450 lines
9.7 KiB
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450 lines
9.7 KiB
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
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<HTML><HEAD><TITLE>Man page of SHM_OPEN</TITLE>
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</HEAD><BODY>
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<H1>SHM_OPEN</H1>
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Section: Linux Programmer's Manual (3)<BR>Updated: 2017-09-15<BR><A HREF="#index">Index</A>
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<A HREF="/cgi-bin/man/man2html">Return to Main Contents</A><HR>
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<A NAME="lbAB"> </A>
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<H2>NAME</H2>
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shm_open, shm_unlink - create/open or unlink POSIX shared memory objects
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<A NAME="lbAC"> </A>
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<H2>SYNOPSIS</H2>
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<B>#include <<A HREF="file:///usr/include/sys/mman.h">sys/mman.h</A>></B>
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<BR>
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<B>#include <<A HREF="file:///usr/include/sys/stat.h">sys/stat.h</A>></B> /* For mode constants */
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<BR>
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<B>#include <<A HREF="file:///usr/include/fcntl.h">fcntl.h</A>></B> /* For O_* constants */
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<P>
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<B>int shm_open(const char *</B><I>name</I><B>, int </B><I>oflag</I><B>, mode_t </B><I>mode</I><B>);</B>
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<P>
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<B>int shm_unlink(const char *</B><I>name</I><B>);</B>
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<P>
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Link with <I>-lrt</I>.
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<A NAME="lbAD"> </A>
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<H2>DESCRIPTION</H2>
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<B>shm_open</B>()
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creates and opens a new, or opens an existing, POSIX shared memory object.
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A POSIX shared memory object is in effect a handle which can
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be used by unrelated processes to
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<B><A HREF="/cgi-bin/man/man2html?2+mmap">mmap</A></B>(2)
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the same region of shared memory.
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The
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<B>shm_unlink</B>()
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function performs the converse operation,
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removing an object previously created by
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<B>shm_open</B>().
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<P>
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The operation of
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<B>shm_open</B>()
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is analogous to that of
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<B><A HREF="/cgi-bin/man/man2html?2+open">open</A></B>(2).
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<I>name</I>
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specifies the shared memory object to be created or opened.
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For portable use,
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a shared memory object should be identified by a name of the form
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<I>/somename</I>;
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that is, a null-terminated string of up to
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<B>NAME_MAX</B>
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(i.e., 255) characters consisting of an initial slash,
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followed by one or more characters, none of which are slashes.
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<P>
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<I>oflag</I>
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is a bit mask created by ORing together exactly one of
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<B>O_RDONLY</B>
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or
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<B>O_RDWR</B>
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and any of the other flags listed here:
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<DL COMPACT>
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<DT id="1"><B>O_RDONLY</B>
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<DD>
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Open the object for read access.
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A shared memory object opened in this way can be
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<B><A HREF="/cgi-bin/man/man2html?2+mmap">mmap</A></B>(2)ed
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only for read
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(<B>PROT_READ</B>)
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access.
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<DT id="2"><B>O_RDWR</B>
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<DD>
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Open the object for read-write access.
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<DT id="3"><B>O_CREAT</B>
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<DD>
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Create the shared memory object if it does not exist.
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The user and group ownership of the object are taken
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from the corresponding effective IDs of the calling process,
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and the object's
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permission bits are set according to the low-order 9 bits of
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<I>mode</I>,
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except that those bits set in the process file mode
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creation mask (see
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<B><A HREF="/cgi-bin/man/man2html?2+umask">umask</A></B>(2))
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are cleared for the new object.
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A set of macro constants which can be used to define
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<I>mode</I>
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is listed in
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<B><A HREF="/cgi-bin/man/man2html?2+open">open</A></B>(2).
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(Symbolic definitions of these constants can be obtained by including
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<I><<A HREF="file:///usr/include/sys/stat.h">sys/stat.h</A>></I>.)
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<DT id="4"><DD>
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A new shared memory object initially has zero length---the size of the
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object can be set using
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<B><A HREF="/cgi-bin/man/man2html?2+ftruncate">ftruncate</A></B>(2).
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The newly allocated bytes of a shared memory
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object are automatically initialized to 0.
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<DT id="5"><B>O_EXCL</B>
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<DD>
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If
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<B>O_CREAT</B>
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was also specified, and a shared memory object with the given
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<I>name</I>
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already exists, return an error.
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The check for the existence of the object, and its creation if it
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does not exist, are performed atomically.
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<DT id="6"><B>O_TRUNC</B>
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<DD>
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If the shared memory object already exists, truncate it to zero bytes.
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</DL>
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<P>
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Definitions of these flag values can be obtained by including
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<I><<A HREF="file:///usr/include/fcntl.h">fcntl.h</A>></I>.
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<P>
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On successful completion
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<B>shm_open</B>()
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returns a new file descriptor referring to the shared memory object.
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This file descriptor is guaranteed to be the lowest-numbered file descriptor
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not previously opened within the process.
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The
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<B>FD_CLOEXEC</B>
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flag (see
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<B><A HREF="/cgi-bin/man/man2html?2+fcntl">fcntl</A></B>(2))
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is set for the file descriptor.
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<P>
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The file descriptor is normally used in subsequent calls
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to
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<B><A HREF="/cgi-bin/man/man2html?2+ftruncate">ftruncate</A></B>(2)
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(for a newly created object) and
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<B><A HREF="/cgi-bin/man/man2html?2+mmap">mmap</A></B>(2).
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After a call to
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<B><A HREF="/cgi-bin/man/man2html?2+mmap">mmap</A></B>(2)
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the file descriptor may be closed without affecting the memory mapping.
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<P>
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The operation
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of
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<B>shm_unlink</B>()
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is analogous to
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<B><A HREF="/cgi-bin/man/man2html?2+unlink">unlink</A></B>(2):
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it removes a shared memory object name, and, once all processes
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have unmapped the object, de-allocates and
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destroys the contents of the associated memory region.
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After a successful
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<B>shm_unlink</B>(),
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attempts to
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<B>shm_open</B>()
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an object with the same
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<I>name</I>
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fail (unless
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<B>O_CREAT</B>
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was specified, in which case a new, distinct object is created).
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<A NAME="lbAE"> </A>
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<H2>RETURN VALUE</H2>
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On success,
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<B>shm_open</B>()
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returns a nonnegative file descriptor.
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On failure,
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<B>shm_open</B>()
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returns -1.
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<B>shm_unlink</B>()
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returns 0 on success, or -1 on error.
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<A NAME="lbAF"> </A>
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<H2>ERRORS</H2>
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On failure,
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<I>errno</I>
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is set to indicate the cause of the error.
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Values which may appear in
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<I>errno</I>
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include the following:
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<DL COMPACT>
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<DT id="7"><B>EACCES</B>
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<DD>
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Permission to
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<B>shm_unlink</B>()
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the shared memory object was denied.
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<DT id="8"><B>EACCES</B>
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<DD>
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Permission was denied to
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<B>shm_open</B>()
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<I>name</I>
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in the specified
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<I>mode</I>,
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or
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<B>O_TRUNC</B>
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was specified and the caller does not have write permission on the object.
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<DT id="9"><B>EEXIST</B>
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<DD>
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Both
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<B>O_CREAT</B>
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and
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<B>O_EXCL</B>
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were specified to
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<B>shm_open</B>()
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and the shared memory object specified by
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<I>name</I>
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already exists.
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<DT id="10"><B>EINVAL</B>
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<DD>
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The
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<I>name</I>
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argument to
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<B>shm_open</B>()
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was invalid.
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<DT id="11"><B>EMFILE</B>
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<DD>
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The per-process limit on the number of open file descriptors has been reached.
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<DT id="12"><B>ENAMETOOLONG</B>
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<DD>
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The length of
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<I>name</I>
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exceeds
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<B>PATH_MAX</B>.
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<DT id="13"><B>ENFILE</B>
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<DD>
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The system-wide limit on the total number of open files has been reached.
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<DT id="14"><B>ENOENT</B>
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<DD>
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An attempt was made to
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<B>shm_open</B>()
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a
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<I>name</I>
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that did not exist, and
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<B>O_CREAT</B>
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was not specified.
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<DT id="15"><B>ENOENT</B>
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<DD>
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An attempt was to made to
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<B>shm_unlink</B>()
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a
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<I>name</I>
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that does not exist.
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</DL>
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<A NAME="lbAG"> </A>
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<H2>VERSIONS</H2>
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These functions are provided in glibc 2.2 and later.
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<A NAME="lbAH"> </A>
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<H2>ATTRIBUTES</H2>
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For an explanation of the terms used in this section, see
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<B><A HREF="/cgi-bin/man/man2html?7+attributes">attributes</A></B>(7).
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<TABLE BORDER>
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<TR VALIGN=top><TD><B>Interface</B></TD><TD><B>Attribute</B></TD><TD><B>Value</B><BR></TD></TR>
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<TR VALIGN=top><TD>
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<B>shm_open</B>(),
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<B>shm_unlink</B>()
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</TD><TD>Thread safety</TD><TD>MT-Safe locale<BR></TD></TR>
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</TABLE>
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<P>
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<A NAME="lbAI"> </A>
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<H2>CONFORMING TO</H2>
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POSIX.1-2001, POSIX.1-2008.
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<P>
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POSIX.1-2001 says that the group ownership of a newly created shared
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memory object is set to either the calling process's effective group ID
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or "a system default group ID".
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POSIX.1-2008 says that the group ownership
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may be set to either the calling process's effective group ID
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or, if the object is visible in the filesystem,
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the group ID of the parent directory.
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<A NAME="lbAJ"> </A>
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<H2>NOTES</H2>
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<P>
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POSIX leaves the behavior of the combination of
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<B>O_RDONLY</B>
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and
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<B>O_TRUNC</B>
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unspecified.
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On Linux, this will successfully truncate an existing
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shared memory object---this may not be so on other UNIX systems.
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<P>
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The POSIX shared memory object implementation on Linux makes use
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of a dedicated
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<B><A HREF="/cgi-bin/man/man2html?5+tmpfs">tmpfs</A></B>(5)
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filesystem that is normally mounted under
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<I>/dev/shm</I>.
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<A NAME="lbAK"> </A>
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<H2>SEE ALSO</H2>
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<B><A HREF="/cgi-bin/man/man2html?2+close">close</A></B>(2),
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<B><A HREF="/cgi-bin/man/man2html?2+fchmod">fchmod</A></B>(2),
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<B><A HREF="/cgi-bin/man/man2html?2+fchown">fchown</A></B>(2),
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<B><A HREF="/cgi-bin/man/man2html?2+fcntl">fcntl</A></B>(2),
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<B><A HREF="/cgi-bin/man/man2html?2+fstat">fstat</A></B>(2),
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<B><A HREF="/cgi-bin/man/man2html?2+ftruncate">ftruncate</A></B>(2),
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<B><A HREF="/cgi-bin/man/man2html?2+memfd_create">memfd_create</A></B>(2),
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<B><A HREF="/cgi-bin/man/man2html?2+mmap">mmap</A></B>(2),
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<B><A HREF="/cgi-bin/man/man2html?2+open">open</A></B>(2),
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<B><A HREF="/cgi-bin/man/man2html?2+umask">umask</A></B>(2),
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<B><A HREF="/cgi-bin/man/man2html?7+shm_overview">shm_overview</A></B>(7)
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<A NAME="lbAL"> </A>
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<H2>COLOPHON</H2>
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This page is part of release 5.05 of the Linux
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<I>man-pages</I>
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project.
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A description of the project,
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information about reporting bugs,
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and the latest version of this page,
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can be found at
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<A HREF="https://www.kernel.org/doc/man-pages/.">https://www.kernel.org/doc/man-pages/.</A>
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<P>
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<HR>
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<A NAME="index"> </A><H2>Index</H2>
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<DL>
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<DT id="16"><A HREF="#lbAB">NAME</A><DD>
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<DT id="17"><A HREF="#lbAC">SYNOPSIS</A><DD>
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<DT id="18"><A HREF="#lbAD">DESCRIPTION</A><DD>
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<DT id="19"><A HREF="#lbAE">RETURN VALUE</A><DD>
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<DT id="20"><A HREF="#lbAF">ERRORS</A><DD>
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<DT id="21"><A HREF="#lbAG">VERSIONS</A><DD>
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<DT id="22"><A HREF="#lbAH">ATTRIBUTES</A><DD>
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<DT id="23"><A HREF="#lbAI">CONFORMING TO</A><DD>
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<DT id="24"><A HREF="#lbAJ">NOTES</A><DD>
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<DT id="25"><A HREF="#lbAK">SEE ALSO</A><DD>
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<DT id="26"><A HREF="#lbAL">COLOPHON</A><DD>
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</DL>
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<HR>
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This document was created by
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<A HREF="/cgi-bin/man/man2html">man2html</A>,
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using the manual pages.<BR>
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Time: 00:05:56 GMT, March 31, 2021
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