<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN">
<!--Converted with LaTeX2HTML 96.1-c (Feb 29, 1996) by Nikos Drakos (nikos@cbl.leeds.ac.uk), CBLU, University of Leeds -->
<HTML>
<HEAD>
<TITLE>Kernel shmid_ds structure</TITLE>
<META NAME="description" CONTENT="Kernel shmid_ds structure">
<META NAME="keywords" CONTENT="lpg">
<META NAME="resource-type" CONTENT="document">
<META NAME="distribution" CONTENT="global">
<LINK REL=STYLESHEET HREF="lpg.css">
</HEAD>
<BODY LANG="EN">
<A NAME="tex2html1221" HREF="node69.html"><IMG WIDTH=37 HEIGHT=24 ALIGN=BOTTOM ALT="next" SRC="/icons//next_motif.gif"></A> <A NAME="tex2html1219" HREF="node67.html"><IMG WIDTH=26 HEIGHT=24 ALIGN=BOTTOM ALT="up" SRC="/icons//up_motif.gif"></A> <A NAME="tex2html1215" HREF="node67.html"><IMG WIDTH=63 HEIGHT=24 ALIGN=BOTTOM ALT="previous" SRC="/icons//previous_motif.gif"></A> <A NAME="tex2html1223" HREF="node1.html"><IMG WIDTH=65 HEIGHT=24 ALIGN=BOTTOM ALT="contents" SRC="/icons//contents_motif.gif"></A> <BR>
<B> Next:</B> <A NAME="tex2html1222" HREF="node69.html">SYSTEM CALL: shmget()</A>
<B>Up:</B> <A NAME="tex2html1220" HREF="node67.html">Internal and User Data </A>
<B> Previous:</B> <A NAME="tex2html1216" HREF="node67.html">Internal and User Data </A>
<BR> <P>
<H4><A NAME="SECTION00744210000000000000">Kernel <TT>shmid_ds</TT> structure</A></H4>
<P>
As with message queues and semaphore sets, the kernel maintains a special internal data structure for
each shared memory segment which exists within its addressing space. This structure is of type
<TT>shmid_ds</TT>, and is defined in <TT>linux/shm.h</TT> as follows:
<P>
<P>
<HR><PRE> /* One shmid data structure for each shared memory segment in the system. */
struct shmid_ds {
struct ipc_perm shm_perm; /* operation perms */
int shm_segsz; /* size of segment (bytes) */
time_t shm_atime; /* last attach time */
time_t shm_dtime; /* last detach time */
time_t shm_ctime; /* last change time */
unsigned short shm_cpid; /* pid of creator */
unsigned short shm_lpid; /* pid of last operator */
short shm_nattch; /* no. of current attaches */
/* the following are private */
unsigned short shm_npages; /* size of segment (pages) */
unsigned long *shm_pages; /* array of ptrs to frames -> SHMMAX */
struct vm_area_struct *attaches; /* descriptors for attaches */
};</PRE>
<HR>Operations on this structure are performed by a special system call, and
should not be tinkered with directly. Here are descriptions of the more pertinent fields:
<P>
<DL ><DT><STRONG><TT>shm_perm</TT></STRONG>
<DD>
<P>
This is an instance of the <TT>ipc_perm</TT> structure, which is defined
for us in <TT>linux/ipc.h</TT>. This holds the permission information for
the segment, including the access permissions, and information
about the creator of the segment (uid, etc).
<P>
<DT><STRONG><TT>shm_segsz</TT></STRONG>
<DD>
<P>
Size of the segment (measured in bytes).
<P>
<DT><STRONG><TT>shm_atime</TT></STRONG>
<DD>
<P>
Time the last process attached the segment.
<P>
<DT><STRONG><TT>shm_dtime</TT></STRONG>
<DD>
<P>
Time the last process detached the segment.
<P>
<DT><STRONG><TT>shm_ctime</TT></STRONG>
<DD>
<P>
Time of the last change to this structure (mode change, etc).
<P>
<DT><STRONG><TT>shm_cpid</TT></STRONG>
<DD>
<P>
The PID of the creating process.
<P>
<DT><STRONG><TT>shm_lpid</TT></STRONG>
<DD>
<P>
The PID of the last process to operate on the segment.
<P>
<DT><STRONG><TT>shm_nattch</TT></STRONG>
<DD>
<P>
Number of processes currently attached to the segment.
<P>
</DL><BR> <HR>
<P><ADDRESS>
<I>Converted on: <BR>
Fri Mar 29 14:43:04 EST 1996</I>
</ADDRESS>
</BODY>
</HTML>
