Shared memory between two c program Post: 302582372

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Top Forums Programming Shared memory between two c program Post 302582372 by tafazzi87 on Thursday 15th of December 2011 05:06:56 PM

12-15-2011

Registered User

Quote:

Originally Posted by DreamWarrior

Yes, you either need a key or to pass the segment's ID. IPC_PRIVATE is meant to be used between two programs that can pass the ID, either because one forked the other and the ID is resident, or because they passed it in some other way. So, in IPC_PRIVATE world, you'd shmget in one and then pass the shmid to the other to shmat.

If you use the command "ipcs -m" you'll see all the shared memory segments. The KEY for IPC_PRIVATE (usually) is 0xffffffff, but you'll see a bunch of them. Each time you run this program you'll be creating another one. The ID for each will be different. Ultimately, it's the ID that determines which segment you attach to.

Think of shmget as a way to both create and lookup that ID. If you don't pass the same ID to each shmat call, you won't be looking at the same segment. Make sense?

Also, you probably mean sizeof(buffer) not sizeof(buffer[1]). The first will insure the segment can store the whole array, your way provides the segment only enough space to hold one element. Of course, this will cause an overflow.

edit: P.S. since you're never asking the shared memory segment be removed, it'll linger forever. You'll want to use ipcrm to get rid of it. Of course, you could also shmctl with the IPC_RMID command to ask it be removed. It'll hang around until all applications currently attached to it have detached. The kernel will detach the application when it is killed as well, so while it's good practice to shmdt, if you application dies in the middle the kernel will do it. Of course, on some O/Ses I've seen that fail and segments get "stuck" and the dumb machine needs a reboot to clean them up, lol.

thanks a lot for your exhaustive answer, i will try soon

---------- Post updated at 05:06 PM ---------- Previous update was at 04:26 PM ----------

so, for example i have to declare :

Code:

key_t shmkey=0x200;

and then use that for shmget in both programs?

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LEARN ABOUT DEBIAN

ticker

NAME(1) 						      General Commands Manual							   NAME(1)

NAME

       ticker - scroll messages across the screen

SYNOPSIS

       ticker [options] [message]

DESCRIPTION

       ticker  is  a  program  that  continually scrolls a given message across the screen.  There is also an interface to allow other programs to
       change the message.

KEYS

       +, [up arrow]
	      Increase scroll speed.

       -, [down arrw]
	      Decrease scroll speed.

       [space]
	      Pause. Press any key to unpause.

OPTIONS

       -h, --help
	      Show summary of options.

       -u, --upper
	      Scroll text on the top line of the screen. (Default)

       -l, --lower
	      Scroll text on the bottom line of the screen.

       -fcolor, --foreground=color
	      Use the specified color as the forground color of the text that is scrolled.  The colors that may be used are:
	       black	       gray
	       red	       brightred
	       green	       brightgreen
	       brown	       yellow
	       blue	       brightblue
	       magenta	       brightmagenta
	       cyan	       brightcyan
	       lightgray       white

       -bcolor, --background=color
	      Use the specified color as the background color of the text that is scrolled. On most terminals, the background color  can  only	be
	      one of the colors listed in the first column above.

       -dsecs, --delay=secs
	      Number  of  seconds  delay  between updates of the display. This controls how fast the text scrolls. You may use decimals to specify
	      faster scroll speeds.  The default delay is 1 second; I find 0.1 more pleasing.

       -snum, --sysv=num
	      Read messages to display from the sysv shared memory segment with an id of num. This is only for use by other programs that need	to
	      be able to change the text ticker displays.

       -Snum, --size=num
	      Size of the shared memory segment to read, when using shared memory communication with another program. Default is 80 characters.

       -csecs, --check=secs
	      Minimum time between checks of the shared memory segment for a new message.  Default is every second. It may in fact check consider-
	      ably less often, as it only checks for a new message once per time that the current message scrolls around the screen.

       message
	      The message to scroll. Required unless -s is used, in which case it is optional.

NOTES

       To use the other 23 or so lines of your screen for something useful while the ticker is running, you might want to use splitvt(1)

AUTHOR

       Joey Hess <joey@kitenet.net>

																	   NAME(1)

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