
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<para>
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See also: <function>sem_get</function> and
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<function>sem_acquire</function>.
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</para>
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<note>
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<simpara>
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This function does not work on Windows systems
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</simpara>
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</note>
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</refsect1>
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</refentry>
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<refentry id="function.shm-attach">
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<refnamediv>
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<refname>shm_attach</refname>
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<refpurpose>Creates or open a shared memory segment</refpurpose>
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</refnamediv>
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<refsect1>
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<title>Description</title>
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<funcsynopsis>
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<funcprototype>
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<funcdef>int <function>shm_attach</function></funcdef>
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<paramdef>int <parameter>key</parameter></paramdef>
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<paramdef>int
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<parameter><optional>memsize</optional></parameter>
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</paramdef>
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<paramdef>int
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<parameter><optional>perm</optional></parameter>
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</paramdef>
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</funcprototype>
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</funcsynopsis>
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<para>
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<function>Shm_attach</function> returns an id that that can be
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used to access the System V shared memory with the given key, the
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first call creates the shared memory segment with mem_size
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(default: sysvshm.init_mem in the <link
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linkend="configuration.file">configuration file</link>, otherwise
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10000 bytes) and the optional perm-bits (default: 0666).
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</para>
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<para>
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A second call to <function>shm_attach</function> for the same
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<parameter>key</parameter> will return a different shared memory
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identifier, but both identifiers access the same underlying
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shared memory. <parameter>Memsize</parameter> and
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<parameter>perm</parameter> will be ignored.
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</para>
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<note>
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<simpara>
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This function does not work on Windows systems
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</simpara>
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</note>
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</refsect1>
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</refentry>
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<refentry id="function.shm-detach">
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<refnamediv>
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<refname>shm_detach</refname>
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<refpurpose>Disconnects from shared memory segment</refpurpose>
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</refnamediv>
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<refsect1>
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<title>Description</title>
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<funcsynopsis>
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<funcprototype>
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<funcdef>int <function>shm_detach</function></funcdef>
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<paramdef>int <parameter>shm_identifier</parameter></paramdef>
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</funcprototype>
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</funcsynopsis>
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<para>
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<function>Shm_detach</function> disconnects from the shared
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memory given by the <parameter>shm_identifier</parameter> created
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by <function>shm_attach</function>. Remember, that shared memory
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still exist in the Unix system and the data is still present.
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</para>
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</refsect1>
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</refentry>
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<refentry id="function.shm-remove">
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<refnamediv>
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<refname>shm_remove</refname>
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<refpurpose>Removes shared memory from Unix systems</refpurpose>
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</refnamediv>
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<refsect1>
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<title>Description</title>
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<funcsynopsis>
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<funcprototype>
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<funcdef>int <function>shm_remove</function></funcdef>
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<paramdef>int <parameter>shm_identifier</parameter></paramdef>
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</funcprototype>
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</funcsynopsis>
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<para>
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Removes shared memory from Unix systems. All data will be
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destroyed.
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</para>
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<note>
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<simpara>
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This function does not work on Windows systems
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</simpara>
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</note>
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</refsect1>
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</refentry>
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<refentry id="function.shm-put-var">
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<refnamediv>
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<refname>shm_put_var</refname>
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<refpurpose>Inserts or updates a variable in shared
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memory</refpurpose>
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</refnamediv>
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<refsect1>
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<title>Description</title>
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<funcsynopsis>
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<funcprototype>
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<funcdef>int <function>shm_put_var</function></funcdef>
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<paramdef>int <parameter>shm_identifier</parameter></paramdef>
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<paramdef>int <parameter>variable_key</parameter></paramdef>
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<paramdef>mixed <parameter>variable</parameter></paramdef>
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</funcprototype>
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</funcsynopsis>
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<para>
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Inserts or updates a <parameter>variable</parameter> with a given
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<parameter>variable_key</parameter>. All variable-types (double,
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int, string, array) are supported.
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</para>
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<note>
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<simpara>
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This function does not work on Windows systems
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</simpara>
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</note>
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</refsect1>
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</refentry>
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<refentry id="function.shm-get-var">
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<refnamediv>
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<refname>shm_get_var</refname>
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<refpurpose>Returns a variable from shared memory
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</refpurpose>
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</refnamediv>
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<refsect1>
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<title>Description</title>
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<funcsynopsis>
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<funcprototype>
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<funcdef>mixed <function>shm_get_var</function></funcdef>
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<paramdef>int <parameter>id</parameter></paramdef>
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<paramdef>int <parameter>variable_key</parameter></paramdef>
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</funcprototype>
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</funcsynopsis>
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<para>
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<function>Shm_get_var</function> returns the variable with a given
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<parameter>variable_key</parameter>. The variable is still present
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in the shared memory.
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</para>
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<note>
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<simpara>
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This function does not work on Windows systems
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</simpara>
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</note>
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</refsect1>
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</refentry>
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<refentry id="function.shm-remove-var">
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<refnamediv>
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<refname>shm_remove_var</refname>
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<refpurpose>Removes a variable from shared memory
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</refpurpose>
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</refnamediv>
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<refsect1>
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<title>Description</title>
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<funcsynopsis>
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<funcprototype>
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<funcdef>int <function>shm_remove_var</function></funcdef>
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<paramdef>int <parameter>id</parameter></paramdef>
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<paramdef>int <parameter>variable_key</parameter></paramdef>
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</funcprototype>
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</funcsynopsis>
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<para>
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Removes a variable with a given <parameter>variable_key</parameter>
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and frees the occupied memory.
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</para>
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<note>
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<simpara>
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This function does not work on Windows systems
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</simpara>
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</note>
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</refsect1>
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</refentry>
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</reference>
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<!-- Keep this comment at the end of the file
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Local variables:
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mode: sgml
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sgml-omittag:t
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sgml-shorttag:t
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sgml-minimize-attributes:nil
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sgml-always-quote-attributes:t
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sgml-indent-step:1
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sgml-indent-data:t
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sgml-parent-document:nil
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sgml-default-dtd-file:"../../manual.ced"
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sgml-exposed-tags:nil
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sgml-local-catalogs:nil
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sgml-local-ecat-files:nil
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End:
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-->
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