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<A NAME="PAGENUM-1145"><P>Page 1145</P></A>
<P>
The mode member of the ipc_perm structure defines, with its lower nine bits, the access permissions to the resource for
a process executing an ipc system call. The permissions are interpreted as follows:
</P>
<TABLE>
<TR><TD>
0400
</TD><TD>
Read by user.
</TD></TR><TR><TD>
0200
</TD><TD>
Write by user.
</TD></TR><TR><TD>
0040
</TD><TD>
Read by group.
</TD></TR><TR><TD>
0020
</TD><TD>
Write by group.
</TD></TR><TR><TD>
0004
</TD><TD>
Read by others.
</TD></TR><TR><TD>
0002
</TD><TD>
Write by others.
</TD></TR></TABLE>
<P>Bits 0100, 0010, and 0001 (the execute bits) are unused by the system. Furthermore "write" effectively means "alter" for
a semaphore set.
</P>
<P>The same system header file defines also the following symbolic constants:
</P>
<TABLE>
<TR><TD>
IPC_CREAT
</TD><TD>
Create entry if key doesn't exists.
</TD></TR><TR><TD>
IPC_EXCL
</TD><TD>
Fail if key exists.
</TD></TR><TR><TD>
IPC_NOWAIT
</TD><TD>
Error if request must wait.
</TD></TR><TR><TD>
IPC_PRIVATE
</TD><TD>
Private key.
</TD></TR><TR><TD>
IPC_RMID
</TD><TD>
Remove resource.
</TD></TR><TR><TD>
IPC_SET
</TD><TD>
Set resource options.
</TD></TR><TR><TD>
IPC_STAT
</TD><TD>
Get resource options.
</TD></TR></TABLE>
<P>Note that IPC_PRIVATE is a key_t type, whereas all the others symbolic constants are flag fields ORable into an
int type variable.
</P>
<P><B>
MESSAGE QUEUES
</B></P>
<P>A message queue is uniquely identified by a positive integer (its
msqid) and has an associated data structure of type
struct msquid_ds, defined in <sys/msg.h>, containing the following members:
</P>
<!-- CODE //-->
<PRE>
struct ipc_perm msg_perm;
ushort msg_qnum; /* no of messages on queue */
ushort msg_qbytes; /* bytes max on a queue */
ushort msg_lspid; /* pid of last msgsnd call */
ushort msg_lrpid; /* pid of last msgrcv call */
time_t msg_stime; /* last msgsnd time */
time_t msg_rtime; /* last msgrcv time */
time_t msg_ctime; /* last change time */
</PRE>
<!-- END CODE //-->
<TABLE>
<TR><TD>
msg_perm
</TD><TD>
ipc_perm structure that specifies the access permissions on the message queue.
</TD></TR><TR><TD>
msg_qnum
</TD><TD>
Number of messages currently on the message queue.
</TD></TR><TR><TD>
msg_qbytes
</TD><TD>
Maximum number of bytes of message text allowed on the message queue.
</TD></TR><TR><TD>
msg_lspid
</TD><TD>
ID of the process that performed the last
msgsnd system call.
</TD></TR><TR><TD>
msg_lrpid
</TD><TD>
ID of the process that performed the last
msgrcv system call.
</TD></TR><TR><TD>
msg_stime
</TD><TD>
Time of the last msgsnd system call.
</TD></TR><TR><TD>
msg_rtime
</TD><TD>
Time of the last msgcv system call.
</TD></TR><TR><TD>
msg_ctime
</TD><TD>
Time of the last system call that changed a member of the
msqid_ds structure.
</TD></TR></TABLE>
<P><B>
SEMAPHORE SETS
</B></P>
<P>A semaphore set is uniquely identified by a positive integer (its
semid) and has an associated data structure of type
struct semid_ds, defined in <sys/sem.h>, containing the following members:
</P>
<!-- CODE SNIP //-->
<PRE>
struct ipc_perm sem_perm;
time_t sem_otime; /* last operation time */
</PRE>
<!-- END CODE SNIP //-->
<A NAME="PAGENUM-1146"><P>Page 1146</P></A>
<!-- CODE SNIP //-->
<PRE>
time_t sem_ctime; /* last change time */
ushort sem_nsems; /* count of sems in set */
</PRE>
<!-- END CODE SNIP //-->
<TABLE>
<TR><TD>
sem_perm
</TD><TD>
ipc_perm structure that specifies the access permissions on the semaphore set.
</TD></TR><TR><TD>
sem_otime
</TD><TD>
Time of last semop system call.
</TD></TR><TR><TD>
sem_ctime
</TD><TD>
Time of last semctl system call that changed a member of the above structure or of
one semaphore belonging to the set.
</TD></TR><TR><TD>
sem_nsems
</TD><TD>
Number of semaphores in the set. Each semaphore of the set is referenced by a
non-negative integer ranging from 0 to sem_nsems_1.
</TD></TR></TABLE>
<P>A semaphore is a data structure of type struct
sem containing the following members:
</P>
<!-- CODE //-->
<PRE>
ushort semval; /* semaphore value */
short sempid; /* pid for last operation */
ushort semncnt; /* no. of awaiting semval to increase */
ushort semzcnt; /* no. of awaiting semval = 0 */
</PRE>
<!-- END CODE //-->
<TABLE>
<TR><TD>
semval
</TD><TD>
Semaphore value: a non-negative integer.
</TD></TR><TR><TD>
sempid
</TD><TD>
ID of the last process that performed a semaphore operation on this semaphore.
</TD></TR><TR><TD>
semncnt
</TD><TD>
Number of processes suspended awaiting for
semval to increase.
</TD></TR><TR><TD>
semznt
</TD><TD>
Number of processes suspended awaiting for
semval to become zero.
</TD></TR></TABLE>
<P><B>
SHARED MEMORY SEGMENTS
</B></P>
<P>A shared memory segment is uniquely identified by a positive integer (its
shmid) and has an associated data structure of
type struct shmid_ds, defined in <sys/shm.h>, containing the following members:
</P>
<!-- CODE //-->
<PRE>
struct ipc_perm shm_perm;
int shm_segsz; /* size of segment */
ushort shm_cpid; /* pid of creator */
ushort shm_lpid; /* pid, last operation */
short shm_nattch; /* no. of current attaches */
time_t shm_atime; /* time of last attach */
time_t shm_dtime; /* time of last detach */
time_t shm_ctime; /* time of last change */
</PRE>
<!-- END CODE //-->
<TABLE>
<TR><TD>
shm_perm
</TD><TD>
ipc_perm structure that specifies the access permissions on the shared memory segment.
</TD></TR><TR><TD>
shm_segsz
</TD><TD>
Size in bytes of the shared memory segment.
</TD></TR><TR><TD>
shm_cpid
</TD><TD>
ID of the process that created the shared memory segment.
</TD></TR><TR><TD>
shm_lpid
</TD><TD>
ID of the last process that executed a
shmat or shmdt system call.
</TD></TR><TR><TD>
shm_nattch
</TD><TD>
Number of current alive attaches for this shared memory segment.
</TD></TR><TR><TD>
shm_atime
</TD><TD>
Time of the last shmat system call.
</TD></TR><TR><TD>
shm_dtime
</TD><TD>
Time of the last shmdt system call.
</TD></TR><TR><TD>
shm_ctime
</TD><TD>
Time of the last shmctl system call that changed
shmid_ds.
</TD></TR></TABLE>
<P><B>
SEE ALSO
</B></P>
<P>ftok(3), msgctl(2), msgget(2), msgrcv(2),
msgsnd(2), semctl(2), semget(2), semop(2), shmat(2),
shmctl(2), shmget(2), shmdt (2)
</P>
<P>Linux 0.99.13, 1 November 1993
</P>
<H3><A NAME="ch05_ 28">
issue
</A></H3>
<P>issue—Issue identification file.
</P>
<A NAME="PAGENUM-1147"><P>Page 1147</P></A>
<P><B>
DESCRIPTION
</B></P>
<P>The file /etc/issue is a text file that contains a message or system identification to be printed before the login prompt.
It may contain various @char and \char sequences if supported by
getty(1).
</P>
<P><B>
FILES
</B></P>
<P>/etc/issue</P>
<P><B>
SEE ALSO
</B></P>
<P>getty(1), motd(5)
</P>
<P>Linux, 24 July 1993
</P>
<H3><A NAME="ch05_ 29">
lilo.conf
</A></H3>
<P>lilo.conf—Configuration file for LILO.
</P>
<P><B>
DESCRIPTION
</B></P>
<P>This file, by default /etc/lilo.conf, is read by the boot loader installer LILO (see
lilo(8)).
</P>
<P>It might look as follows:
</P>
<!-- CODE //-->
<PRE>
boot = /dev/hda
delay = 40
compact
vga = normal
root = /dev/hda1
read-only
image = /zImage-1.5.99
label = try
image = /zImage-1.0.9
label = 1.0.9
image = /tamu/vmlinuz
label = tamu
root = /dev/hdb2
vga = ask
other = /dev/hda3
label = dos
table = /dev/hda
</PRE>
<!-- END CODE //-->
<P>This configuration file specifies that LILO uses the Master Boot Record on
/dev/hda. (For a discussion of the various ways to use LILO and the interaction with other operating systems, see
user.tex from the LILO documentation.)
<P>When booting, the boot loader will wait 4 seconds (40 deciseconds) for you to press Shift. If you don't, then the first
kernel image mentioned (/zImage-1.5.99, which you probably installed just 5 minutes ago) will be booted. If you do, the
boot loader will ask you which image to boot. In case you forgot the possible choices, press Tab (or ? if you have a U.S.
keyboard), and you will be presented with a menu. You now have the choice of booting this brand new kernel, an old trusted kernel,
or a kernel on another root file system (just in case you did something stupid on your usual root) or booting a
different operating system. There can be up to 16 images mentioned in
lilo.conf.
</P>
<P>As can be seen previously, a configuration file starts with a number of global options (the top six lines in the
example), followed by descriptions of the options for the various images. An option in an image description will override a
global option.
</P>
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