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:
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:
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..
Dear Reader,
Is is necessary to attach / dettach the shared memory segments for write operations , if more than one program is accessing same shared memory segments..
I have used semaphore mutex and still I'm getting segmentation fault when I write to the segment when other program is already... (1 Reply)
I am running HP-UX B.11.11.
I'm increasing a parameter for a database engine so that it uses more memory to buffer the disk drive (to speed up performance). I have over 5GB of memory not being used.
But when I try to start the DB with the increased buffer parameter I get told.
"Not... (1 Reply)
Hi all :confused: ,
I am new to unix.I have been asked to implement shared memory in user's mode.What does this mean?What is the difference it makes in kernel mode and in users mode?What are the advantages of this impemenation(user's mode)?
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#include <sys/types.h>
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hi,
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I need to create a shared library to access an in memory DB. The DB is not huge, but big enough to make it cumbersome to carry around in every single process using the shared library. Luckily, it is pretty static information, so I don't need to worry much about synchronizing the data between... (12 Replies)
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Obviously, I'll have to worry about mutual exclusion. (0 Replies)
Hello.
I am new to this forum and I would like to ask for advice about low level POSIX programming.
I have to implement a POSIX compliant C shared library.
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Discussion started by: iamjag
5 Replies
LEARN ABOUT OSX
shmget
SHMGET(2) BSD System Calls Manual SHMGET(2)NAME
shmget -- get shared memory area identifier
SYNOPSIS
#include <sys/shm.h>
int
shmget(key_t key, size_t size, int shmflg);
DESCRIPTION
shmget() returns the shared memory identifier associated with the key key.
A shared memory segment is created if either key is equal to IPC_PRIVATE, or key does not have a shared memory segment identifier associated
with it, and the IPC_CREAT bit is set in shmflg.
If a new shared memory segment is created, the data structure associated with it (the shmid_ds structure, see shmctl(2)) is initialized as
follows:
o shm_perm.cuid and shm_perm.uid are set to the effective uid of the calling process.
o shm_perm.gid and shm_perm.cgid are set to the effective gid of the calling process.
o shm_perm.mode is set to the lower 9 bits of shmflg.
o shm_lpid, shm_nattch, shm_atime, and shm_dtime are set to 0
o shm_ctime is set to the current time.
o shm_segsz is set to the value of size.
o The ftok(3) function may be used to generate a key from a pathname.
RETURN VALUES
Upon successful completion a positive shared memory segment identifier is returned. Otherwise, -1 is returned and the global variable errno
is set to indicate the error.
ERRORS
The shmget() system call will fail if:
[EACCES] A shared memory segment is already associated with key and the caller has no permission to access it.
[EEXIST] Both IPC_CREAT and IPC_EXCL are set in shmflg, and a shared memory segment is already associated with key.
[EINVAL] No shared memory segment is to be created, and a shared memory segment exists for key, but the size of the segment associ-
ated with it is less than size, which is non-zero.
[ENOENT] IPC_CREAT was not set in shmflg and no shared memory segment associated with key was found.
[ENOMEM] There is not enough memory left to created a shared memory segment of the requested size.
[ENOSPC] A new shared memory identifier could not be created because the system limit for the number of shared memory identifiers
has been reached.
LEGACY SYNOPSIS
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>
int
shmget(key_t key, int size, int shmflg);
All of these include files are necessary. The type of size has changed.
SEE ALSO ftok(3), shmat(2), shmctl(2), shmdt(2), compat(5)BSD August 17, 1995 BSD