maps a file, its contents are shared among them. If the mappings allow shared write access, then data written into the memoryobject through the address space of one process appears in the address spaces of all processes that similarly map the same portionof the memory object.</p><p><sup>[<a href="javascript:open_code('SHM')">SHM</a>]</sup> <img src="../images/opt-start.gif" alt="[Option Start]" border="0">Shared memory objects are named regions of storage that may be independent of the file system and can be mapped into the addressspace of one or more processes to allow them to share the associated memory. <img src="../images/opt-end.gif" alt="[Option End]"border="0"></p><p>An <a href="../functions/unlink.html"><i>unlink</i>()</a> of a file <sup>[<a href="javascript:open_code('SHM')">SHM</a>]</sup><img src="../images/opt-start.gif" alt="[Option Start]" border="0"> &nbsp;or <a href="../functions/shm_unlink.html"><i>shm_unlink</i>()</a> of a shared memory object, <img src="../images/opt-end.gif" alt="[Option End]" border="0"> while causing the removal of the name, does not unmap any mappings established for the object. Once thename has been removed, the contents of the memory object are preserved as long as it is referenced. The memory object remainsreferenced as long as a process has the memory object open or has some area of the memory object mapped.</p><h5><a name="tag_02_08_03_03"></a>Memory Protection</h5><p><sup>[<a href="javascript:open_code('MPR MF')">MPR MF</a>]</sup> <img src="../images/opt-start.gif" alt="[Option Start]" border="0"> The functionality described in this section is dependent on support of the Memory Protection and Memory Mapped Files option(and the rest of this section is not further marked for these options). <img src="../images/opt-end.gif" alt="[Option End]" border="0"></p><p>When an object is mapped, various application accesses to the mapped region may result in signals. In this context, SIGBUS isused to indicate an error using the mapped object, and SIGSEGV is used to indicate a protection violation or misuse of anaddress:</p><ul><li><p>A mapping may be restricted to disallow some types of access.</p></li><li><p>Write attempts to memory that was mapped without write access, or any access to memory mapped PROT_NONE, shall result in aSIGSEGV signal.</p></li><li><p>References to unmapped addresses shall result in a SIGSEGV signal.</p></li><li><p>Reference to whole pages within the mapping, but beyond the current length of the object, shall result in a SIGBUS signal.</p></li><li><p>The size of the object is unaffected by access beyond the end of the object (even if a SIGBUS is not generated).</p></li></ul><h5><a name="tag_02_08_03_04"></a>Typed Memory Objects</h5><p><sup>[<a href="javascript:open_code('TYM')">TYM</a>]</sup> <img src="../images/opt-start.gif" alt="[Option Start]" border="0">The functionality described in this section is dependent on support of the Typed Memory Objects option (and the rest of thissection is not further marked for this option). <img src="../images/opt-end.gif" alt="[Option End]" border="0"></p><p>Implementations may support the Typed Memory Objects option without supporting the Memory Mapped Files option or the SharedMemory Objects option. Typed memory objects are implementation-configurable named storage pools accessible from one or moreprocessors in a system, each via one or more ports, such as backplane buses, LANs, I/O channels, and so on. Each valid combinationof a storage pool and a port is identified through a name that is defined at system configuration time, in animplementation-defined manner; the name may be independent of the file system. Using this name, a typed memory object can be openedand mapped into process address space. For a given storage pool and port, it is necessary to support both dynamic allocation fromthe pool as well as mapping at an application-supplied offset within the pool; when dynamic allocation has been performed,subsequent deallocation must be supported. Lastly, accessing typed memory objects from different ports requires a method forobtaining the offset and length of contiguous storage of a region of typed memory (dynamically allocated or not); this allows typedmemory to be shared among processes and/or processors while being accessed from the desired port.</p><h4><a name="tag_02_08_04">  2.8.4 </a>Process Scheduling</h4><p><sup>[<a href="javascript:open_code('PS')">PS</a>]</sup> <img src="../images/opt-start.gif" alt="[Option Start]" border="0"> Thefunctionality described in this section is dependent on support of the Process Scheduling option (and the rest of this section isnot further marked for this option). <img src="../images/opt-end.gif" alt="[Option End]" border="0"></p><h5><a name="tag_02_08_04_01"></a>Scheduling Policies</h5><p>The scheduling semantics described in this volume of IEEE&nbsp;Std&nbsp;1003.1-2001 are defined in terms of a conceptual modelthat contains a set of thread lists. No implementation structures are necessarily implied by the use of this conceptual model. Itis assumed that no time elapses during operations described using this model, and therefore no simultaneous operations arepossible. This model discusses only processor scheduling for runnable threads, but it should be noted that greatly enhancedpredictability of realtime applications results if the sequencing of other resources takes processor scheduling policy intoaccount.</p><p>There is, conceptually, one thread list for each priority. A runnable thread will be on the thread list for that thread'spriority. Multiple scheduling policies shall be provided. Each non-empty thread list is ordered, contains a head as one end of itsorder, and a tail as the other. The purpose of a scheduling policy is to define the allowable operations on this set of lists (forexample, moving threads between and within lists).</p><p>Each process shall be controlled by an associated scheduling policy and priority. These parameters may be specified by explicitapplication execution of the <a href="../functions/sched_setscheduler.html"><i>sched_setscheduler</i>()</a> or <a href="../functions/sched_setparam.html"><i>sched_setparam</i>()</a> functions.</p><p>Each thread shall be controlled by an associated scheduling policy and priority. These parameters may be specified by explicitapplication execution of the <a href="../functions/pthread_setschedparam.html"><i>pthread_setschedparam</i>()</a> function.</p><p>Associated with each policy is a priority range. Each policy definition shall specify the minimum priority range for thatpolicy. The priority ranges for each policy may but need not overlap the priority ranges of other policies.</p><p>A conforming implementation shall select the thread that is defined as being at the head of the highest priority non-emptythread list to become a running thread, regardless of its associated policy. This thread is then removed from its thread list.</p><p>Four scheduling policies are specifically required. Other implementation-defined scheduling policies may be defined. Thefollowing symbols are defined in the Base Definitions volume of IEEE&nbsp;Std&nbsp;1003.1-2001, <a href="../basedefs/sched.h.html"><i>&lt;sched.h&gt;</i></a>:</p><dl compact><dt>SCHED_FIFO</dt><dd>First in, first out (FIFO) scheduling policy.</dd><dt>SCHED_RR</dt><dd>Round robin scheduling policy.</dd><dt>SCHED_SPORADIC</dt><dd><sup>[<a href="javascript:open_code('SS')">SS</a>]</sup> <img src="../images/opt-start.gif" alt="[Option Start]" border="0">Sporadic server scheduling policy. <img src="../images/opt-end.gif" alt="[Option End]" border="0"></dd><dt>SCHED_OTHER</dt><dd>Another scheduling policy.</dd></dl><p>The values of these symbols shall be distinct.</p><h5><a name="tag_02_08_04_02"></a>SCHED_FIFO</h5><p>Conforming implementations shall include a scheduling policy called the FIFO scheduling policy.</p><p>Threads scheduled under this policy are chosen from a thread list that is ordered by the time its threads have been on the listwithout being executed; generally, the head of the list is the thread that has been on the list the longest time, and the tail isthe thread that has been on the list the shortest time.</p><p>Under the SCHED_FIFO policy, the modification of the definitional thread lists is as follows:</p><ol><li><p>When a running thread becomes a preempted thread, it becomes the head of the thread list for its priority.</p></li><li><p>When a blocked thread becomes a runnable thread, it becomes the tail of the thread list for its priority.</p></li><li><p>When a running thread calls the <a href="../functions/sched_setscheduler.html"><i>sched_setscheduler</i>()</a> function, theprocess specified in the function call is modified to the specified policy and the priority specified by the <i>param</i>argument.</p></li><li><p>When a running thread calls the <a href="../functions/sched_setparam.html"><i>sched_setparam</i>()</a> function, the priority ofthe process specified in the function call is modified to the priority specified by the <i>param</i> argument.</p></li><li><p>When a running thread calls the <a href="../functions/pthread_setschedparam.html"><i>pthread_setschedparam</i>()</a> function,the thread specified in the function call is modified to the specified policy and the priority specified by the <i>param</i>argument.</p></li><li><p>When a running thread calls the <a href="../functions/pthread_setschedprio.html"><i>pthread_setschedprio</i>()</a> function, thethread specified in the function call is modified to the priority specified by the <i>prio</i> argument.</p></li><li><p>If a thread whose policy or priority has been modified other than by <a href="../functions/pthread_setschedprio.html"><i>pthread_setschedprio</i>()</a> is a running thread or is runnable, it then becomes thetail of the thread list for its new priority.</p></li><li><p>If a thread whose policy or priority has been modified by <a href="../functions/pthread_setschedprio.html"><i>pthread_setschedprio</i>()</a> is a running thread or is runnable, the effect on itsposition in the thread list depends on the direction of the modification, as follows:</p><ol type="a"><li><p>If the priority is raised, the thread becomes the tail of the thread list.</p></li><li><p>If the priority is unchanged, the thread does not change position in the thread list.</p></li><li><p>If the priority is lowered, the thread becomes the head of the thread list.</p></li></ol></li><li><p>When a running thread issues the <a href="../functions/sched_yield.html"><i>sched_yield</i>()</a> function, the thread becomesthe tail of the thread list for its priority.</p></li><li><p>At no other time is the position of a thread with this scheduling policy within the thread lists affected.</p></li></ol><p>For this policy, valid priorities shall be within the range returned by the <a href="../functions/sched_get_priority_max.html"><i>sched_get_priority_max</i>()</a> and <a href="../functions/sched_get_priority_min.html"><i>sched_get_priority_min</i>()</a> functions when SCHED_FIFO is provided as theparameter. Conforming implementations shall provide a priority range of at least 32 priorities for this policy.</p><h5><a name="tag_02_08_04_03"></a>SCHED_RR</h5><p>Conforming implementations shall include a scheduling policy called the ``round robin'' scheduling policy. This policy shall beidentical to the SCHED_FIFO policy with the additional condition that when the implementation detects that a running thread hasbeen executing as a running thread for a time period of the length returned by the <a href="../functions/sched_rr_get_interval.html"><i>sched_rr_get_interval</i>()</a> function or longer, the thread shall become the tailof its thread list and the head of that thread list shall be removed and made a running thread.</p><p>The effect of this policy is to ensure that if there are multiple SCHED_RR threads at the same priority, one of them does notmonopolize the processor. An application should not rely only on the use of SCHED_RR to ensure application progress among multiplethreads if the application includes threads using the SCHED_FIFO policy at the same or higher priority levels or SCHED_RR threadsat a higher priority level.</p><p>A thread under this policy that is preempted and subsequently resumes execution as a running thread completes the unexpiredportion of its round robin interval time period.</p><p>For this policy, valid priorities shall be within the range returned by the <a href="../functions/sched_get_priority_max.html"><i>sched_get_priority_max</i>()</a> and <a href="../functions/sched_get_priority_min.html"><i>sched_get_priority_min</i>()</a> functions when SCHED_RR is provided as theparameter. Conforming implementations shall provide a priority range of at least 32 priorities for this policy.</p><h5><a name="tag_02_08_04_04"></a>SCHED_SPORADIC</h5><p><sup>[<a href="javascript:open_code('SS')">SS|TSP</a>]</sup> <img src="../images/opt-start.gif" alt="[Option Start]" border="0">The functionality described in this section is dependent on support of the Process Sporadic Server or Thread Sporadic Serveroptions (and the rest of this section is not further marked for these options). <img src="../images/opt-end.gif" alt="[Option End]"border="0"></p><p>If _POSIX_SPORADIC_SERVER or _POSIX_THREAD_SPORADIC_SERVER is defined, the implementation shall include a scheduling policyidentified by the value SCHED_SPORADIC.</p><p>The sporadic server policy is based primarily on two parameters: the replenishment period and the available execution capacity.The replenishment period is given by the <i>sched_ss_repl_period</i> member of the <b>sched_param</b> structure. The availableexecution capacity is initialized to the value given by the <i>sched_ss_init_budget</i> member of the same parameter. The sporadicserver policy is identical to the SCHED_FIFO policy with some additional conditions that cause the thread's assigned priority to beswitched between the values specified by the <i>sched_priority</i> and <i>sched_ss_low_priority</i> members of the<b>sched_param</b> structure.</p><p>The priority assigned to a thread using the sporadic server scheduling policy is determined in the following manner: if theavailable execution capacity is greater than zero and the number of pending replenishment operations is strictly less than<i>sched_ss_max_repl</i>, the thread is assigned the priority specified by <i>sched_priority</i>; otherwise, the assigned priorityshall be <i>sched_ss_low_priority</i>. If the value of <i>sched_priority</i> is less than or equal to the value of<i>sched_ss_low_priority</i>, the results are undefined. When active, the thread shall belong to the thread list corresponding toits assigned priority level, according to the mentioned priority assignment. The modification of the available execution capacityand, consequently of the assigned priority, is done as follows:</p><ol><li><p>When the thread at the head of the <i>sched_priority</i> list becomes a running thread, its execution time shall be limited toat most its available execution capacity, plus the resolution of the execution time clock used for this scheduling policy. Thisresolution shall be implementation-defined.</p></li><li><p>Each time the thread is inserted at the tail of the list associated with <i>sched_priority</i>- because as a blocked thread itbecame runnable with priority <i>sched_priority</i> or because a replenishment operation was performed-the time at which thisoperation is done is posted as the <i>activation_time</i>.</p></li><li><p>When the running thread with assigned priority equal to <i>sched_priority</i> becomes a preempted thread, it becomes the head ofthe thread list for its priority, and the execution time consumed is subtracted from the available execution capacity. If theavailable execution capacity would become negative by this operation, it shall be set to zero.</p></li>