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Full Discussion: Shared memory between two c program
Top Forums Programming Shared memory between two c program Post 302582372 by tafazzi87 on Thursday 15th of December 2011 05:06:56 PM
Old 12-15-2011
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 CENTOS

paps

PAPS(1) 						      General Commands Manual							   PAPS(1)

NAME

       paps - UTF-8 to PostScript converter using Pango

SYNOPSIS

       paps [options] files...

DESCRIPTION

       paps reads a UTF-8 encoded file and generates a PostScript language rendering of the file. The rendering is done by creating outline curves
       through the pango ft2 backend.

OPTIONS

       These programs follow the usual GNU command line syntax, with long options starting with  two  dashes  (`-').   A  summary  of  options	is
       included below.

       --landscape
	      Landscape output. Default is portrait.

       --columns=cl
	      Number of columns output. Default is 1.

       --font=desc
	      Set the font description. Default is Monospace 12.

       --rtl  Do rtl layout.

       --paper ps
	      Choose paper size. Known paper sizes are legal, letter, a4. Default is A4.

       --bottom-margin=bm
	      Set bottom margin in postscript points (1/72 inch). Default is 36.

       --top-margin=tm
	      Set top margin. Default is 36.

       --left-margin=lm
	      Set left margin. Default is 36.

       --right-margin=rm
	      Set right margin. Default is 36.

       --help Show summary of options.

       --header
	      Draw page header for each page.

       --markup
	      Interpret the text as pango markup.

       --encoding=ENCODING
	      Assume the documentation encoding is ENCODING.

       --lpi  Set the lines per inch. This determines the line spacing.

       --cpi  Set the characters per inch. This is an alternative method of specifying the font size.

       --stretch-chars
	      Indicates  that  characters  should be stretched in the y-direction to fill up their vertical space. This is similar to the texttops
	      behaviour.

AUTHOR

       paps was written by Dov Grobgeld <dov.grobgeld@gmail.com>.

       This manual page was written by Lior Kaplan <kaplan@debian.org>, for the Debian project (but may be used by others).

								  April  17, 2006							   PAPS(1)
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