Problems with shared memory and lists Post: 302329516

Sponsored Content

Top Forums Programming Problems with shared memory and lists Post 302329516 by BeNdErR on Sunday 28th of June 2009 01:49:54 PM

06-28-2009

Registered User

Problems with shared memory and lists

Hi, I need to put in shared memory a list made with object of this structure:

Code:

typedef struct Obj{
char objname[20];
struct Obj *nextObj;
}Object

I've filled my list with (for example) 10 elements, but when i try to put it in shared memory to be read by another process i get segmentation fault after first read record..
I've read in another thread (here -> dynamic shared memory )that malloc gives sone problems with shared memory

Quote:

If you called malloc to create linked_list_header->ptr, then the address returned by malloc is private to the calling (the process that calls malloc) process. Same with NODE->buffer. You will have to allocate these things in shared "by hand" without calling malloc.

but i don't understand what i have to do now and how to solve my problem.

i tried to share object in this way:

Code:

Obj *head;     //pointer to the head of my FILLED list
int shmid;
Obj *sh;
...
shmid = shmget(SHMKEY,(sizeof(Obj)*10),0666|IPC_CREAT|IPC_EXCL);
...
sh = (Obj *)shmat(shmid, 0, 0666);
...
*sh = *head;

probably I miss something or maybe I'm totally wrong... how can I allocate Obj in shared "by hand" without calling malloc?

Thanks for your support, best regards.

BeNdErR

View Public Profile for BeNdErR

Find all posts by BeNdErR

10 More Discussions You Might Find Interesting

1. Programming

Shared memory

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...

2. UNIX for Advanced & Expert Users

Shared memory shortage but lots of unused memory

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...

3. Linux

all about shared memory

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...

4. Programming

memory sharing - not shared memory -

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...

5. Programming

Shared memory in shared library

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...

6. Programming

Shared memory for shared library

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.

7. Programming

[C] Problems with shared memory

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...

8. UNIX for Advanced & Expert Users

Shared Memory

Hi, Using ipcs we can see shared memory, etc.. details. How can I add/remove shared memory(command name)? Thanks, Naga:cool:

9. AIX

shared memory

1.How to know wich process is using the shared memory? 2.How to flush (release) the process from the shared memory?

10. Programming

Shared library with acces to shared memory.

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...

LEARN ABOUT OSF1

shm_unlink

shm_unlink(3)						     Library Functions Manual						     shm_unlink(3)

NAME

       shm_unlink - Removes a shared memory object created by a call to the shm_open function (P1003.1b)

LIBRARY

       Realtime Library (librt.so, librt.a)

SYNOPSIS

       #include <sys/mman.h>

       int shm_unlink (
       const char *name);

PARAMETERS

       *name   Points to the name of the shared memory object.

DESCRIPTION

       The  shm_unlink	function  removes the name of the shared memory object pointed to by name.  If this file descriptor is used by other func-
       tions, all references to the shared memory object are removed, but content removal is delayed until all open and mapped references  to  the
       shared memory object are removed.

RETURN VALUES

       On a successful call to the shm_unlink function, a value of 0 (zero) is returned.  Otherwise, a value of -1 is returned and errno is set to
       indicate the error.

ERRORS

       The shm_unlink function fails under the following conditions:

       [EACCES] 	   Permission to unlink the shared memory object is denied.

       [ENAMETOOLONG]	   The length of the name argument exceeds PATH_MAX, or a pathname component is longer than NAME_MAX while _POSIX_NO_TRUNC
			   is in effect.

       [ENOENT] 	   The named shared memory object does not exist.

RELATED INFORMATION

       Functions: close(2), mmap(2), munmap(2), shm_open(3)

       Guide to Realtime Programming delim off

																     shm_unlink(3)

10 More Discussions You Might Find Interesting

1. Programming

Shared memory

Discussion started by: joseph_shibu

2. UNIX for Advanced & Expert Users

Shared memory shortage but lots of unused memory

Discussion started by: cjcamaro

3. Linux

all about shared memory

Discussion started by: vijaya2006

4. Programming

memory sharing - not shared memory -

Discussion started by: elzalem