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Full Discussion: Shared memory between two c program
Top Forums Programming Shared memory between two c program Post 302582619 by DreamWarrior on Friday 16th of December 2011 02:26:25 PM
Old 12-16-2011
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 LINUX

umask

UMASK(2)						     Linux Programmer's Manual							  UMASK(2)

NAME

       umask - set file mode creation mask

SYNOPSIS

       #include <sys/types.h>
       #include <sys/stat.h>

       mode_t umask(mode_t mask);

DESCRIPTION

       umask()	sets  the calling process's file mode creation mask (umask) to mask & 0777 (i.e., only the file permission bits of mask are used),
       and returns the previous value of the mask.

       The umask is used by open(2), mkdir(2), and other system calls that create files to modify the permissions placed on newly created files or
       directories.  Specifically, permissions in the umask are turned off from the mode argument to open(2) and mkdir(2).

       The constants that should be used to specify mask are described under stat(2).

       The  typical default value for the process umask is S_IWGRP | S_IWOTH (octal 022).  In the usual case where the mode argument to open(2) is
       specified as:

	   S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP | S_IROTH | S_IWOTH

       (octal 0666) when creating a new file, the permissions on the resulting file will be:

	   S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH

       (because 0666 & ~022 = 0644; i.e., rw-r--r--).

RETURN VALUE

       This system call always succeeds and the previous value of the mask is returned.

CONFORMING TO

       SVr4, 4.3BSD, POSIX.1-2001.

NOTES

       A child process created via fork(2) inherits its parent's umask.  The umask is left unchanged by execve(2).

       The umask setting also affects the permissions assigned to POSIX IPC objects (mq_open(3), sem_open(3), shm_open(3)), FIFOs (mkfifo(3)), and
       Unix  domain  sockets (unix(7)) created by the process.	The umask does not affect the permissions assigned to System V IPC objects created
       by the process (using msgget(2), semget(2), shmget(2)).

SEE ALSO

       chmod(2), mkdir(2), open(2), stat(2)

COLOPHON

       This page is part of release 3.27 of the Linux man-pages project.  A description of the project, and information about reporting bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.

Linux								    2008-01-09								  UMASK(2)
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