95 lines
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95 lines
3.9 KiB
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<TITLE>Kernel shmid_ds structure</TITLE>
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<A NAME="tex2html1221" HREF="node69.html"><IMG WIDTH=37 HEIGHT=24 ALIGN=BOTTOM ALT="next" SRC="next_motif.gif"></A> <A NAME="tex2html1219" HREF="node67.html"><IMG WIDTH=26 HEIGHT=24 ALIGN=BOTTOM ALT="up" SRC="up_motif.gif"></A> <A NAME="tex2html1215" HREF="node67.html"><IMG WIDTH=63 HEIGHT=24 ALIGN=BOTTOM ALT="previous" SRC="previous_motif.gif"></A> <A NAME="tex2html1223" HREF="node1.html"><IMG WIDTH=65 HEIGHT=24 ALIGN=BOTTOM ALT="contents" SRC="contents_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME="tex2html1222" HREF="node69.html">SYSTEM CALL: shmget()</A>
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<B>Up:</B> <A NAME="tex2html1220" HREF="node67.html">Internal and User Data </A>
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<B> Previous:</B> <A NAME="tex2html1216" HREF="node67.html">Internal and User Data </A>
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<BR> <P>
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<H4><A NAME="SECTION00744210000000000000">Kernel <TT>shmid_ds</TT> structure</A></H4>
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<P>
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As with message queues and semaphore sets, the kernel maintains a special internal data structure for
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each shared memory segment which exists within its addressing space. This structure is of type
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<TT>shmid_ds</TT>, and is defined in <TT>linux/shm.h</TT> as follows:
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<P>
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<P>
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<HR><PRE> /* One shmid data structure for each shared memory segment in the system. */
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struct shmid_ds {
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struct ipc_perm shm_perm; /* operation perms */
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int shm_segsz; /* size of segment (bytes) */
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time_t shm_atime; /* last attach time */
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time_t shm_dtime; /* last detach time */
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time_t shm_ctime; /* last change time */
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unsigned short shm_cpid; /* pid of creator */
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unsigned short shm_lpid; /* pid of last operator */
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short shm_nattch; /* no. of current attaches */
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/* the following are private */
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unsigned short shm_npages; /* size of segment (pages) */
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unsigned long *shm_pages; /* array of ptrs to frames -> SHMMAX */
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struct vm_area_struct *attaches; /* descriptors for attaches */
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};</PRE>
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<HR>Operations on this structure are performed by a special system call, and
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should not be tinkered with directly. Here are descriptions of the more pertinent fields:
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<P>
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<DL ><DT><STRONG><TT>shm_perm</TT></STRONG>
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<DD>
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<P>
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This is an instance of the <TT>ipc_perm</TT> structure, which is defined
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for us in <TT>linux/ipc.h</TT>. This holds the permission information for
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the segment, including the access permissions, and information
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about the creator of the segment (uid, etc).
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<P>
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<DT><STRONG><TT>shm_segsz</TT></STRONG>
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<DD>
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<P>
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Size of the segment (measured in bytes).
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<P>
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<DT><STRONG><TT>shm_atime</TT></STRONG>
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<DD>
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<P>
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Time the last process attached the segment.
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<P>
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<DT><STRONG><TT>shm_dtime</TT></STRONG>
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<DD>
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<P>
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Time the last process detached the segment.
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<P>
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<DT><STRONG><TT>shm_ctime</TT></STRONG>
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<DD>
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<P>
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Time of the last change to this structure (mode change, etc).
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<P>
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<DT><STRONG><TT>shm_cpid</TT></STRONG>
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<DD>
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<P>
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The PID of the creating process.
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<P>
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<DT><STRONG><TT>shm_lpid</TT></STRONG>
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<DD>
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<P>
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The PID of the last process to operate on the segment.
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<P>
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<DT><STRONG><TT>shm_nattch</TT></STRONG>
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<DD>
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<P>
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Number of processes currently attached to the segment.
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<P>
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</DL><BR> <HR>
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<P><ADDRESS>
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<I>Converted on: <BR>
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Fri Mar 29 14:43:04 EST 1996</I>
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</ADDRESS>
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