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283 lines
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<title>shm_open</title>
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<basefont size="3"> <a name="shm_open"></a> <a name="tag_03_673"></a><!-- shm_open -->
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<!--header start-->
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<center><font size="2">The Open Group Base Specifications Issue 6<br>
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IEEE Std 1003.1-2001<br>
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Copyright © 2001 The IEEE and The Open Group, All Rights reserved.</font></center>
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<!--header end-->
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<hr size="2" noshade>
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<h4><a name="tag_03_673_01"></a>NAME</h4>
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<blockquote>shm_open - open a shared memory object (<b>REALTIME</b>)</blockquote>
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<h4><a name="tag_03_673_02"></a>SYNOPSIS</h4>
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<blockquote class="synopsis">
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<div class="box"><code><tt><sup>[<a href="javascript:open_code('SHM')">SHM</a>]</sup> <img src="../images/opt-start.gif" alt=
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"[Option Start]" border="0"> #include <<a href="../basedefs/sys/mman.h.html">sys/mman.h</a>><br>
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<br>
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int shm_open(const char *</tt><i>name</i><tt>, int</tt> <i>oflag</i><tt>, mode_t</tt> <i>mode</i><tt>); <img src=
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"../images/opt-end.gif" alt="[Option End]" border="0"></tt></code></div>
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<tt><br>
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</tt></blockquote>
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<h4><a name="tag_03_673_03"></a>DESCRIPTION</h4>
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<blockquote>
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<p>The <i>shm_open</i>() function shall establish a connection between a shared memory object and a file descriptor. It shall
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create an open file description that refers to the shared memory object and a file descriptor that refers to that open file
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description. The file descriptor is used by other functions to refer to that shared memory object. The <i>name</i> argument points
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to a string naming a shared memory object. It is unspecified whether the name appears in the file system and is visible to other
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functions that take pathnames as arguments. The <i>name</i> argument conforms to the construction rules for a pathname. If
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<i>name</i> begins with the slash character, then processes calling <i>shm_open</i>() with the same value of <i>name</i> refer to
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the same shared memory object, as long as that name has not been removed. If <i>name</i> does not begin with the slash character,
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the effect is implementation-defined. The interpretation of slash characters other than the leading slash character in <i>name</i>
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is implementation-defined.</p>
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<p>If successful, <i>shm_open</i>() shall return a file descriptor for the shared memory object that is the lowest numbered file
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descriptor not currently open for that process. The open file description is new, and therefore the file descriptor does not share
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it with any other processes. It is unspecified whether the file offset is set. The FD_CLOEXEC file descriptor flag associated with
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the new file descriptor is set.</p>
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<p>The file status flags and file access modes of the open file description are according to the value of <i>oflag</i>. The
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<i>oflag</i> argument is the bitwise-inclusive OR of the following flags defined in the <a href=
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"../basedefs/fcntl.h.html"><i><fcntl.h></i></a> header. Applications specify exactly one of the first two values (access
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modes) below in the value of <i>oflag</i>:</p>
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<dl compact>
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<dt>O_RDONLY</dt>
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<dd>Open for read access only.</dd>
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<dt>O_RDWR</dt>
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<dd>Open for read or write access.</dd>
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</dl>
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<p>Any combination of the remaining flags may be specified in the value of <i>oflag</i>:</p>
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<dl compact>
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<dt>O_CREAT</dt>
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<dd>If the shared memory object exists, this flag has no effect, except as noted under O_EXCL below. Otherwise, the shared memory
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object is created; the user ID of the shared memory object shall be set to the effective user ID of the process; the group ID of
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the shared memory object is set to a system default group ID or to the effective group ID of the process. The permission bits of
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the shared memory object shall be set to the value of the <i>mode</i> argument except those set in the file mode creation mask of
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the process. When bits in <i>mode</i> other than the file permission bits are set, the effect is unspecified. The <i>mode</i>
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argument does not affect whether the shared memory object is opened for reading, for writing, or for both. The shared memory object
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has a size of zero.</dd>
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<dt>O_EXCL</dt>
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<dd>If O_EXCL and O_CREAT are set, <i>shm_open</i>() fails if the shared memory object exists. The check for the existence of the
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shared memory object and the creation of the object if it does not exist is atomic with respect to other processes executing
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<i>shm_open</i>() naming the same shared memory object with O_EXCL and O_CREAT set. If O_EXCL is set and O_CREAT is not set, the
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result is undefined.</dd>
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<dt>O_TRUNC</dt>
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<dd>If the shared memory object exists, and it is successfully opened O_RDWR, the object shall be truncated to zero length and the
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mode and owner shall be unchanged by this function call. The result of using O_TRUNC with O_RDONLY is undefined.</dd>
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</dl>
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<p>When a shared memory object is created, the state of the shared memory object, including all data associated with the shared
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memory object, persists until the shared memory object is unlinked and all other references are gone. It is unspecified whether the
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name and shared memory object state remain valid after a system reboot.</p>
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</blockquote>
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<h4><a name="tag_03_673_04"></a>RETURN VALUE</h4>
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<blockquote>
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<p>Upon successful completion, the <i>shm_open</i>() function shall return a non-negative integer representing the lowest numbered
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unused file descriptor. Otherwise, it shall return -1 and set <i>errno</i> to indicate the error.</p>
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</blockquote>
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<h4><a name="tag_03_673_05"></a>ERRORS</h4>
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<blockquote>
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<p>The <i>shm_open</i>() function shall fail if:</p>
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<dl compact>
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<dt>[EACCES]</dt>
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<dd>The shared memory object exists and the permissions specified by <i>oflag</i> are denied, or the shared memory object does not
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exist and permission to create the shared memory object is denied, or O_TRUNC is specified and write permission is denied.</dd>
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<dt>[EEXIST]</dt>
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<dd>O_CREAT and O_EXCL are set and the named shared memory object already exists.</dd>
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<dt>[EINTR]</dt>
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<dd>The <i>shm_open</i>() operation was interrupted by a signal.</dd>
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<dt>[EINVAL]</dt>
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<dd>The <i>shm_open</i>() operation is not supported for the given name.</dd>
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<dt>[EMFILE]</dt>
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<dd>Too many file descriptors are currently in use by this process.</dd>
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<dt>[ENAMETOOLONG]</dt>
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<dd>
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The length of the <i>name</i> argument exceeds {PATH_MAX} or a pathname component is longer than {NAME_MAX}.</dd>
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<dt>[ENFILE]</dt>
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<dd>Too many shared memory objects are currently open in the system.</dd>
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<dt>[ENOENT]</dt>
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<dd>O_CREAT is not set and the named shared memory object does not exist.</dd>
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<dt>[ENOSPC]</dt>
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<dd>There is insufficient space for the creation of the new shared memory object.</dd>
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</dl>
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</blockquote>
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<hr>
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<div class="box"><em>The following sections are informative.</em></div>
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<h4><a name="tag_03_673_06"></a>EXAMPLES</h4>
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<blockquote>
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<p>None.</p>
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</blockquote>
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<h4><a name="tag_03_673_07"></a>APPLICATION USAGE</h4>
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<blockquote>
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<p>None.</p>
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</blockquote>
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<h4><a name="tag_03_673_08"></a>RATIONALE</h4>
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<blockquote>
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<p>When the Memory Mapped Files option is supported, the normal <a href="../functions/open.html"><i>open</i>()</a> call is used to
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obtain a descriptor to a file to be mapped according to existing practice with <a href="../functions/mmap.html"><i>mmap</i>()</a>.
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When the Shared Memory Objects option is supported, the <i>shm_open</i>() function shall obtain a descriptor to the shared memory
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object to be mapped.</p>
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<p>There is ample precedent for having a file descriptor represent several types of objects. In the POSIX.1-1990 standard, a file
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descriptor can represent a file, a pipe, a FIFO, a tty, or a directory. Many implementations simply have an operations vector,
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which is indexed by the file descriptor type and does very different operations. Note that in some cases the file descriptor passed
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to generic operations on file descriptors is returned by <a href="../functions/open.html"><i>open</i>()</a> or <a href=
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"../functions/creat.html"><i>creat</i>()</a> and in some cases returned by alternate functions, such as <a href=
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"../functions/pipe.html"><i>pipe</i>()</a>. The latter technique is used by <i>shm_open</i>().</p>
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<p>Note that such shared memory objects can actually be implemented as mapped files. In both cases, the size can be set after the
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open using <a href="../functions/ftruncate.html"><i>ftruncate</i>()</a>. The <i>shm_open</i>() function itself does not create a
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shared object of a specified size because this would duplicate an extant function that set the size of an object referenced by a
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file descriptor.</p>
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<p>On implementations where memory objects are implemented using the existing file system, the <i>shm_open</i>() function may be
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implemented using a macro that invokes <a href="../functions/open.html"><i>open</i>()</a>, and the <a href=
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"../functions/shm_unlink.html"><i>shm_unlink</i>()</a> function may be implemented using a macro that invokes <a href=
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"../functions/unlink.html"><i>unlink</i>()</a>.</p>
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<p>For implementations without a permanent file system, the definition of the name of the memory objects is allowed not to survive
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a system reboot. Note that this allows systems with a permanent file system to implement memory objects as data structures internal
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to the implementation as well.</p>
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<p>On implementations that choose to implement memory objects using memory directly, a <i>shm_open</i>() followed by an <a href=
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"../functions/ftruncate.html"><i>ftruncate</i>()</a> and <a href="../functions/close.html"><i>close</i>()</a> can be used to
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preallocate a shared memory area and to set the size of that preallocation. This may be necessary for systems without virtual
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memory hardware support in order to ensure that the memory is contiguous.</p>
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<p>The set of valid open flags to <i>shm_open</i>() was restricted to O_RDONLY, O_RDWR, O_CREAT, and O_TRUNC because these could be
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easily implemented on most memory mapping systems. This volume of IEEE Std 1003.1-2001 is silent on the results if the
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implementation cannot supply the requested file access because of implementation-defined reasons, including hardware ones.</p>
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<p>The error conditions [EACCES] and [ENOTSUP] are provided to inform the application that the implementation cannot complete a
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request.</p>
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<p>[EACCES] indicates for implementation-defined reasons, probably hardware-related, that the implementation cannot comply with a
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requested mode because it conflicts with another requested mode. An example might be that an application desires to open a memory
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object two times, mapping different areas with different access modes. If the implementation cannot map a single area into a
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process space in two places, which would be required if different access modes were required for the two areas, then the
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implementation may inform the application at the time of the second open.</p>
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<p>[ENOTSUP] indicates for implementation-defined reasons, probably hardware-related, that the implementation cannot comply with a
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requested mode at all. An example would be that the hardware of the implementation cannot support write-only shared memory
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areas.</p>
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<p>On all implementations, it may be desirable to restrict the location of the memory objects to specific file systems for
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performance (such as a RAM disk) or implementation-defined reasons (shared memory supported directly only on certain file systems).
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The <i>shm_open</i>() function may be used to enforce these restrictions. There are a number of methods available to the
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application to determine an appropriate name of the file or the location of an appropriate directory. One way is from the
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environment via <a href="../functions/getenv.html"><i>getenv</i>()</a>. Another would be from a configuration file.</p>
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<p>This volume of IEEE Std 1003.1-2001 specifies that memory objects have initial contents of zero when created. This is
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consistent with current behavior for both files and newly allocated memory. For those implementations that use physical memory, it
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would be possible that such implementations could simply use available memory and give it to the process uninitialized. This,
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however, is not consistent with standard behavior for the uninitialized data area, the stack, and of course, files. Finally, it is
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highly desirable to set the allocated memory to zero for security reasons. Thus, initializing memory objects to zero is
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required.</p>
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</blockquote>
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<h4><a name="tag_03_673_09"></a>FUTURE DIRECTIONS</h4>
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<blockquote>
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<p>None.</p>
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</blockquote>
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<h4><a name="tag_03_673_10"></a>SEE ALSO</h4>
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<blockquote>
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<p><a href="close.html"><i>close</i>()</a> , <a href="dup.html"><i>dup</i>()</a> , <a href="exec.html"><i><a href=
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"../functions/exec.html">exec</a></i>()</a> , <a href="fcntl.html"><i>fcntl</i>()</a> , <a href="mmap.html"><i>mmap</i>()</a> , <a
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href="shmat.html"><i>shmat</i>()</a> , <a href="shmctl.html"><i>shmctl</i>()</a> , <a href="shmdt.html"><i>shmdt</i>()</a> , <a
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href="shm_unlink.html"><i>shm_unlink</i>()</a> , <a href="umask.html"><i>umask</i>()</a> , the Base Definitions volume of
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IEEE Std 1003.1-2001, <a href="../basedefs/fcntl.h.html"><i><fcntl.h></i></a>, <a href=
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"../basedefs/sys/mman.h.html"><i><sys/mman.h></i></a></p>
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</blockquote>
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<h4><a name="tag_03_673_11"></a>CHANGE HISTORY</h4>
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<blockquote>
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<p>First released in Issue 5. Included for alignment with the POSIX Realtime Extension.</p>
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</blockquote>
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<h4><a name="tag_03_673_12"></a>Issue 6</h4>
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<blockquote>
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<p>The <i>shm_open</i>() function is marked as part of the Shared Memory Objects option.</p>
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<p>The [ENOSYS] error condition has been removed as stubs need not be provided if an implementation does not support the Shared
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Memory Objects option.</p>
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</blockquote>
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<div class="box"><em>End of informative text.</em></div>
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