Advanced Programming in the UNIX® Environment

ptg10805159

Section 15.6 XSI IPC 559

Permission Bit

user-read 0400

user-write (alter) 0200

group-read 0040

group-write (alter) 0020

other-read 0004

other-write (alter) 0002

Figure 15.24XSI IPC permissions

Some implementations define symbolic constants to represent each permission, but

these constants arenot standardized by the Single UNIX Specification.

15.6.3 Configuration Limits

All three forms of XSI IPC have built-in limits that we may encounter.Most of these

limits can be changed by reconfiguring the kernel. We describe the limits when we

describe each of the three forms of IPC.

Each platform provides its own way to report and modify a particular limit. FreeBSD 8.0,

Linux 3.2.0, and Mac OS X 10.6.8 provide thesysctlcommand to view and modify kernel

configuration parameters. On Solaris 10, changes to kernel IPC limits aremade with the

prctlcommand.

On Linux, you can display the IPC-related limits by runningipcs -l.OnFreeBSD and Mac

OS X, the equivalent command isipcs -T.OnSolaris, you can discover the tunable

parameters by runningsysdef -i.

15.6.4 Advantages and Disadvantages

Afundamental problem with XSI IPC is that the IPC structures aresystemwide and do

not have a reference count. For example, if we create a message queue, place some

messages on the queue, and then terminate, the message queue and its contents arenot

deleted. Theyremain in the system until specifically read or deleted by some process

callingmsgrcvormsgctl, by someone executing theipcrm( 1 )command, or by the

system being rebooted. Comparethis with a pipe, which is completely removed when

the last process to reference it terminates.With a FIFO, although the name stays in the

file system until explicitly removed, any data left in a FIFO is removed when the last

process to reference the FIFO terminates.

Another problem with XSI IPC is that these IPC structures arenot known by names

in the file system.We can’t access them and modify their properties with the functions

we described in Chapters 3 and 4. Almost a dozen new system calls (msgget,semop,

shmat,and so on) wereadded to the kernel to support these IPC objects.We can’t see

the IPC objects with anlscommand, we can’t remove them with thermcommand, and

we can’t change their permissions with thechmod command. Instead, two new

commands —ipcs( 1 )andipcrm( 1 )—wereadded.