3) I'm a little messed up with shared memory. I mean, what I suppose about how it works is - (hope you'll understand my english-for-dummies-language-level) - like a "box" where I can put things. But if i try to put inside a pointer I'll get maaany problems. So the list I've putted in (with my method) are no longer behaving as a list, but more likely as a """""string""""" (watch for "" ).
example:
now i've a box made of 7 slots of x-dimension. once I attach memory
(...shmat..) X becomes of a true value, in my case Obj. then I have 7 slots of Obj-size each.
when i fill my box up, i fill slots individually moving through shared memory with sh++, and not with pointer to the next obj.
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)?
And also i would like to know why exactly shared... (0 Replies)
hi,
this is the problem: i want to swap a linked list between 4 processes (unrelated), is there any way i can do that just by sending a pointer to a structure?
//example
typedef struct node
{
int x;
char c;
struct node *next;
} node;
or i should send the items ( x,c ) by... (9 Replies)
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)
I am writing a shared library in Linux (but compatible with other UNIXes) and I want to allow multiple instances to share a piece of memory -- 1 byte is enough. What's the "best" way to do this? I want to optimize for speed and portability.
Obviously, I'll have to worry about mutual exclusion. (0 Replies)
Hi everbody, i have a problem with shared memory and child-processes in C (unix).
I have a server that do forks to create (N) child processes. This processes work with a shared "stuct" thanks to shared memory and a semaphore. The problem is when a child modify the shared memory and the others... (2 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.
A file will have some variables and the shared library will have some functions which need those variables.
There is one special... (5 Replies)
Discussion started by: iamjag
5 Replies
LEARN ABOUT ULTRIX
shmget
shmget(2) System Calls Manual shmget(2)Name
shmget - get shared memory segment
Syntax
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>
int shmget (key, size, shmflg)
key_t key;
int size, shmflg;
Description
The system call returns the shared memory identifier associated with key.
A shared memory identifier and associated data structure and shared memory segment of size size bytes are created for key, if one of the
following is true:
The key is equal to IPC_PRIVATE. For further information, see
The key does not already have a shared memory identifier associated with it, and (shmflg & IPC_CREAT ) is true.
Upon creation, the data structure associated with the new shared memory identifier is initialized as follows:
The and are set equal to the effective user ID and effective group ID of the calling process.
The low-order nine bits of are set equal to the low-order nine bits of shmflg. The is set equal to the value of size.
The and are set equal to zero (0). The shm_ctime is set equal to the current time.
Return Values
Upon successful completion, a non-negative integer, namely, a shared memory identifier is returned. Otherwise, a value of -1 is returned
and errno is set to indicated the error.
Diagnostics
The system call fails if any of the following is true:
[EINVAL] The size is less than the system-imposed minimum or greater than the system-imposed maximum.
[EACCES] A shared memory identifier exists for key, but operations permission, as specified by the low-order nine bits of shmflg,
would not be granted. For further information, see
[EINVAL] A shared memory identifier exists for key, but the size of the segment associated with it is less than size and size is not
equal to zero.
[ENOENT] A shared memory identifier does not exist for key, and (shmflg & IPC_CREAT ) is false.
[ENOSPC] A shared memory identifier is to be created, but the system-imposed limit on the maximum number of allowed shared memory
identifiers would be exceeded.
[ENOMEM] A shared memory identifier and the associated shared memory segment are to be created, but the amount of available physical
memory is not sufficient to fill the request.
[EEXIST] A shared memory identifier exists for key, but ((shmflg & IPC_CREAT ) and (shmflg & IPC_EXCL )) is true.
See Alsoshmctl(2), shmop(2), ftok(3)shmget(2)
06-29-2009
I solved my problem in another way.. but I'll try your solution too 
pratice makes perfect!