Shared memory between two c program Post: 302582619

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Top Forums Programming Shared memory between two c program Post 302582619 by DreamWarrior on Friday 16th of December 2011 02:26:25 PM

12-16-2011

Registered User

Did you change the sizeof? You don't even use buffer in the first program, so you can get rid of that. Then when you shmget pass (sizeof(int) * 2) as the size argument.

Also, to the third argument, shmflg, you have to OR in IPC_CREAT. So the call should be:

Code:

key_t shmkey=0x200;
int id;
..,
id = shmget(shmkey, sizeof(int) * 2, 0666 | IPC_CREAT);

Without IPC_CREAT, it's likely that shmget is returning -1 which, since you're not doing error checking, is then passed to shmat which is returning NULL. When you attempt to reference through the NULL pointer you are seg faulting.

I suggest reading the man pages for shmget!

edit: I also suggest not using 0666 and using the proper mode flags; but that's your prerogative. For example:

Code:

#include <sys/stat.h>
...
mode_t mode = S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP | S_IROTH | S_IWOTH;
...
id = shmget(shmkey, sizeof(int) * 2, mode | IPC_CREAT);

edit2: haha, beaten by hours, didn't even notice there was a second page.

Last edited by DreamWarrior; 12-16-2011 at 03:33 PM..

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LEARN ABOUT DEBIAN

shmidcat

SHMIDCAT(1)						     Shared Memory Trampoline						       SHMIDCAT(1)

NAME

       shmidcat - Copies stdin/file to a shared memory block for gtkwave(1)

SYNTAX

       shmidcat [VCDFILE]

DESCRIPTION

       Copies  either  the  file specified at the command line or stdin (if no file specified) line by line to a shared memory block.  stdout will
       contain a shared memory ID which should be passed on to gtkwave(1).

EXAMPLES

       To run this program the standard way type:

       cat whatever.vcd | shmidcat
	      The shared memory ID is emitted to stdout.

       shmidcat whatever.vcd | gtkwave -v -I whatever.sav
	      GTKWave directly grabs the ID from stdin.

LIMITATIONS

       This program is mainly for illustrative and testing purposes only.  Its primary use is for people who wish to write interactive VCD dumpers
       for gtkwave(1) as its source code may be examined, particularly the emit_string() function.  It can also be used to test if an existing VCD
       file will load properly in interactive mode.  Note that it can also be used to redirect VCD files which are written into  a  pipe  to  gtk-
       wave(1) in a non-blocking fashion.

AUTHORS

       Anthony Bybell <bybell@rocketmail.com>

SEE ALSO

       gtkwave(1)

Anthony Bybell							       3.0.8							       SHMIDCAT(1)

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