pointers and linked list with shared memory - memory

Question

I want to get 2 programs to communicate, one (server) would store datas, and the other (client) would just access it.
I'll have to use a linked list to store datas because it won't stop storing, and then I was wondering if I could access to the whole linked list if only the first node is shared in memory.
What I mean is… are we allowed to access from the client program to the memory pointed by a shared pointer?
Sorry it seems obvious that we can not, so should I store my linked list into the shared memory, or do you think that would be awkward?
Because if I do so, I'll have to declare a shared memory for every node right?
So, to add shared memory to both programs I need the same keys, but I don't know how many keys there will be, and I can't just store it for both programs, unless I would have had already a linked list…
so I used a very very VERY awkward method that I don't even know if it works right, but I wish you can tell, which is to use ftok that is supposed to take an (url,pid) and return a key. So I assumed it would send the exact same key if I used the same url and pid, using a fake pid starting from 0 that I would increment for every element I add to the linked list… what do you think about it? Any other way to do it which would seem less… crap?
typedef struct s_shared_elem
{
char c;
struct s_shared_elem* next;
struct s_shared_elem* previous;
}shared_elem;
typedef struct s_shared_list
{
s_shared_elem* first;
s_shared_elem* last;
}shared_list;
int forthekey = 0;
char* url="/home/toor/Projet_cgi/";
shared_elem* shared_malloc(int pid, const char* url)
{
shared_elem* shm;
int shmid;
int key=ftok(url,pid);
if((shmid=shmget(key,1,IPC_CREAT | 0666)) < 0)
{
perror("shmget");
exit(1);
}
if ((shm = shmat(shmid,NULL,0)) == (shared_elem*)-1)
{
perror("shmat");
exit(1);
}
return shm;
}
void Init_shared_list(shared_list* liste)
{
liste->first = NULL;
liste->last = NULL;
}
void Add_elem(shared_list* liste)
{
shared_elem* new = shared_malloc(pid,url);
new->next = NULL;
new->previous = liste->last;
if(liste->first == NULL)
{
liste->first = new;
liste->last = new;
}
else
{
liste->last->next = new;
liste->last = new;
}
forthekey++;
}
void shared_free(shared_elem* todelete,int pid, const char* url)
{
shared_elem* shm;
int shmid;
int key=ftok(url,pid);
if((shmid=shmget(key,1,IPC_CREAT | 0666)) < 0)
{
perror("shmget");
exit(1);
}
shmdt(todelete);
shmctl(shmid,IPC_RMID,NULL);
forthekey--;
}
void Delete_list(shared_list* liste)
{
while(liste->last != liste->first)
{
shared_elem* tmp=liste->last;
liste->last=liste->last->previous;
Shared_free(tmp,pid,url);
}
Shared_free(liste->first,pid,url);
}

Answer 1

In share memory you can insert a whole linked list. It is useful in many cases. You do not need to create a linked list of share memory (e.g. using previous key, next key ). All you need to copy each node of linked list to the shared memory.
for example .....
process2.c
#include<stdio.h>
#include<stdlib.h>
#include<string.h>
#include<sys/types.h>
#include<sys/ipc.h>
#include<sys/shm.h>
int main(int argc, char *argv[])
{
int shmid,i;
node *data;
if ((shmid = shmget(10, SHM_SIZE, 0644 | IPC_CREAT)) == -1) {
perror("shmget");
exit(1);
}
data = (node *)shmat(shmid, (void *)0, 0); // node is linked list
for(i=0;i<2;i++)
printf("%d\n",(data++)->item_code);
if (shmdt(data) == -1) {
perror("shmdt");
exit(1);
}
return 0;
}
process1.c
#include<stdio.h>
#include<stdlib.h>
#include<string.h>
#include<sys/types.h>
#include<sys/ipc.h>
#include<sys/shm.h>
int main(int argc, char *argv[])
{
node *SELL=NULL; // node is linked list (structure) SELL is header
insert(&SELL,"Soap",1,12.5,10);
insert(&SELL,"Pen",2,20.75,8);
display(SELL);
int shmid,i;
node *data;
if ((shmid = shmget(10, 2*sizeof(node), 0644 | IPC_CREAT)) == -1) {
perror("shmget");
exit(1);
}
data = (node *) shmat(shmid, (void *)0, 0);
for(i=0;i<2;i++)
{
*(data++)=*SELL;
SELL=SELL->next;
}
getchar();
if (shmdt(data) == -1) {
perror("shmdt");
exit(1);
}
shmctl(shmid, IPC_RMID, NULL);
return 0;
}
Run process1.c 1st then run process2.c